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.