Marijuana Usage May Affect IQ in Teens

Share This:


Almost one out of every four American high schoolers smokes marijuana–more than nicotine cigarettes–believing it to be safe and free of any serious side effects. According to the results of a recent study, they may be wrong.

IQ Drop May Be Specific to Adolescents

A new study published in the prestigious journal Proceedings of the National Academy of Sciences examined the effect of persistent marijuana use before the age of 18, and the authors found several reasons for concern. The investigators looked at over 1,000 individuals who were initially IQ-tested at age 13–presumably before beginning any significant drug use–and who then began to use marijuana before the age of 18. They then followed these individuals up to age 38 and found that, on average, the people who were most closely monitored during this period experienced a loss of almost 10 IQ points, from about 100 (an average IQ) to below 91(an IQ which puts them near the bottom 25 percent of the general population). A comparable control group who didn’t use marijuana in adolescence experienced no drop in IQ.

Other psychological and cognitive measures, such as memory and attention, also declined in young marijuana users. Perhaps most importantly, those who used marijuana in their teenage years, and subsequently stopped using the drug, didn’t fully restore their neuropsychological function or IQ. People who started using the drug after age 18 didn’t show any decline in IQ, suggesting that the effect is specific to the young, developing brain.

No Known Safe Level of Use Among Teens

Among adults, especially young adults, marijuana is widely considered a relatively benign drug, although side effects are not uncommon. Side effects may include decreased lung capacity; chronic bronchitis; decreased sperm counts and motility; and decreased fetal growth and development with heavy use during pregnancy. Whether or not marijuana causes amotivational syndrome in teenagers–a lowered interest in social interactions and activities–is still controversial.

However, even allowing for the fact that IQ is, at best, a crude measure of intelligence and certainly doesn’t predict success in many areas of life, this newly documented impact on teenagers is noteworthy. It’s important to note that this new study didn’t look at how much marijuana was smoked, nor how potent the drug was. Thus, there is no known safe level of use among adolescents; even once-a-day use was associated with a lowered IQ.

Share This:

The One Medical blog is published by One Medical, an innovative primary care practice with offices in Boston, Chicago, Los Angeles, New York, Phoenix, the San Francisco Bay Area, Seattle, and Washington, DC.

Any general advice posted on our blog, website, or app is for informational purposes only and is not intended to replace or substitute for any medical or other advice. The One Medical Group entities and 1Life Healthcare, Inc. make no representations or warranties and expressly disclaim any and all liability concerning any treatment, action by, or effect on any person following the general information offered or provided within or through the blog, website, or app. If you have specific concerns or a situation arises in which you require medical advice, you should consult with an appropriately trained and qualified medical services provider.