Marijuana's Lasting Effects on the Brain

NIDA Director Nora Volkow

September 2012

We repeatedly hear the myth that marijuana is a benign drug—that it is not addictive (which it is) or that it does not pose a threat to the user’s health or brain (which it does). A major new study published last week in Proceedings of the National Academy of Sciences (and funded partly by NIDA and other NIH institutes) provides objective evidence that, at least for adolescents, marijuana is harmful to the brain.

The new research is part of a large-scale study of health and development conducted in New Zealand. Researchers administered IQ tests to over 1,000 individuals at age 13 (born in 1972 and 1973) and assessed their patterns of cannabis use at several points as they aged. Participants were again tested for IQ at age 38, and their two scores were compared as a function of their marijuana use. The results were striking: Participants who used cannabis heavily in their teens and continued through adulthood showed a significant drop in IQ between the ages of 13 and 38—an average of 8 points for those who met criteria for cannabis dependence. (For context, a loss of 8 IQ points could drop a person of average intelligence into the lowest third of the intelligence range.) Those who started using marijuana regularly or heavily after age 18 showed minor declines. By comparison, those who never used marijuana showed no declines in IQ.

Other studies have shown a link between prolonged marijuana use and cognitive or neural impairment. A recent report in Brain, for example, reveals neural-connectivity impairment in some brain regions following prolonged cannabis use initiated in adolescence or young adulthood. But the New Zealand study is the first prospective study to test young people before their first use of marijuana and again after long-term use (as much as 20+ years later). Indeed, the ruling out of a pre-existing difference in IQ makes the study particularly valuable. Also, and strikingly, those who used marijuana heavily before age 18 showed mental decline even after they quit taking the drug. This finding is consistent with the notion that drug use during adolescence—when the brain is still rewiring, pruning, and organizing itself—can have negative and long-lasting effects on the brain.

While this study cannot exclude all potential contributory factors (e.g., child abuse, subclinical mental illness, mild learning disabilities), the neuropsychological declines following marijuana use were present even after researchers controlled for factors like years of education, mental illness, and use of other substances. Mental impairment was evident not just in test scores but in users’ daily functioning. People who knew the study participants (e.g., friends and relatives) filled out questionnaires and reported that persistent cannabis users had significantly more memory and attention problems: easily getting distracted, misplacing things, forgetting to keep appointments or return calls, and so on.

Unfortunately, the proportion of American teens who believe marijuana use is harmful has been declining for the past several years, which has corresponded to a steady rise in their use of the drug, as shown by NIDA’s annual Monitoring the Future survey of 8th, 10th, and 12th graders. Since it decreases IQ, regular marijuana use stands to jeopardize a young person’s chances of success in school. So as another school year begins, we all must step up our efforts to educate teens about the harms of marijuana so that we can realign their perceptions of this drug with the scientific evidence. 

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