Young people’s memories improved when they stopped using marijuana

Taking a monthlong break from pot helps clear away young people’s memory fog, a small study suggests. The results show that not only does marijuana impair teenagers’ and young adults’ abilities to take in information, but that this memory muddling may be reversible.

Scientists have struggled to find clear answers about how marijuana affects the developing brain, in part because it’s unethical to ask children to begin using a drug for a study. But “you can do the opposite,” says neuropsychologist Randi Schuster. “You can get kids who are currently using, and pay them to stop.”

For a study published October 30 in the Journal of Clinical Psychiatry, Schuster and her colleagues did just that. The team recruited 88 Boston-area youngsters ages 16 to 25 years old who reported using marijuana at least once a week, and offered 62 of them money to quit for a month. Participants were paid more money as the experiment went along, with top earners banking $585 for their month without pot. 

The money “worked exceptionally well,” says Schuster, of Massachusetts General Hospital in Boston and Harvard Medical School. Urine tests showed that 55 of the 62 participants stopped using marijuana for the 30 days of the experiment.

Along with regular drug tests, participants underwent attention and memory tests. Tricky tasks that required close monitoring of number sequences and the directions and locations of arrows revealed that, over the month, young people’s ability to pay attention didn’t seem to be affected by their newfound abstinence.

But former users’ memories were affected, and quickly. Just a week into the experiment, the abstainers performed moderately better on memory tests than they had at the beginning of the study. Young people who continued using marijuana didn’t improve on the memory test. One particular aspect of memory, the ability to take in and remember lists of words, seemed to drive the overall improvement.

Cannabis is probably impairing young people’s ability to handle new information, the results suggest. But there’s good news here, Schuster says. “From these data, we think that at least some of that impairment is not permanent,” she says. “It’s not set in stone.”

The results raise lots of interesting questions, says clinical neuropsychologist April Thames of the University of Southern California in Los Angeles. One has to do with a point of no return for the developing brain, she says. “If somebody is using very heavily over a prolonged period of time, is there a point at which these functions may not recover?”

Schuster and her colleagues plan to conduct longer-term studies designed to begin answering that and such questions as whether stopping pot use for six months tracks with improvements in schoolwork for 13- to 19-year-olds.

But while there’s still much to learn about how marijuana affects developing brains, the latest results suggest caution is needed, especially at a time when, as laws change, marijuana is becoming increasingly available in the United States and other countries. “We should really be urging kids to delay using cannabis, particularly high-potency products, for as long as possible,” Schuster says.

Editor's note: This story was updated November 2, 2018, to correct that the top earners in the study were paid $585, not $535.