The Link Between Adolescent Marijuana Use and Lower IQ: What Science Tells Us
As marijuana use becomes more prevalent and the debate over “medical” marijuana intensifies, people may wonder what the consequences will be for adolescent users of the drug. One of the answers is simple and scientifically verifiable: Those who start using marijuana habitually as youth tend to have lower IQs as adults when compared to non-users over the same span of time.
The National Institute on Drug Abuse states: “A large longitudinal study in New Zealand found that persistent marijuana use disorder with frequent use starting in adolescence was associated with a loss of an average of 6 or up to 8 IQ points measured in mid-adulthood. Those who used marijuana heavily as teenagers and quit using as adults did not recover the lost IQ points.”
Here is another study: “Among nearly 4,000 young adults in the Coronary Artery Risk Development in Young Adults study tracked over a 25-year period until mid-adulthood, cumulative lifetime exposure to marijuana was associated with lower scores on a test of verbal memory but did not affect other cognitive abilities such as processing speed or executive function.”
These were two of several studies cited by the NIDA.
As always, there are caveats with these studies. Marijuana users may often use other drugs as well, thus clouding the results of any study focused on one drug. Researchers must also take into account their subjects’ socioeconomic position, family situation, and much more when drawing conclusions.
The evidence, however, points to the simple fact that when adolescents consistently use marijuana throughout their youth, their IQ is generally lower than those who did not. Marijuana harms brain development and should not be used, especially by our youth.