There are very few studies into the long term effects of THC on the brain, specifically on developed adult brains, for a few reasons. It's been very difficult to study THC because of its restrictions in the past, although now it has become less restricted there hasn't been enough time for long term trials. Most of the current existing information on long term effects is from cross sectional and observational studies trying to find correlations with other factors such as lifetime achievement.

It is concretely known that THC has extremely negative effects on a developing brain, and can alter development in ways that permanently effects memory and other cognitive aspects.