Will smoking pot make you dumb? The link between marijuana use and poorer cognitive function, particularly in teens and young adults, concerns policy-makers and parents – but new research published in JAMA Psychiatry suggests this concern may be unwarranted. 

When it comes to cannabis and its effect on the brain, studies have produced a mixed bag of results. On the one hand, studies have shown that the compound THC, found in marijuana, can boost cognition (at least in mice). Then again, there are those that have shown consistent use of cannabis can hurt your verbal memory and reduce the amount of grey matter in your brain. (To confound matters further, it can also increase the number of neural connections.)

So, J. Cobb Scott, an assistant professor of psychiatry at the Perelman School of Medicine, University of Pennsylvania, and colleagues set about compiling a review and meta-analysis of 69 studies on marijuana use and cognitive ability in teenagers and young adults to see if they could reach a more definitive answer.

The papers used were published between 1973 and 2017 and, combined, involved 2,152 regular cannabis users and a further 6,575 infrequent users with a mean age of 20 to 21. Regular users, perhaps a little unsurprisingly, skewed quite heavily male (68 percent).

The verdict: Yes, frequent and heavy use in teenagers and young adults does have a small but significant effect on cognitive ability. However, this fades surprisingly quickly, in just 72 hours of abstinence.

“That was the biggest surprise,” said Scott, reports Time. “There is biological plausibility that cannabis could cause changes in the brain that is still developing. But the abstinence data we have indicates that while those effects are detectable, they seem to go away after more than three days of abstinence.”

This sounds like very good news for smokers. However, the researchers do point out some limitations to the review.

First of all, the studies were limited to recreational use only, so the findings may not apply to those who use marijuana for medical reasons. Second, of all the 69 papers analyzed, only 15 looked at marijuana's effect on cognitive function for a period longer than 72 hours. For more conclusive proof that cannabis does not cause long-term damage to the brain, a large-scale longitudinal study examining marijuana's effects over a period of many years would be needed. 

Finally, the review doesn't determine causality. “These results are another piece of the puzzle to consider in evaluating the risks versus potential benefits of cannabis," Scott added. "It’s not the final say, and we still have a lot to learn about the longer-term effects of heavy cannabis use on brain and behavior.” 

[H/T: Time]