Abstract
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Rationale Long-term heavy cannabis use can result in
memory impairment. Adolescent users may be especially
vulnerable to the adverse neurocognitive effects of cannabis.
Objectives and methods In a cross-sectional and prospective
neuropsychological study of 181 adolescents aged
16–20 (mean 18.3 years), we compared performance
indices from one of the most widely used measures of
learning and memory—the Rey Auditory Verbal Learning
Test—between cannabis users (n=52; mean 2.4 years of
use, 14 days/month, median abstinence 20.3 h), alcohol
users (n=67) and non-user controls (n=62) matched for
age, education and premorbid intellectual ability (assessed
prospectively), and alcohol consumption for cannabis and
alcohol users.
Results Cannabis users performed significantly worse than
alcohol users and non-users on all performance indices. They recalled significantly fewer words overall (p<0.001), demonstrating
impaired learning (p<0.001), retention (p<0.001)
and retrieval (p<0.05) (Cohen’s d 0.43–0.84). The degree of
impairment was associated with the duration, quantity,
frequency and age of onset of cannabis use, but was
unrelated to alcohol exposure or other drug use. No gender
effects were detected and the findings remained after
controlling for premorbid intellectual ability. An earlier age
of onset of regular cannabis use was associated with worse
memory performance after controlling for extent of exposure
to cannabis.
Conclusions Despite relatively brief exposure, adolescent
cannabis users relative to their age-matched counterparts
demonstrated similar memory deficits to those reported in
adult long-term heavy users. The results indicate that
cannabis adversely affects the developing brain and
reinforce concerns regarding the impact of early exposure.