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Teens and young adults who regularly use cannabis but abstained for a whole month showed marked improvements in memory functions that are important for learning. This was the first time that researchers tracked cognitive changes over time associated with quitting cannabis use.

“Our findings provide two pieces of convincing evidence,” Randi Schuster, director of Neuropsychology at the Center for Addiction Medicine in the Massachusetts General Hospital’s Department of Psychiatry, lead author of the paper.

“The first is that adolescents learn better when they are not using cannabis. The second – which is the good news part of the story – is that at least some of the deficits associated with cannabis use are not permanent and actually improve pretty quickly after cannabis use stops.”

According to the researchers, 13% of middle and high school students use cannabis, with daily use increasing between grade 8 and 12.

Previously, the same team of researchers found that cannabis users aged 16 and under had problems assimilating new information, something that wasn’t observed among users 17 or older. This suggests that the psychoactive compounds found in marijuana may interfere in some way with the cognitive development of certain groups of teens, whose brains are still in development. Another 2014 study of 16- to 19-year-olds who use cannabis found abnormalities in their brain’s gray matter.

Schuster and colleagues enlisted 88 participants aged 16 to 25, all of whom smoked cannabis at least once a week. The aim of the study was to compare the cognitive performance of young cannabis users who stopped drug use for 30 days with a group that carried on as usual with cannabis use. The two groups were randomized in order to control for factors such as pre-existing differences in mood, cognition, and motivation, but also the frequency and intensity of cannabis use.

Participants were financially rewarded in order to incentivize their abstinence. Regular urine tests were performed in order to ensure that participants in the abstinence group stayed on the track and didn’t skew the results.

According to the results of cognitive testing, the ability to recall new information and to learn improved in the group that stopped cannabis use. No such effect was observed in the group that carried on as usual. In particular, the greatest improvement occurred in the first week of abstinence. A month of cannabis abstinence was not linked to any improvements in attention.

“The ability to learn or ‘map down’ new information, which is a critical facet of success in the classroom, improved with sustained non-use of cannabis.” Schuster says. “Young cannabis users who stop regular – weekly or more – use may be better equipped to learn efficiently and therefore better positioned for academic success. We can confidently say that these findings strongly suggest that abstaining from cannabis helps young people learn, while continuing cannabis use may interfere with the learning process.”

Next, the researchers plan on studying whether attention and memory continue to improve after longer periods of abstinence.

The findings appeared in the Journal of Clinical Psychiatry.