A Review of the Research on the Risks and Harms Associated to the Use of Marijuana

Jordan Diplock, Irwin Cohen, and Darryl Plecas
School of Criminology and Criminal Justice,
University College of the Fraser Valley, Abbotsford, British Columbia, Canada
The truth about the risks and harms associated to personal marijuana use is rarely a feature of the ongoing debate over the legal status of the drug, with advocates on both sides at fault. Some consensus over the potential harms needs to be reached before any meaningful discussion can occur on this issue. This article reviews research published between 2000 and 2007 and suggests that there are many risks associated to marijuana use with regards to impairment, academic and social development, general and mental health, and continued drug use. Although some findings highlight very serious concerns for users, the numbers that become adversely affected by marijuana use do not represent the majority of users. A debate on the legal status of marijuana based on the facts about the risks and harms of this drug will greatly aid in determining the appropriate actions to address personal marijuana use around the world.
Keywords: Academic Performance; Gateway; Harms; Health; Impairment; Marijuana; Mental Health; Risks
The debate over the personal use of marijuana in North America and around the world is extremely contentious with supporters for decriminalization and legalization, and others who assert the importance of strict prohibition. The exceptionally adversarial nature of this debate is likely one of the main obstacles to determining the most appropriate way to address marijuana use within society. As a result of interested parties remaining resolute in their particular positions, the marijuana debate often becomes characterized by selective reporting or the misuse or misinterpretation of the available information. In addition, the popular debate rarely transcends ideological arguments on marijuana’s potential harms. With proponents of legalization championing marijuana as a benign drug and prohibitionists stressing its dangerousness, the debate often fails to consider the totality of the empirical research evidence. The purpose of this review is to discuss the harms associated with marijuana use from an objective viewpoint to provide a basis for the development of further research on how to best address the issues of marijuana use.
As research on marijuana use and its effects is constantly providing additional information, the full extent of the effects of marijuana on users will likely not be known conclusively in the near future. This should not be regarded negatively, as it is the nature of research that future studies improve upon the methodologies and results of previous research. For example, in 1997, The Independent, a popular British newspaper, was a strong supporter of the decriminalization of marijuana in the United Kingdom. In part, this support led to a pro-cannabis march that pressured the government to downgrade the classification of marijuana . Ten years later, that newspaper printed a public apology for its leadership role in the legalization campaign with a headline stating “If only we had known then what we can reveal today”. This example demonstrates the importance of considering new evidence and being willing to refine one’s position based on the best available information. By reviewing the current research on the potential harms associated with marijuana use, this review intends to synthesize the best evidence to inform the debate.
Ensuring that one considers the most current research on marijuana use is not only important because of the changing nature of academic research, but also because the drug under study has changed over the years. In other words, marijuana does not refer to cannabis with a particular level of -Tetrahydrocannabinol (THC). Over time, the level of THC in marijuana has changed; typically, it has increased. However, because there have been very few studies on the changes in potency of marijuana over the years, it cannot be confirmed conclusively that marijuana users in the 1970s were typically consuming a different drug than today’s users. The information that does exist suggests that, on average, marijuana users today are exposed to higher levels of THC than in past decades. Research on potency trends of seized marijuana between 1980 and 1997 concluded that average THC levels of marijuana seized in the United States increased from less the 1.5% in 1980 to approximately 3% in the early 1990s, to over 4% in 1997 . Moreover, in an article published by the Drug Enforcement Administration (DEA), Newell reported that average THC concentrations in marijuana from 36 samples seized in the state of Florida in 2002 were over 6%. These levels were determined to be at par with the averages reported by the Marijuana Potency Monitoring Project . In Canada , the Royal Canadian Mounted Police [RCMP] reported that on average seizures of marijuana in Canada had THC concentrations over 10%. Seizures in Europe of imported marijuana typically had THC levels between 2% and 8%, but the potency of hydroponically-grown “skunk” may be as high as double that of the imported marijuana . However, it must be kept in mind that the nature of marijuana production and distribution is such that a regular user would likely be exposed to marijuana of various different concentration levels of THC. As the majority of marijuana production remains the industry of criminals, many of whom use hydroponic operations and compete with each other to produce the most and the ‘best’ marijuana, there is no reason to believe that the quality of street marijuana has remained consistent over time.
In addition to levels of THC, the understanding of the number of different constituents of marijuana and their potential to interact with each other changes over time. ElSohly and Slade reported that the number of known natural compounds in marijuana increased from 423 to 489 between 1980 and 2005. Of those numerous chemicals, 70 were Cannabinoids, 9 of which were discovered since 1980 . The changes in knowledge about the complex chemical makeup of marijuana further complicate the study of the potential dangers of its use.
Because marijuana is used around the world by approximately 160 million people, there has been a great deal of research conducted on its effects on users . The use of marijuana results in a variety of changes within the user’s body that can have a range of effects . Given this, the focus of this review is limited to the research evidence on potential harms associated with marijuana use in the areas of: impairment; academic and social development; general physical health; mental health; and continuing drug use. Although there is also a substantial body of research on the medical use of marijuana for particular patients, a review and discussion of the research on medical marijuana is not included in this study. This exclusion is not meant to suggest that marijuana is universally accepted as a safe or effective treatment for any illness, as Voth has clearly demonstrated that the wider debate over the use of marijuana extends into the issue of the drug’s medical use. The discussion presented in this review will concentrate on the use of marijuana within the general population and the empirical evidence for how marijuana use effects the general population in the five previously listed areas.

To ensure that this review considered the most current research, information was collected from articles published from 2000 to 2007. Articles were identified by searching a number of databases, including Medline, Pub.Med, PsychINFO, and Google Scholar. To ensure a more complete search, a variety of keywords were combined with ‘marijuana’ to search the databases. In particular, these keywords related to the five aforementioned areas. An extremely partial list of keywords included ‘impairment’, ‘academics’, ‘heart disease’, ‘respiratory’, ‘cancer’, ‘psychosis’, and ‘gateway’.
Once an article was identified, it was assessed for appropriateness based on a review of the article’s title and abstract. One potential limitation of this review was that only full-text-available articles written in English were considered for this review. However, in order to expand the number of articles considered, both original research studies and articles that reviewed topics related to the harms of marijuana use were included. In order to ensure objectivity in the selection process, the inclusion or rejection of articles occurred without consideration of authorship or the conclusions or recommendations made by the authors. Given this, the articles considered in this review represented the continuum of current research on the harms that may be associated with marijuana use. Because of the scope of this topic and the amount of literature on marijuana use, the articles included in this review do not represent all available research on the effects of marijuana use. However, because many of the articles included in this review included extensive reviews of previous literature, the areas of focus for this review were well represented.
Finally, when considering the evidence presented in this review, it is critical to keep in mind that many of the studies based their results and conclusions on self-reported effects of marijuana use by the users themselves. While self-report studies are extremely valuable, they are susceptible to a variety of methodological problems, such as social desirability effects, errors in memory, exaggeration, and deception, which must be considered when evaluating results or conclusions . In addition, it is also extremely difficult to link or establish a direct causal relationship between drug use and other specific behaviours as it is likely that behaviours or outcomes are the result of multiple factors, rather than exclusively one factor, such as drug use.
Marijuana Related Impairment
One of the important debates in the research literature is the effect of marijuana use on cognitive and motor skills. Several studies have focused on determining whether there are any negative effects on cognitive or motor skills within hours of marijuana use . A number of studies have more specifically focused on the effect of marijuana use on abilities related to operating a motor vehicle . In addition to studies of short-term impairment, research has been conducted on long-term impairments associated with prolonged marijuana use .
Short-term Impairment
Impairment immediately after the consumption of marijuana may be a concern for users and the community at large. Short-term impairment has generally been assessed anywhere from 5 – 10 minutes to several hours after use. Testing the effects of marijuana on working and episodic memory determined that focusing attention and response accuracy were impaired immediately after smoking marijuana, even marijuana with less than 4% THC. The authors concluded that the marijuana resulted in difficulty maintaining a coherent train of thought and disruptions to selective filtering processes, both of which impaired memory. Similarly, another study reported that acute marijuana intoxication was accompanied by impairment of brain function related to goal-oriented activities. Further, it was suggested that marijuana consumption inhibited impulse and anger control in some users implying a possible link between marijuana use and violent or antisocial behaviour in some individuals . However, impaired attention was not found in a study of marijuana’s effects on auditory focused attention tasks where participants responded to a tone by pressing a button as quickly as possible. Results of an examination of brain functioning hours after using marijuana found that heavy marijuana users did not present impaired abilities on simple spatial working memory tasks, as deficits were compensated for by employing regions of the brain not commonly used during such tasks.
Although the research reported that short-term cognitive impairment could occur among marijuana users, the level of impairment and its seriousness was not significant. However, this does not suggest that there are no or few short-term risks of impairment. Instead, this conclusion may be due to the small sample sizes of only 10 to 12 participants in the studies examined . In effect, the sample sizes in these studies limited the ability to draw any firm conclusions about the range or seriousness of short-term cognitive impairments associated with marijuana consumption.
Researchers also examined the relationship between marijuana induced cognitive impairment and common abilities, activities, or behaviours, such as operating a motor vehicle. Ramaekers and co-workers concluded that decision-making, planning, tracking, reaction time, and impulse control were impaired by high-potency marijuana. Although the 20 subjects were considered only light users, substantial impairment of executive and motor functioning for a period of at least six hours was found. Although the 13% THC level in the marijuana used in this study was higher than the averages reported by the DEA and RCMP , this study demonstrated that serious impairment lasting for many hours was common when consuming high potency forms of marijuana.
Operating a motor vehicle can be dangerous at any time. However, doing so while impaired by marijuana significantly increases the risks of accident. Although some studies revealed that recent marijuana use was a causal factor for only a small proportion of accidents, short-term marijuana impairment does contribute to serious motor vehicle accidents To better determine marijuana impairment among drivers, standardized field sobriety tests have been designed to detect impairment by marijuana in a manner similar to alcohol. Research on field tests concluded that, as expected, impairment increases with the level of THC . Even low levels of THC can moderately impair driving abilities, but driving is severely impaired when either higher levels of THC marijuana is consumed or marijuana with lower levels of THC is consumed with even small amounts of alcohol . Considering the research examined for this review on the relationship between marijuana consumption and impairment, there appears to be a strong consensus that marijuana use has a negative and potentially harmful effect on driving.
Long-term Impairment
There are few studies on the long-term impairment of chronic marijuana consumption compared to the acute effects of marijuana use. Still, some researchers examined the potential for impairment as a result of long-term use, even during periods of abstinence . From the results of one study of older participants (33-50 years old), it appeared that, although heavy marijuana users showed impaired cognitive abilities after a week of abstinence, there were no noticeable impairments after twenty-eight days of abstinence . When compared to a control group, long-term marijuana using teens (aged 16 – 18) had equivalent task performance on a go/no-go task after twenty-eight days of abstinence . However, marijuana users committed more errors on cognitive tests and showed increased brain processing effort during the inhibition task . When comparing early-onset users to late-onset users, even after twenty-eight days of abstinence, early-onset frequent marijuana users had a greater likelihood of suffering a range of cognitive functioning impairments, in particular verbal IQ, compared to late-onset and non-users .
One interesting finding about long-term marijuana users was that there was an increase in brain activity in more regions of the brain when performing a variety of cognitive tests when compared to non-users. The researchers concluded that this finding was the result of the brain working harder and differently to overcome the deficits resulting from the marijuana use . In addition to working harder and differently, significantly increased blood volumes in various regions of the brain have been discovered , even after a period of abstinence of six to thirty-six hours. The researchers indicated that it remained unknown how these changes affected brain functioning and whether these changes were permanent, long-lasting, or temporary. However, these findings do suggest that there is a potential for some type of long-term brain impairment. Nonetheless, with the exception of impairments caused by psychosis and other mental illnesses discussed later in this review, when considering the totality of the research literature on the relationship between marijuana use and long-term cognitive or motor impairment, there appears to be little evidence to support the assertion that serious impairment is a likely result from long-term marijuana use, especially after a period of abstinence.
The Effects of Marijuana Use on Academic and Social Development
As marijuana is the drug of choice for many young people, it is necessary to understand whether marijuana has any negative effects on academic performance and the transition from adolescence to adulthood. The evidence for both immediate impairment and the possibility of longer-term impairment supports the notion that marijuana use may have negative consequences on the development of young users. In a consideration of academic performance and graduation, a number of studies have focused on the relationship between marijuana use and absenteeism , I.Q. , and academic achievement . By examining the lifestyles of adults who reported being heavy marijuana users in their youth, other researchers have attempted to assess the effects of marijuana use on social development . The following section provides a discussion of the literature in these areas.
Marijuana and School Performance
There are many factors that contribute to academic achievement, such as general intelligence, interest/curiosity, motivation, lifestyle, and social relationships/networks. Since the adolescent human brain is still developing, it is possible that recreational marijuana use may disrupt ‘normal’ development, which may manifest in, among other things, poorer school performance. Survey research revealed that students who were absent on the day of a school-based survey were more likely to use marijuana, alcohol, and cigarettes than students who were present. Although it is unsupportable to conclude that one specific day of absence from school was caused by or related to marijuana use, this study provides some small support for the more impressive findings of Lynskey and Hall’s review of cross-sectional studies on marijuana and school-related issues. Their review of over 50 research studies concluded that marijuana appeared to have a strong relationship with absenteeism, lack of retention, and not graduating.
An examination of the relationship between academic achievement and drug use in a diverse sample of 18,726 students concluded that marijuana use, when examined alone, was statistically significantly related to lower standardized test scores in math, science, reading, and social studies. Average scores on the math comprehension test for marijuana users were further below the mean than on any other test, while reading comprehension appeared to be affected the least. However, when marijuana was combined with alcohol or cigarettes, the results were much less robust. In effect, both regular smoking and alcohol intoxication explained much more of the variance, thus reducing the influence of marijuana on test scores. The explanation provided for this finding was the relatively small number of students who reported ever being under the influence of marijuana at school compared to the number of students who regularly used alcohol and/or cigarettes at school . Similarly, a study by Diego and colleagues found that grade point averages decreased as the reported frequency of marijuana use increased. Marijuana use had a larger negative correlation with grade point average as frequency of use increased than alcohol or cigarettes. While these findings suggested a link between marijuana use and academic achievement, the research could not establish a direct causal relationship or the direction of the relationship. Nonetheless, for the most part, social scientists agree that marijuana use is detrimental to school performance .
Since marijuana has been linked to short-term impairment and a decrease in school performance, some researchers have studied the effects of marijuana on IQ (29). However, measuring the direct effects of marijuana use on IQ has been difficult as there is rarely a baseline measure of a subject’s IQ prior to their initiation into marijuana use. One longitudinal study that had baseline measures of IQ prior to the subject ever using marijuana reported a statistically significant decrease in IQ score among individuals who smoked five or more marijuana cigarettes per week. On average, a 4.1 point decrease was measured between the time the subject was 9 – 12 years old (no prior use) and 17 – 20 years old (current and/or past use). However, when considering the degree of marijuana use for the sample of 70 marijuana users, only those characterised as heavy users showed any decreases in IQ compared to slight users, former users, and non-users who demonstrated increases in IQ . These results suggested that marijuana use has an effect on general intelligence but is more severe for regular and chronic marijuana users.
Marijuana Use and Later Social Development
Success in adulthood is related to a wide range of developmental and social variables throughout childhood and adolescence. It has been hypothesised that many of these contributing dynamics could be negatively affected by the use of marijuana. For example, some people contend that one of the possible outcomes of marijuana use is chronic low motivation. In effect, the hypothesis is that marijuana use among young people contributed to the development of low motivation which has long-term effects on school and employment performance. In their research, however, Lynskey and Hall concluded that there was little evidence to support the low motivational syndrome hypothesis because the majority of supportive evidence was based on older uncontrolled studies of case histories and observational reports, while controlled field or laboratory studies did not find compelling evidence of such a syndrome. Moreover, long-term (over 20 years), regular marijuana use among males was not associated with any specific negative socio-demographic effects such as alcohol or nicotine abuse or dependence, hospitalizations, and health-related quality of life .
However, other researchers have found several adverse associations between marijuana use and social development. A study of the relationship between marijuana use in 2,842 high school students and later occupational attainment concluded that marijuana had some differential negative associations with occupational attainment for males and females . Specifically, for males, self-reported abstinence or low frequency use of marijuana had no effect on occupational attainment, although high prestige jobs typically had a greater percentage of non-users or former low frequency users. However, for male users, after a certain threshold level was passed, success in occupational attainment decreased with increased early marijuana use. The threshold for this relationship in this study was ambiguous as the linear relationship began with the category associated to between 3 and 39 occasions of marijuana use in one year. Among females, early marijuana use was found to have strong negative outcomes on occupational attainment, but the pattern was different from that of males, lacking the easily identifiable threshold and negative linear relationship .
Green and Ensminger examined the effects of marijuana use on a variety of social variables among a cohort of 530 African Americans. Frequent adolescent marijuana use was associated with poorer academic achievement, a lack of stable employment, and family dysfunction. These results suggested that using marijuana 20 or more times during adolescence was associated with being unemployed, unmarried, and becoming a parent while unmarried. Early marijuana use was also linked to dropping out of school and continued marijuana use as an adult . Although this study was specific to African Americans, when considered with other studies on occupational attainment and school performance, these results contribute to the body of literature indicating that marijuana use among young people can have a detrimental outcome on their future. However, these findings do not confirm a causal relationship between marijuana use and poor performance in school or life. Still, the evidence does suggest that, even in the absence of a direct causal link, the use of marijuana during adolescence, for many young people, is often accompanied by other factors, such as the development of delinquent peer associations or a general lack of commitment to pro-social activities and institutions, which can lead to problems with social development.
General Health Consequences of Marijuana Use
The use of marijuana introduces foreign substances into the body and produces a number of chemical changes in the user’s brain and body. Given this, there is a large amount of literature focusing on the physical effects of marijuana. To begin, there is little evidence to suggest that marijuana use poses a serious risk for an overdose death or its infrequent use is related to the development of long-term health problems . Given this research, the majority of health-related studies focused on the potential harmful health outcomes associated with long-term and heavy marijuana use. One of the most widely studied issues is the relationship between smoking marijuana and the development of respiratory ailments .
In addition, the short-term and long-term effects of marijuana use on the circulatory system have also been extensively studied . Other researchers have focused on potential reproductive harms , the effects of marijuana use on the immune system , and the risks for cancers . There is also a burgeoning research literature on the degree to which marijuana users can develop a dependency and experience withdrawal symptoms . The following section will review the research literature on these important issues.
Respiratory Ailments Related to Marijuana Use
The most common way of using marijuana is by smoking it. A direct consequence of this method of consumption is that smoke must enter the airways and lungs of the user. As a result, researchers are interested in the amount and type of harm that smoking marijuana has on the respiratory system of users. This is particularly important because marijuana smoke contains many of the same poisons found in tobacco smoke. Given this, research has focused on determining whether the respiratory outcomes of smoking marijuana are similar or worse than those associated with smoking tobacco . Taylor et al. reported that respiratory symptoms were significantly more prominent in marijuana-dependent users than in non-users. The sample consisted of 21 year old subjects from the 1970s who self-reported short histories of smoking marijuana . The associated self-reported respiratory problems included wheezing, shortness of breath after exercise, nocturnal chest tightness, and early morning phlegm and mucus. These symptoms, which are typically indicative of chronic bronchitis, were also found to be associated with smoking marijuana in other research .
In their review of the research literature, Taylor and Hall argued that marijuana should be considered as damaging to the airways as tobacco and that there was a strong possibility that smoking marijuana was a contributing factor to the development of chronic lung disease. Further research concluded that long-term marijuana smoking was also associated with an increase in airflow obstruction and obstructive lung disease. A comparison of the effects of marijuana cigarettes to tobacco cigarettes concluded that one marijuana cigarette can have the obstructing effects on the lungs equal to that of two to five tobacco cigarettes. Lower lung density and increased total lung capacity were also recorded for marijuana smokers, but macroscopic emphysema was not found to be a common symptom . These findings suggested that serious negative respiratory outcomes should be expected for regular marijuana smokers, regardless of the marijuana’s THC levels, even among youth or young adults.
Since many of the detrimental effects on the respiratory system are the direct result of smoking, there have been several studies examining whether vaporizers provide a less harmful way to consume marijuana . Based on self-reported respiratory symptoms after using vaporizers to inhale marijuana cannabinoids, Earleywine and Barnwell concluded that vaporizers did provide some measure of safety, especially as the amount of marijuana inhaled increased. Hazekamp et al. reached a similar conclusion.
While the use of vaporizers may reduce or eliminate some of the respiratory ailments for users, the THC in marijuana may pose a respiratory risk. In response to the presence of THC, human airways experience cellular changes, especially to mitochondrial energetics, which are responsible, in part, for the health of cells and their energy production . Sarafina et al. described these changes as deleterious effects, as changes to the mitochondria of lung cells affects the viability and functioning of those cells. These changes were more significant with higher concentrations of THC and longer exposure times . In effect, as a result of THC in the lungs and airways, the risk of adverse pulmonary conditions is substantially increased by the potential for damage to the airway epithelial cells .
Potential Harms of Marijuana Use on the Heart and Circulatory System
One direct outcome from using marijuana is an immediate increase in heart rate. It is estimated that marijuana use increases the heart rate 20% to 50% immediately following consumption . This has led researchers to examine the short and long-term implications of marijuana use on the heart and the circulatory system. The majority of research in this area relies on case studies . Although the conditions documented in the research literature may be serious, it must be kept in mind that there is little evidence to suggest that the outcomes discussed in the case studies are typical or the norm for marijuana users.
Based on their case study of a 34-year-old man who reported heart fluttering and near syncope after marijuana use, Rezkalla and coworkers suggested that marijuana was a likely contributor to the decrease in coronary blood flow and ventricular tachycardia experienced by their subject. Another study described two cases; one in which a man with a history of heart problems suffered arrhythmia precipitated by marijuana use, the second described a young patient who suffered an onset of myocardial infarction. The researchers concluded that marijuana was a serious concern for those who may be predisposed to heart-related illnesses. Similarly, Caldicott et al. documented the case of a young patient who suffered a heart attack after marijuana use, despite having no other identifiable risk factors for a cardiac event.
Findings may be more informative when referring to larger samples that identify cardiac risks associated with marijuana use. One study concluded that, although it was less common than other stressors, marijuana use was a trigger for myocardial infarction . In this study , the risk of onset of myocardial infarction increased approximately five-fold in the first hour after use.
The conclusion of existing research is that marijuana use may, in rare instances, trigger a heart attack. However, it is important to recognise that the evidence in support of this conclusion may be confounded by the subject’s participation in a wide range of other unhealthy habits that may also contribute to a greater or lesser degree to a heart attack. Still, there is some evidence to conclude that marijuana is harmful to the heart and researchers, such as Aryana and Williams (, have stated a belief that heart problems related to marijuana use may be more common than is currently recognized. In addition, they warned that as the population of marijuana users aged, continued use may increase the risk for a number of adverse cardiovascular issues, such as tachyarrhythmia, acute coronary syndrome, vascular complication, and congenital heart defects .
Consequences of Marijuana Use on Reproduction and Pregnancy
There is a growing body of literature on the effects of drug use on sperm and egg development and the short and long-term outcomes for the foetus. This literature focuses on the relationship between drug use and implications for fertility and healthy, successful pregnancy. For example, several studies have investigated the effects of marijuana use on male sperm fertility and female hormones . Scheul et al. found that the presence of THC in the reproductive fluids of both males and females could inhibit the ability of sperm to complete fertilization. Other research reported that THC inhibited male fertility by binding to sperm cells and impairing sperm functions. In females, marijuana was found to disrupt the endocrine system and produce an estrogenic effect, which can have detrimental effects on specific elements of the female reproductive system . It should be noted, however, that the effects were more the result of the contaminants of smoking the drug than the psychoactive chemicals . In addition, marijuana use negatively affected female reproductive hormones which could lead to delayed ovulation . In considering these studies, the conclusion is that marijuana use may have some negative effects on human reproduction and that these outcomes are increased for those already at risk for infertility or other reproductive conditions.
Research also examined the degree to which marijuana use by pregnant mothers affected the unborn foetus and whether maternal marijuana use led to negative outcomes for the child. Kuczkowski reported that THC crosses the placental barrier, but that there was no confirmation that it had a teratogenic effect. In other words, there is no evidence that marijuana use by a pregnant mother contributes or causes birth defects or malformations. However, research by Wang et al. determined that some impairment was present in foetuses exposed to marijuana. This finding led the researchers to conclude that some long-term emotional and behavioural implications existed for children exposed to marijuana while in the womb.
Fried and Smith’s review of literature concluded that the effects of prenatal exposure to marijuana were subtle, with little evidence supporting growth or behavioural effects prior to age three. Others concluded that there was a statistically significant association between prenatal exposure to marijuana and later use; however, they concluded that there were many other potential factors that could have contributed to later marijuana use among those exposed to the drug while in the womb. One common theme among the research conducted to date was that they all called for more study on this issue. Although further research is needed in this area, to date, no substantial dangers have been confirmed to be associated to smoking marijuana while pregnant. However, marijuana smoke contains hazardous chemicals and materials, many of which exist in tobacco smoke. Therefore, just as health providers caution that tobacco should not be used by pregnant mothers, the caution should extend to marijuana use.
Marijuana Use as a Potential Threat to the Immune System
THC from marijuana may act upon the immune system similarly to the way it does on cells in the reproductive system . If the immune system is compromised by the use of marijuana, there may be significant implications for health care systems around the world . The relationship between marijuana use and deficiencies of the immune system is based, in part, on the findings that THC inhibits the ability of T-cells and alveolar macrophages to protect the body from foreign pathogens . Alveolar macrophages are a main defence against infections in the lungs. A review of the research literature in this area by Copeland et al. suggested, however, that it might require high doses of THC to substantially impair immune system functioning. Still, when considering the number of respiratory problems associated with smoking marijuana, and the possibility of serious carcinogenic properties in the drug, compromising the immune system may further compound the harms of marijuana use, especially among those already suffering from weakened immune systems.
Cancer Causing Effects of Marijuana
Because marijuana smoke contains many of the same harmful carcinogens as tobacco smoke, there is a possibility that marijuana use may be associated with the onset of various types of cancers, especially lung cancer as the most common method of consuming marijuana is by smoking it . To date, however, the research does not support the association between marijuana use and cancer. In their study, Hashibe and colleagues failed to find substantial evidence for an association between marijuana use and lung or upper areodigestive tract cancers. A review of research on lung cancer and marijuana use by Mehra et al. revealed many of the methodological difficulties in attributing outcomes specifically to smoking marijuana. For example, in many instances, marijuana users also smoke tobacco, there is the challenge of determining proper thresholds for marijuana use, and the research has typically included only small sample sizes. Mehra et al. suggested that because the plausibility of an association between marijuana smoking and cancer is so apparent, improved studies are required to test this possible link. Other research has reached similar conclusions about the link between marijuana use and cancer . Although a 1999 study by Zhang and colleagues reported a potential for marijuana use to increase the risk of squamous cell carcinoma of the head and neck, the evidence for a link between marijuana use and head and neck cancers has been limited and conflicting . In a recent study, marijuana was not found to increase the risk of head and neck cancer, although the duration of use under study might have been too limited to rule out the possibility of a longer-term effect . Another large-sample study concluded that marijuana was not associated to oral squamous cell carcinoma. There was also no link between maternal or paternal marijuana use and risk of childhood acute myeloid leukaemia .
Although there is currently no evidence to confirm that marijuana use increases the risk for any type of cancer, there will likely be continued research. Already, there are many researchers who believe that the changes to a variety of cells in the body caused by marijuana use may contribute to the development of cancers including lung cancer, oral cancers, and breast cancer .
Marijuana Dependency and Withdrawal
Despite the commonly held belief that marijuana use does not lead to addiction, existing research has often referred to a dependency on the drug . Although many people use marijuana on a regular basis, Looby and Earleywine reported that fewer than half of all daily users exhibited the behaviours necessary to meet the established criteria for being classified as drug dependent. These criteria include tolerance, withdrawal, taking the drug for longer periods of time or larger doses than intended, inability to stop or reduce use, increasing the time spent obtaining the drug and recovering from its effects, ignoring other important activities, and continuing use despite undesirable consequences. The authors argued that frequent use does not necessarily result in dependence, but that it may be a contributing factor. Their research suggested that negative effects of marijuana use, such as dissatisfaction with life, low motivation, and unhappiness, were more related to dependence on the drug than regular use . When considering the results of this research with findings from Copersino et al. on withdrawal symptoms, strong support is established for the idea that a proportion of frequent marijuana users suffer negative effects resulting from a dependency.
In terms of factors that most likely contribute to the development of a marijuana dependency, Hall reported that initiation to drug use at an early age was the most significant. However, in terms of public policy, if THC levels are indeed increasing and continue to increase, there will likely be a growing number of users who find themselves dependent on marijuana. Furthermore, as the National Institute on Drug Abuse’s definition of addiction focuses on the “uncontrollable, compulsive craving, seeking and use of drugs”, the physical effects of dependency and withdrawal may be only part of the problem, as addiction can occur without physical signs of dependency. This may prove more problematic if future research establishes additional negative health consequences of long-term use as users may experience more difficulty abstaining from use even in the face of exacerbating social and health problems.
Marijuana Use and Mental Health
In addition to some potentially serious physical health problems, marijuana use has also been associated with mental health problems. The link between marijuana use and psychosis or later schizophrenia has possibly received the most attention in the research literature. This body of research focuses on the role of marijuana in triggering psychosis the risk of developing schizophrenia among those who suffered marijuana-induced psychoses, the dangers of marijuana use for those already suffering from psychosis , and a number of hypotheses on whether marijuana use contributes to the presence of psychoses or schizophrenia or whether mental health issues contribute to the onset of marijuana use To a lesser degree, researchers have also investigated the relationship between marijuana use and depression and anxiety .
Marijuana-Precipitated Psychosis and Schizophrenia
An association between marijuana use and the onset of psychosis recently emerged as a serious concern. Given this, it is necessary to understand the potential for marijuana to contribute to psychosis and what proportion of marijuana users are at risk for developing psychosis. Research suggests that 8% to 10% of all cases of psychosis may be triggered by the use of marijuana Others concluded that marijuana use was linked to psychosis independent of any previous mental pathology. Given this, there is a growing consensus that, although it is relatively rare, marijuana induced psychosis is a potential threat to users, specifically to those who are already vulnerable for this type of mental affliction In order to explain this relationship, Caspi et al. reported that there may be an interaction between the chemicals typically present in marijuana and a number of ‘susceptible’ genes in the user that contributes to the onset of marijuana-induced psychosis and schizophrenia.
Research findings suggested that if marijuana use triggered psychosis, it might be a risk factor for schizophrenia in determining whether those who suffered from an episode of marijuana-induced psychosis were at risk of developing later schizophrenia, a group of such individuals was compared to a group of people referred for schizophrenia-spectrum disorders for the first time who had no history of marijuana psychosis . Although suffering from some recognized methodological problems, this study found that marijuana-induced psychosis was an important risk factor for developing schizophrenia and that it often had an earlier age of onset compared to those who self-reported no marijuana use. In partial support, Solowij and Michie found similarities between the cognitive effects of marijuana use and the cognitive endophenotypes of schizophrenia. This suggested that there was little reason to believe that marijuana is a direct cause of schizophrenia, but that marijuana likely aggravates pre-existing susceptibilities to schizophrenia . This hypothesis may explain why those prone to suffering from marijuana-related psychosis are also more susceptible to later schizophrenia.
One of the complications for fully understanding marijuana’s association with psychosis and later schizophrenia is that people with mental illness may continue to use the drug. The effects of marijuana use in patients who had recently suffered from psychosis were studied to determine whether symptoms were prolonged and worsened by the drug . Findings suggested that those who continued to use marijuana were at a greater risk of having more symptoms and a continuous course of mental illness . It could not be confirmed from the study, however, if marijuana caused the symptoms to worsen or the degree to which marijuana directly contributed to the symptoms.
There were a number of relational hypotheses tested in the research literature . The most common hypotheses were that: marijuana use caused psychosis and schizophrenia without any existing predisposition; marijuana use triggered the onset of these symptoms in people who were previously vulnerable; marijuana use exacerbated the symptoms in those already suffering; and those already suffering from these symptoms were more likely to self-medicate with marijuana. Although the current state of the research does not support the hypothesis that the relationship between marijuana and psychotic symptoms is one of self-medication , other hypotheses found more support.
The strongest support was for the second and third hypotheses. However, the causal hypothesis remains debatable. Degenhardt and Hall found that cases of schizophrenia in the general population did not rise with an increase in reported marijuana use, thus weakening the case for the causal hypothesis. Although further research is needed to more fully understand the causal association between marijuana use and psychosis, based on the research to date, psychosis and later schizophrenia as a result of marijuana use is a risk for a small portion of the marijuana using population.
Depression and Anxiety Among Marijuana Users
Although psychosis and schizophrenia were researched more than other mental health issues associated with marijuana use, there is a body of research on other issues such as depression, anxiety, and violence. Research found that increased marijuana use among high school students was associated with increased self-reports of depression. However, others found that past-year marijuana use was not a significant predictor of future development of depression. Similarly, research by Bonn-Miller et al. found that marijuana use was a predictor of anxiety symptoms, but not of depression. Again, it remains a challenge to determine whether marijuana use is a cause of these symptoms or if the symptoms play a contributing role in marijuana use.
Marijuana’s Role in Continuing Drug Use
The discussion of potential harms of marijuana use presented thus far indicated that marijuana poses a number of potential risks to the general population of users and some specific negative outcomes for a relatively small subgroup. The risk or actual harms associated with marijuana use can be seriously compounded by the use of other drugs and can become overshadowed by the dangers associated with becoming addicted to ‘harder drugs’. Moreover, there has long been the suggestion that marijuana can act as a ‘gateway’ for much harder drug use. It would appear that the probability that marijuana acts as a gateway to other illicit drugs is much higher than the other way around . According to Fergusson and Horwood , when adjusting for other common covariate factors such as childhood, family, and life-style factors, regular marijuana use (fifty or more times in a year) was strongly related to the onset of further illicit drug use. However, others found that the opportunities presented by the lifestyle accompanying marijuana use were just as likely as the actual use of marijuana to predict the use of other illicit drugs. Currently, there is no evidence to prove or disprove that any biological effects of marijuana use increases the likelihood of using other illicit drugs, although researchers continue to test this hypothesis . Based on twin studies, it is well established that marijuana use is a strong predictor of future illicit drug use regardless of the familial and environmental similarities between twins .
Still, since the majority of marijuana users do not continue on to other illicit drugs , it is important to understand what factors distinguish between those who do and those who do not go on to use harder drugs. The appropriate policy and control responses may be very different depending on whether the relationship was based on the biological effects of marijuana use or on the lifestyles that accompanied marijuana and other illicit drug use. Currently, it can be concluded that, for those who use marijuana, there is a risk of using other illicit drugs. However, without a better understanding of what causes or correlates with an increased risk, it is impossible to determine what effects changes to marijuana’s current legal status would have on patterns and rates of drug use.
The debate over the most appropriate policy to have with respect to the personal use of marijuana has generally been polarized because of differing positions on the drug’s harms. In addition to the unknown extent of the potential for harm caused by existence and interaction of over 800 natural chemical components of marijuana, including 70 cannabinoids, it can be concluded that marijuana does pose some considerable confirmed risks to users. Some concern over marijuana is merited by findings regarding its ability to create short-term impairment, specifically on driving ability. Academic performance and social development appear to be negatively affected by marijuana use, but the causal role that the drug plays in the lack of future success of young people remains unconfirmed. As expected, smoking the drug contributes to considerable harm to the lungs and airways. Even though the use of vaporizers removes the contaminants of combustion and reduces some major respiratory problems, THC exposure to the lungs appears to be unhealthy. The immune system is also compromised by the use of marijuana, specifically the ability of the lungs to defend against foreign pathogens. Although cancers, heart problems, and threats to human reproduction are not common among marijuana users, most experts contend that further investigation is required, and the potential for risk should not be dismissed. The development of psychosis and later schizophrenia should also remain a concern for a small proportion of those who use marijuana. Dependency and regular, long-term use of the drug are also factors that likely exacerbate the potential for the majority of the harms previously identified in this review. Of course, these harms are often compounded by the fact the marijuana users have an increased likelihood of continuing on to other illicit drugs.
It is important to remain cognizant of the fact that the harms associated with marijuana use, though very serious in some cases, are not experienced by the majority of users, although prolonged regular use will generally put a person at a greater risk than occasional use. The debate over marijuana use requires advocates of both decriminalization and prohibition to concede that marijuana is neither harmless, nor is it particularly dangerous to the majority of users. It should be acknowledged by all that the lives of a small proportion of the population will be seriously disrupted by marijuana use.
With an understanding of the potential harms associated to marijuana use forming the basis of the debate, politicians, policymakers, and citizens can begin to answer the important questions that will form the basis for discussing policy options. For example, what can be learned from other jurisdictions about ways to respond to the social and personal harms associated with marijuana use? What lessons can be learned from the experiences with alcohol that might apply to marijuana? Are there other or better approaches than prohibition to manage the problems that marijuana use creates? Further research will also be required to better understand whether decriminalization promotes increased use. In other words, would the decriminalization of marijuana create better opportunities to regulate the drug, or would it result in greater social harm?
To date, the research evidence shows that marijuana has a number of associated harms. In some cases, these harms are worse than those associated with regulated substances such as alcohol or tobacco. Based on the course of research, it is likely that future studies will further refine our understanding of the harms of marijuana use. However, because marijuana continues to be a popular recreational drug, it is necessary that researchers disseminate their latest findings in a wide range of ways in order for the public to have the best information at their disposal about the harms and risks associated with using marijuana.

Source: Journal of Global Drug Policy and Practice Vol. 3. Issue 2 Summer 2009

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