BERLIN – Occasional to moderate cannabis use at a young age is not associated with detrimental effects on cognition or educational performance, according to the findings of a large U.K. study.
Adolescents with heavier use – defined in the study as self-reported lifetime use of cannabis 50 times or more by age 15 – had a modest 2.9% decrease in educational performance on a compulsory school exam given at age 15 or 16, compared with never-users. However, heavier use had no impact at all on IQ scores measured at age 15 after adjustment for potential confounding factors, Claire Mokrysz reported at the annual congress of the European College of Neuropsychopharmacology.
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Adolescent marijuana use may not be as detrimental to mental health as previously believed.
These results from the Avon Longitudinal Study of Parents and Children (ALSPAC), also known as “the Children of the 90’s Study,” stand in sharp contrast to those from an earlier Duke University study, which concluded that cannabis use likely has a neurotoxic effect on the adolescent brain that is apparent in midlife (Proc. Natl. Acad. Sci. USA 2012;109: E2657-64).
“Previous research findings showing poorer cognitive performance in cannabis users may have resulted from the lifestyle, behavior, and personal history typically associated with cannabis use rather than cannabis use itself,” said Ms. Mokrysz, of University College London.
She reported on 2,612 children in ALSPAC who had their IQ tested at ages 8 and 15.
Adolescents with heavier cannabis use by age 15 had a nearly a 3-point lower IQ at that age than did never users, after adjustment for IQ at age 8. However, upon further adjustment for maternal education, pregnancy, and early-life factors, and use of tobacco, alcohol, and other recreational drugs, the difference in IQ between heavier and never-users vanished.
Heavier users of cannabis scored an initially impressive 11% lower than never-users on the standardized educational performance exam in an unadjusted analysis. After adjustment for the potential confounders, however, the difference shrank to a modest 2.9%.
“People often believe that using cannabis can be very damaging to intellectual ability in the long term, but it is extremely difficult to separate the direct effects of cannabis from other potential explanations,” she said. “This is a potentially important public health message: The belief that cannabis is particularly harmful may detract focus from and awareness of other potentially harmful behaviors.”
ECNP President Guy Goodwin called the ALSPAC analysis potentially important.
“It suggests that the current focus on the alleged harms of cannabis may be obscuring the fact that its use is often correlated with that of other even more freely available drugs and possibly lifestyle factors. These may be as or more important than cannabis itself,” said Dr. Goodwin, professor of psychiatry at the University of Oxford (England).
The ALSPAC study is funded by the U.K. Medical Research Council, the Wellcome Trust, and the University of Bristol. Ms. Mokrysz reported having no financial conflicts.