Talk about a buzzkill. When researchers looked at the marijuana habits of a group of people ages 18 to 30, and then again 25 years later, one finding rose above the fog: For every five years of lighting up, your ability to recall common words slides.

At the beginning of the study, which was published in JAMA Internal Medicine, 84 percent of the group reported using marijuana, while less than 12 percent reported still smoking it 25 years later.
During the follow-up, participants were given standard cognitive tests that measured three things: processing speed (for example, solving simple math problems), verbal memory (word recall), and higher-level skills that show how well different parts of the brain are working together.

The longer people used marijuana, the worse they did on all three fronts. However, after accounting for other factors that could affect brain performance, such as education level, only the association between long-term marijuana use and verbal memory persisted. Specifically, for every five years that someone uses marijuana, they recall one less word from a list of 15.

The study authors admit they were a bit surprised to find such a consistent association, and say they plan to invite participants back for a 30-year visit, at which time they’ll undergo cognitive testing.
https://www.yahoo.com/news/years-smoking-weed-may-leave-134920406.html