Smart Approaches to Marijuana’s 2017 publication references academic studies which suggest that marijuana primes the brain for other types of drug usage.  Here’s the summary on that subject from page 4, Marijuana and Other Drugs: A Link We Can’t Ignore :

MORE THAN FOUR in 10 people who ever use marijuana will go on to use other illicit drugs, per a large, nationally representative sample of U.S. adults.(1) The CDC also says that marijuana users are three times more likely to become addicted to heroin.(2)

Although 92% of heroin users first used marijuana before going to heroin, less than half used painkillers before going to heroin.

And according to the seminal 2017 National Academy of Sciences report, “There is moderate evidence of a statistical association between cannabis use and the development of substance dependence and/or a substance abuse disorder for substances including alcohol, tobacco, and other illicit drugs.”(3)

RECENT STUDIES WITH animals also indicate that marijuana use is connected to use and abuse of other drugs. A 2007 Journal of Neuropsychopharmacology study found that rats given THC later self -administered heroin as adults, and increased their heroin usage, while those rats that had not been treated with THC maintained a steady level of heroin intake.(4) Another 2014 study found that adolescent THC exposure in rats seemed to change the rodents’ brains, as they subsequently displayed “heroin-seeking” behaviour. Youth marijuana use could thus lead to “increased vulnerability to drug relapse in adulthood.”(5)

National Institutes of Health Report

The National Institutes of Health says that research in this area is “consistent with animal experiments showing THC’s ability to ‘prime’ the brain for enhanced responses to other drugs. For example, rats previously administered THC show heightened behavioral response not only when further exposed to THC, but also when exposed to other drugs such as morphine—a phenomenon called cross-sensitization.”(6)

Suggestions that one addictive substance replaces another ignores the problem of polysubstance abuse, the common addiction of today.

Additionally, the majority of studies find that marijuana users are often polysubstance users, despite a few studies finding limited evidence that some people substitute marijuana for opiate medication. That is, people generally do not substitute marijuana for other drugs. Indeed, the National Academy of Sciences report found that “with regard to opioids, cannabis use predicted continued opioid prescriptions 1 year after injury.  Finally, cannabis use was associated with reduced odds of achieving abstinence from alcohol, cocaine, or polysubstance use after inpatient hospitalization and treatment for substance use disorders” [emphasis added].(7)

Moreover, a three-year 2016 study of adults also found that marijuana compounds problems with alcohol. Those who reported marijuana use during the first wave of the survey were more likely than adults who did not use marijuana to develop an alcohol use disorder within three years.(8) Similarly, alcohol consumption in Colorado has increased slightly since legalization. (9)

Source:   http://www.poppot.org/2017/07/03/replacing-one-addiction-another-will-not-work/