Cannabis Doesn’t Cause Cognitive Decline, Twin Study Finds

This article was originally published on The Cannigma, and appears here with permission.

Does cannabis use, medicinal or otherwise, have a long-term, negative impact on cognitive function? Years of studies have provided complex and often conflicting answers, particularly when it comes to teenage cannabis use. One new long-term study, looking at sets of twins where one sibling uses cannabis and the other doesn’t, offers new insights.

While slight IQ drops are indeed correlated to cannabis use, the new study suggests, those drops are not caused by it. Instead, there may be a connection between a genetic predisposition for cognitive decline and likelihood of using cannabis in the first place.

A Methodological Problem?

There is a growing number of studies that suggest cannabis use, particularly in adolescence, can lead to reduced cognitive function and IQ later on in life. 

These studies mostly rely on tracking the long-term outcomes of those who begin using cannabis earlier in life, to see whether there are statistically significant changes in cognitive functioning between those who use cannabis and those who don’t. 

One such longitudinal study, published in 2012, found relatively small, but significant changes in cognitive function for those who started using cannabis at a young age. The more marijuana they used, the greater the change in cognitive function. 

New research suggests that we may have been misinterpreting previous studies, or that they may have methodological limitations. While there is a correlation between cannabis use and cognitive decline, evidence from a new study suggests that the association isn’t causal. 

Instead of tracking cannabis use and cognitive functioning in individuals throughout their youth and young adulthood, the researchers working on the new study looked at these factors in hundreds of pairs of twins. They looked specifically at cases where one twin used cannabis and other did not, to see whether cognitive functioning changes occurred in only one twin, or in both.

Twins: The Ultimate Control Group

The benefit to twin studies is that you can control for more factors which might be clouding the data. In the 2012 study researchers controlled for years of education, schizophrenia and other drug use, but couldn’t account for things like genetic differences …

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