Verbal learning and memory in adolescent cannabis users, alcohol users and non-users

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.