Proponents of cannabis use can highlight many benefits of consuming the plant or its extracts, such as relief from chronic and neuropathic pain, treatment of debilitating seizure disorders, and alleviating the symptoms of anxiety. However, few drugs are without side effects. Researchers have confirmed a number of inconvenient and even worrisome effects and conditions that arise from heavy marijuana use – like the extremely unpleasant cannabis hyperemesis syndrome – and have theorized links to many more.

One of the more controversial biochemical consequences of cannabis is cognitive impairment. A landmark study published in 2016 suggested that many years of heavy use can lead to memory declines, yet data about potential short-term impacts remains less certain. Given how many adolescents enjoy using the drug, many researchers are keen to determine whether or not a current cannabis habit will affect a young individual’s ability to perform at school or work.

So, Randi Schuster of the Massachusetts General Hospital (MGH) Psychiatry department assembled a team to do just that. Their paper, published in the Journal of Clinical Psychology, supports the hypothesis that memory capacity is suppressed by cannabis. But, encouragingly, the study also revealed that the impairment dissipates quickly after the drug is discontinued.

"Our findings provide two pieces of convincing evidence," Schuster said in a statement. "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."

For their experiment, Schuster’s team enrolled 88 Boston-area adolescents (aged 16-25) who all reported using cannabis at least once a week. Half of the group was told to completely stop consuming cannabis products for 30 days, whereas the other half – which had been balanced to account for pre-existing differences in cognitive ability, mood, motivation, and degree of cannabis use – were told to not alter their intake. And because taking people at their word would not have cut it in this situation, participants’ use or abstinence was monitored with urine tests.

To gauge attention and memory, the team administered a well-known and highly used cognitive performance test before the cannabis use assignments went into effect, and then once a week for four weeks. After analyzing these results, the researchers found that marijuana abstainers showed a significant improvement on the verbal learning and recall sections of the test compared to baseline, and they continued to score higher throughout the month. Continued users did not improve. Moreover, abstainers had better overall memory scores than users at weeks one, two, and three. Neither cannabis use or abstinence were associated with a change in attention scores.

"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 said. "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."

According to an MGH press release, two follow-up trials have already been initiated. One will compare the cognitive ability of cannabis abstainers, aged 13 to 19, to peers who have never used the drug. The other will follow young people who quit marijuana products for a longer time period.