More Author Affiliations: ORYGEN
Research Centre (Drs Yu¨ cel,
Whittle, and Lubman) and
Melbourne Neuropsychiatry
Centre, Department of
Psychiatry, The University of
Melbourne and Melbourne
Health (Drs Yu¨ cel, Whittle,
Fornito, and Pantelis),
Melbourne, Australia; School of
Psychology and Illawarra
Institute for Mental Health,
University of Wollongong,
Wollongong, Australia
(Dr Solowij and
Ms Respondek); and
Schizophrenia Research
Institute, Sydney, Australia
(Dr Solowij).
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recent structural magnetic resonance imaging (MRI) studies
have also reported contradictory findings, ranging from
no global or regional changes in brain tissue volume or
composition14-16 to gray and white matter density changes,
either globally17 or in focal regions, most notably in the
hippocampal and parahippocampal areas.18,19 However,
these previous studies used imaging techniques with relatively
coarse spatial and anatomical resolution and typically
focused on samples with multiple substance use or
comorbid psychiatric disorders and on only moderate levels
of cannabis use (ie, _2 joints per day). Indeed, despite
strong evidence of neurotoxicity in the animal literature,
6-9 to our knowledge, no neuroimaging study has
examined the neurobiologic sequelae of long-term heavy
cannabis use while controlling for the important confounds
of polydrug abuse and co-occurring psychiatric
disorders.
In this study, we used high-resolution 3-T MRI to assess
volumetric changes in 2 cannabinoid-rich regions
of the brain (the hippocampus and the amygdala) known
to be susceptible to the neurotoxic effects of cannabis exposure
in a sample of long-term heavy users carefully
screened for polysubstance abuse and mental disorders.
Given the growing literature regarding an association between
cannabis use and the development of psychosis20
and cognitive impairment,16,21 we also assessed for subthreshold
psychotic symptoms and verbal learning ability
in this otherwise psychologically healthy sample.
METHODS
PARTICIPANTS
Male cannabis users with long histories of regular and heavy
cannabis use (n=15) and nonusing healthy male volunteers
(n=16) matched on age, estimated premorbid intelligence (National
Adult Reading Test),22 years of education, and state and
trait anxiety (Spielberger State-Trait Anxiety Inventory)23 were
recruited from the general community via a variety of advertisements
(Table). Cannabis users had lower Global Assessment
of Functioning scale scores and greater depressive symptoms
(as measured using the Hamilton Depression Rating Scale)24
than the comparison group; however, there were no current
or lifetime histories of diagnosable medical, neurologic, or psychiatric
conditions as assessed using the Structured Clinical Interview
for DSM-IV Axis I Disorders, Patient Edition.25 All the
control subjects also underwent a Structured Clinical Interview
for DSM-IV Axis I Disorders, Non-Patient Edition.25 Subthreshold
psychotic symptoms were probed using the Scale for the
Assessment of Positive Symptoms26 and the Scale for the Assessment
of Negative Symptoms.27 Regarding alcohol use, the
groups did not differ in levels of current consumption, lifetime
use, or history of abuse or dependence; and no participant
drank more than 24 standard alcoholic drinks per week.
Significantly more cannabis users were also tobacco smokers
(_2=22.9, P_.001) (Table). For all users, cannabis was the primary
drug of abuse, with only limited experimental use of other
illicit drugs (generally _10 lifetime episodes).
PROCEDURE
Participants were assessed on 2 occasions, usually 1 week apart.
In the first test session, participants completed demographic,
clinical, and substance use history assessments. In the second
test session, they completed the Rey Auditory Verbal Learning
Test (RAVLT) and underwent structural MRI.
Participants were asked to abstain from using substances for
at least 12 hours before each test session, and cannabis users
reported abstaining from cannabis for a mean of 21.3 hours before
the first test session (median, 14 hours; range, 10-72 hours)
and a mean of 19.8 hours before the second test session (median,
17 hours; range, 12-48 hours). Urine samples were obtained
from users on 4 occasions and from controls on 2 occasions
to corroborate self-reported abstinence. Specifically, for
cannabis users, samples were obtained on the evening before
each test session and on the day of testing. For controls, samples
were collected only on the day of testing. Examination of these
samples demonstrated that all but 1 cannabis user had cannabinoid
metabolites (11-nor-_9-tetrahydrocannabinol-9-
carboxylic acid creatinine normalized) detected in urine samples
from the first test session, and levels were generally high
(evening: median, 467 ng/mg [range, 0-2320 ng/mg]; day of
testing: median, 447 ng/mg [range, 0-11 293 ng/mg]). From the
second test session, 2 users returned a 0 reading; otherwise,
cannabinoid metabolite levels were again high (evening: median,
456 ng/mg [range, 0-3511 ng/mg]; day of testing: median,
389 ng/mg [range, 0-4470 ng/mg]). The levels of urinary
cannabinoid metabolites generally corroborate the selfreported
patterns of heavy cannabis use in the sample. All but
2 control subjects returned a 0 reading for cannabinoid metabolites
across both test sessions. The 2 controls with positive
urine samples reported only minimal and very occasional
exposure to cannabis. The median level of cannabinoid metabolites
in controls at the first test session was 0 ng/mg (range,
0-184 ng/mg) and at the second test session was 0 ng/mg (range,
0-180 ng/mg).
STRUCTURAL MRI
The MRI data were obtained using a 3-T scanner (Intera; Phillips
Medical Systems NA, Bothell, Washington) at the Symbion
Clinical Research Imaging Centre, Prince of Wales Medical
Research Institute, Sydney. A 3-dimensional volumetric
spoiled gradient–recalled echo sequence generated 180 contiguous
coronal slices. The imaging parameters were as follows:
echo time, 2.9 milliseconds; repetition time, 6.4 milliseconds;
flip angle, 8°; matrix size, 256_256; and 1-mm3 voxels.
Hippocampal, amygdala, whole brain, and intracranial volumes
were measured using established reliable protocols28-31
and were delineated by a trained rater (S.W.) masked to group
information. Specifically, the hippocampal boundaries were as
follows: posterior, the slice with the greatest length of continuous
fornix; medial, the open end of the hippocampal fissure
posteriorly, the uncal fissure in the hippocampal body, and the
medial aspect of the ambient gyrus anteriorly; lateral, the temporal
horn of the lateral ventricle; inferior, the white matter inferior
to the hippocampus; superior, the superior border of the
hippocampus; and anterior, the alveus was used to differentiate
the hippocampal head from the amygdala. The anterior border
was the most difficult to identify consistently and was aided
by moving between slices before and after the index slice. The
amygdala boundaries were as follows: posterior, the appearance
of amygdala gray matter above the temporal horn; superolateral,
the thin strip of white matter that separates the amygdala
from the claustrum and the tail of the caudate; medial, the
angular bundle, which separates the amygdala from the entorhinal
cortex; superomedial, the semilunar gyrus; inferior, the
hippocampus; inferolateral, the temporal lobe white matter and
the extension of the temporal horn; and anterior, the slice anterior
to the appearance of the optic chiasm. Whole brain volumes
were estimated using the Brain Extraction Tool method32
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to separate brain from nonbrain tissue. After brain/nonbrain
segmentation, each voxel was classified into gray matter, white
matter, or cerebrospinal fluid using FAST Model statistical software.
33 Only gray and white matter were used in the estimate
of whole brain volumes. The intracranial cavity was delineated
from a sagittal reformat of the original 3-dimensional data
set. The major anatomical boundary was the dura mater below
the inner table, which was generally visible as a white line.
Where the dura mater was not visible, the cerebral contour was
outlined. Other landmarks included the undersurfaces of the
frontal lobes, the dorsum sellae, the clivus, and the posterior
arch of the craniovertebral junction.
Interrater and intrarater reliabilities were assessed by means
of the intraclass correlation coefficient (ICC) (absolute agreement)
using 15 brain images from a separate MRI database established
specifically for this purpose and that has previously
been delineated by another expert rater. For the hippocampus,
interrater ICC reliabilities were 0.92 (right) and 0.91 (left)
and intrarater ICC reliabilities were 0.98 (right) and 0.95 (left).
For the amygdala, interrater ICC reliabilities were 0.85 (right)
and 0.88 (left) and intrarater ICC reliabilities were 0.93 (right)
and 0.97 (left). Once reliability was established, the rater (S.W.)
delineated the regions of interest for the images acquired from
the present study.
STATISTICAL ANALYSES
Whole brain volume, age, educational level, and estimated IQ
were not significantly different between the 2 groups and were,
therefore, not used as covariates (Table). Regional gray matter
volumes for the hippocampus and amygdala were corrected for
the effect of the intracranial cavity using a previously described
formula34 and were analyzed using analyses of variance,
with hemisphere (left or right) and region (hippocampus
and amygdala) as within-subject factors and group as the
between-subject factor. Main effects and interactions were evalu-
Table. Demographic, Clinical, Drug Use, and MRI Volumetric Measures
Measure
Long-term Cannabis Users
(n=15)
Nonusing Control Subjects
(n=16) P Valuea
Age, mean (SD), y 39.8 (8.9) 36.4 (9.8) .31
IQ, mean (SD) 109.2 (6.3) 113.9 (8.1) .09
RAVLT score, mean (SD)
Sum of 5 learning trials 43.8 (8.8) 57.4 (10.1) _.001
20-min delay 8.9 (4.1) 12.3 (3.7) .009b
Educational level, mean (SD), y 13.4 (3.2) 14.8 (3.7) .28
GAF scale score, mean (SD) 72.0 (11.2) 80.8 (9.4) .02
HAM-D score, mean (SD) 5.87 (3.2) 2.56 (1.9) _.001b
STAI, mean (SD)
State anxiety 34.3 (9.8) 32.9 (9.4) .67
Trait anxiety 39.3 (9.7) 39.0 (8.2) .92
SAPS score, mean (SD) 8.1 (7.9) 0.6 (1.2) _.001b
SANS score, mean (SD) 11.7 (8.5) 1.4 (1.4) _.001b
Cannabis use
Duration of regular use, mean (SD) [range], yc 19.7 (7.3) [10-32] NA NA
Age started regular use, mean (SD) [range], yc 20.1 (6.9) [12-34] NA NA
Current use, mean (SD), d/mod 28 (4.6) NA NA
Current use, mean (SD), cones/mod,e 636 (565) NA NA
Cumulative exposure, past 10 y, mean (SD)f 77 816 (66 542) NA NA
Cumulative exposure, lifetime, mean (SD)f 186 184 (210 022) 12.7 (12.2) _.001
Estimated episodes of use, median (range) 62 000 (4600-288 000) 11 (0-30) _.001
Alcohol use, mean (SD), standard drinks/wk 9.6 (6.1) 6.8 (5.0) .19
Tobacco use, mean (SD), cigarettes/d 16.5 (8.9) 7.5 (9.2) .20
Brain volumes, mean (SD), mm3
Intracranial cavity 1 546 237 (94 018) 1 607 590 (136 386) .14
Whole brain 1 310 780 (90 778) 1 374 123 (105 673) .09
Hippocampus .002g
Left hemisphere 2849 (270) 3240 (423)
Right hemisphere 2949 (244) 3348 (400)
Amygdala .01g
Left hemisphere 1766 (98) 1878 (190)
Right hemisphere 1601 (143) 1744 (158)
Abbreviations: GAF, Global Assessment of Functioning; HAM-D, Hamilton Depression Rating Scale; MRI, magnetic resonance imaging; NA, not applicable;
RAVLT, Rey Auditory Verbal Learning Test; SAPS, Scale for the Assessment of Positive Symptoms; SANS, Scale for the Assessment of Negative Symptoms;
STAI, State-Trait Anxiety Inventory.
aTwo-tailed t test unless otherwise indicated.
bMann-Whitney test.
cRegular use was defined as at least twice a month.
dCannabis users had used at this level for most of their drug-using history.
eA cone is the small funnel into which cannabis is packed to consume through a water pipe in a single inhalation. Without the loss of sidestream smoke, the
quantity of tetrahydrocannabinol delivered by this method is estimated as equating 3 cones to 1 cigarette-sized joint. Thus, the cannabis users in this study
smoked the equivalent of 212 joints per month, or approximately 7 joints per day.
fExpressed as cones for users and as episodes for controls. Estimates of lifetime exposure beyond 10 years in these very long-term users became skewed and
unreliable; hence, the 10-year estimate was used in correlational analyses.
gRegion_group analysis of variance.
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ated using Greenhouse-Geisser–corrected degrees of freedom,
with _=.05. Effect sizes, expressed as Cohen d, are also reported
for pairwise contrasts. Only effects involving group (cannabis
users vs nonusers) and associations with cannabis use
parameters are reported because this was the primary focus of
the present study. Group comparisons of performance on the
RAVLT and measures of subthreshold psychotic symptoms
(using the Scale for the Assessment of Positive Symptoms and
the Scale for the Assessment of Negative Symptoms) were conducted
using independent-samples t tests or Mann-Whitney tests
for nonnormally distributed data. Pearson product moment correlational
analyses were conducted to examine the behavioral
(ie, symptom and cognitive) relevance of any identified group
differences in regional brain volumes and the association
between these brain changes and parameters of cannabis use.
These analyses were necessarily exploratory given the limited
sample size.
RESULTS
GROUP CONTRASTS
In the analysis of regional gray matter volumes, there
was a significant main effect of group (F1,29=12.98,
P=.001) and a region_group interaction (F1,29=6.25,
P=.02). This result and the post hoc pairwise analyses
demonstrated reduced hippocampal volumes in cannabis
users (F1,29=11.14, P=.002 corrected; a reduction of
12.1% in the left and 11.9% in the right hippocampus
relative to controls), with a very large effect size (Cohen
d: left hippocampus, 1.17; and right hippocampus,
1.27) (Figure 1). Cannabis users also had smaller
amygdala volumes (F1,29=7.31, P=.01 corrected; a
reduction of 6.0% in the left amygdala and 8.2% in the
right amygdala relative to controls), with large effect
sizes (Cohen d: left amygdala, 0.80; and right amygdala,
0.99). The region _ group interaction reflects that the
overall reduction in hippocampal volume was relatively
(and significantly) greater than the reduction in amygdala
volume (12.0% in the hippocampus vs 7.1% in the
amygdala). In the analysis of subthreshold psychotic
symptoms, cannabis users reported significantly higher
positive symptoms (Scale for the Assessment of Positive
Symptoms; z=−3.57, P_.001) and negative symptoms
(Scale for the Assessment of Negative Symptoms;
z=−3.66, P_.001) than nonusing controls. Regarding
verbal learning, cannabis users displayed significantly
poorer performance than controls on the RAVLT measures
(sum of words recalled across the 5 learning trials:
z=−3.97, P_.001; and free recall after a 20-minute
delay: z=−2.61, P=.009).
CORRELATIONAL ANALYSES
There was a significant inverse association between left
hippocampal volume and cumulative cannabis exposure
during the previous 10 years (r=−0.62, P=.01; accounting
for 38% of the variance in left hippocampal volume)
(Figure 2A). When 1 participant with relatively
higher cumulative cannabis exposure and small hippocampal
volume was excluded, 22% of the variance was
still accounted for despite falling short of significance in
the reduced sample (r=−0.47, P=.09). There was also an
association between left hippocampal volume and positive
symptoms (r=−0.77, P_.001) (Figure 2B) and between
positive symptoms and cumulative cannabis exposure
(r=0.52, P=.048) (Figure 2C). The associations
between left hippocampal volume and cumulative cannabis
exposure and between left hippocampal volume and
positive symptoms remained after controlling for the effects
of global functioning (Global Assessment of Functioning
scale) and depressive symptoms (Hamilton Depression
Rating Scale). No other associations were found
between other brain volumetric measures, cannabis use,
and psychotic symptoms, and they did not vary as a function
of alcohol or tobacco use. Measures of RAVLT performance
did not correlate with hippocampal or amygdala
volumes in either controls or cannabis users.
COMMENT
To our knowledge, this is the first human study of longterm
heavy cannabis users to demonstrate marked
exposure-related hippocampal volume reductions.
These findings corroborate previous animal research,6-9
suggesting that long-term heavy cannabis use is associated
with significant and localized hippocampal volume
reductions that relate to increasing cumulative cannabis
exposure. In addition, the present findings are consis-
tent with the view that cannabis use increases the risk
of psychotic symptoms and informs the debate concerning
the potential long-term hazardous effects of cannabis
in this regard. The bilateral reduction in amygdala
volume is a novel but not unexpected finding given the
dense concentration of cannabinoid receptors in this
region.35
Although these findings are consistent with those of
a previous study,18 it is difficult to directly compare these
results with those of other human studies given that past
work used MRI with lower magnetic field strength and
spatial resolution and did not conduct region-of-interest–
based analyses (eg, performed whole-brain voxel-based
analyses18). Tzilos et al14 conducted the only other study,
to our knowledge, that investigated cannabis users with
a relatively long history of use (specifically, an average
duration of use of 22.6 years, or 18.9 years of daily use)
and their study is, therefore, most comparable with the
present study. Although they found no effects of longterm
cannabis use on hippocampal volume, the authors
acquired their images at a lower field strength and with
a coarser spatial resolution (1.5 T with 3-mm-thick slices
vs 3 T with 1-mm-thick slices in the present study), an
important consideration given the size of the brain structures
investigated. Moreover, their region of interest was
less specific to the hippocampus relative to the present
measure because they also included the parahippocampal
gyrus. Furthermore, there was a relatively large age
discrepancy between their users and controls (38.1 vs 29.5
years), and the minimum duration of exposure to cannabis
was considerably lower in their sample (as little as
1 year of cannabis exposure), but, overall, their sample
reported an average of 20 100 lifetime episodes of use.
In contrast, the minimum duration of exposure to cannabis
in the present sample was 10 years, with an average
of 62 000 episodes of use. Thus, despite a similar mean
duration of use, the present sample used more than 3 times
as much cannabis, which may explain the finding of a
dose-response relationship between hippocampal volume
and cumulative cannabis use. Further highresolution
MRI work is necessary to characterize precisely
the dosage of cannabis required for significant brain
changes to occur.
The pattern of use in the present sample is consistent
with heavy cannabis use patterns that have previously
been reported in other Australian studies. For example,
Copeland and colleagues36 reported median daily intake
of 8 cones (the small funnel into which cannabis is packed
to consume through a water pipe in a single inhalation)
in an Australian sample of cannabis users seeking treatment
for cannabis dependence, ranging up to 125 cones
per day in the heaviest user, with 11% reporting cannabis
smoking throughout the day. The heaviest user herein
reported smoking 80 cones per day (approximately 25
joints smoked throughout the day). This pattern of cannabis
use is not dissimilar to the heaviest cannabis users
from other studies of non–treatment-seeking samples of
Australian cannabis users.37,38
Despite the large magnitude of effects observed, it remains
unclear whether these volumetric reductions
reflect neuronal or glial loss, a change in cell size, or a
reduction in synaptic density (eg, dendritic arborization),
all of which have been reported in rodent studies.
6-9 For example, Scallet and colleagues9 found striking
tetrahydrocannabinol-induced residual decreases in
the mean volume of hippocampal neurons and their nuclei
and a 44% reduction in the number of synapses up
to 7 months after the last exposure to tetrahydrocannabinol.
Moreover, Landfield and colleagues7 administered
tetrahydrocannabinol 5 times a week for 8 months
(approximately 30% of the rat lifespan, and comparable
in frequency and duration to the present sample) and
found significant tetrahydrocannabinol-induced decreases
in neuronal density in the hippocampus. Such
findings may help explain the mechanisms underlying
gross hippocampal and amygdala volume loss seen in this
sample of long-term heavy cannabis users.
Left Hippocampal Volume, mm3
In the present study, hippocampal volume in the cannabis-
using group was inversely correlated with cumulative
exposure to the drug in the left, but not right, hemisphere.
Previous functional imaging studies16,39 have found
reduced left hippocampal activation during cognitive performance
in cannabis users, and there is evidence to suggest
that hippocampal abnormalities in psychiatric disorders
such as schizophrenia are more prominent in the
left hemisphere.40 These findings converge to suggest that
the left hippocampus may be particularly vulnerable to
the effects of cannabis exposure and may be more closely
related to the emergence of psychotic symptoms. In this
context, it is interesting that we found a significant inverse
correlation between left hippocampal volume and
positive symptoms. Cannabis use was also positively correlated
with positive symptoms, suggesting that there are
complex associations among exposure to cannabis, hippocampal
volume reductions, and psychotic symptoms.
Given these relationships, it is possible that the exposurerelated
hippocampal reduction may reflect heavy cannabis
use in response to preexisting or developing psychotic
symptoms. However, there is limited empirical
support for long-term self-medication of subthreshold psychotic
symptoms with cannabis and stronger support for
the induction of psychotic symptoms subsequent to cannabis
exposure.20 As such, it seems more likely that prolonged
heavy use of cannabis induced subthreshold psychotic
symptoms and that both of these factors are
associated with hippocampal volume loss. These symptoms
were subthreshold because these cannabis-using participants
were carefully screened for current and past history
of mental disorders. Furthermore, the fact that the
mean age of the present cannabis-using sample was nearly
40 years suggests that these symptoms are unlikely to reflect
a prodrome. One speculation is that the present participants
were less genetically vulnerable to developing
a psychotic disorder subsequent to cannabis use,41,42 allowing
them to smoke heavily for many years. Future longitudinal
work assessing the emergence of hippocampal
reductions and psychotic symptoms with continued exposure
to cannabis, and how these are related to polymorphic
variations in susceptibility genes for psychotic
disorders, will prove useful in better characterizing these
relationships.
Given that cannabis users had significantly greater depressive
symptom scores than controls and that there is
an association between depression and hippocampal volume
reduction,43 it could also be argued that depressive
symptoms may be another mediating factor in the relationship
between cannabis use and hippocampal volume
reduction. However, there are a variety of important
considerations that make this unlikely. First, there
was no significant association between hippocampal volumes
and depressive symptom scores. Second, the relationship
between left hippocampal volume and quantity
of cannabis used was maintained after statistically
controlling for depressive symptoms. Finally, the overwhelming
evidence suggests that hippocampal reductions
in major depressive disorder tend to occur in the
more persistent forms of the disorder (eg, multiple episodes,
repeated relapses, or long illness duration).43,44 This
was not the case in the present sample of cannabis users,
who scored less than 6.0 on the Hamilton Depression
Rating Scale, had never been diagnosed as having
major depression, and did not seek treatment for any depressive
disorder.
Cannabis users showed poorer performance on measures
of verbal learning, consistent with previous findings.
Although some functional imaging studies have
found reduced left hippocampal blood flow and activation
during verbal (and visual) learning tasks in cannabis
users, we found no correlation between RAVLT
performance measures and hippocampal volume in either
controls or cannabis users. It is likely that anatomical volume
is a less sensitive measure than brain activation for
identifying correlations with behavioral performance. This
is a particularly pertinent consideration given that the
performance measures on the RAVLT are likely to reflect
the operation of numerous cognitive processes not
necessarily related to hippocampal function. Future work
using experimental tasks designed to more specifically
probe memory functions mediated by the hippocampus
may be useful in this regard.
The bilateral reduction in amygdala volume is a novel
but not unexpected finding given the dense concentration
of cannabinoid receptors in this region.35 There were
no cognitive, psychotic, or depressive symptom associations
with reduced volume in the amygdala. However,
this region has been significantly implicated in cannabinoid-
associated emotional and reward-related learning
and memory processes.47,48 Given that these aspects of
learning have not been examined in human cannabis users,
they would seem to serve as a potentially informative
avenue forward to help elucidate the functional relevance
of such volumetric reduction in the amygdala.
The relationship between long-term cannabis use and
brain abnormalities is complex. Although a limitation of
this study may be the residual effects of cannabis in light
of the fact that the cannabis users in this study were required
to be cannabis free for only 12 to 24 hours before
MRI, such issues are likely to be more pertinent for studies
examining more dynamic aspects of brain functioning
(eg, activations and cognition).49 The present structural
findings are unlikely to relate to the recent effects
of cannabis use because we are unaware of any evidence
that suggests that the hippocampus and amygdala can
change in volume by 6% to 12% in short periods. However,
although we maintain that the present results reflect
brain changes associated with long-term heavy cannabis
use rather than the consequences of recent exposure,
further longitudinal work is required to assess whether
such changes are reversible across more protracted periods
of abstinence.
Another limitation of this study is the relatively small
sample size, although the sample was exceptionally unique
in that participants were very long-term and heavy cannabis
users (mean of 5-7 joints per day for _10 years)
without polydrug use or co-occurring neurologic or diagnosable
mental disorders. As such, we conducted the
first, to our knowledge, “pure” examination of the effects
of heavy and protracted exposure to cannabis in humans.
The large effect sizes of the main findings suggest
that these results are robust and reproducible. These findings
are further strengthened by the observed dose-
response relationships between hippocampal volume reductions
and cumulative cannabis use.
There is ongoing controversy concerning the longterm
effects of cannabis on the brain.  These findings
challenge the widespread perception of cannabis as having
limited or no neuroanatomical sequelae. Although
modest use may not lead to significant neurotoxic effects,
these results suggest that heavy daily use might indeed
be toxic to human brain tissue. Further prospective,
longitudinal research is required to determine the
degree and mechanisms of long-term cannabis-related
harm and the time course of neuronal recovery after abstinence.
Correspondence: MuratYu¨ cel, PhD,MAPS,ORYGENResearch
Centre, 35 Poplar Rd (Locked Bag 10), Melbourne,