A recent study found no association between teenage marijuana use and cognitive decline.
The study recruited almost 3,000 American twins and assessed their IQs when they were 9-12 years old and then again when they were 17-20 years old. The twins said whether or not they smoked marijuana and in several hundreds of cases, one twin did and the other didn’t.
Using twins in research is useful because it eliminates any genetic factors or bias that could be associated with a disease or condition.
Overall, the IQ of non-users was 4 points higher than that of users. However, marijuana-using twins did not have greater IQ decline than their non-using sibling. This means that an external factor could be causing cognitive decline in adolescents who smoke marijuana and perhaps it’s not the drug itself.
This doesn’t mean than smoking pot as a teenager is problem-free. Let’s not forget that the researchers only looked at IQ and did not examine marijuana’s impact on working memory or executive function. Also, there are other studies that have found a link to cognitive decline, like this one. And the study was too short to rule out any long-term health effects.
Plus, maybe we should focus our research question to envelop the reasons as to which these 13-year-old kids feel the need to smoke marijuana?
2015 study: Longitudinal twin study finds no association between marijuana use and IQ decline
2012 study: Persistent cannabis use is linked to cognitive decline