Abstaining from cannabis for a month can boost the memory performance of regular users, according to a study of young people who used the drug at least once a week.

Researchers found that four weeks without cannabis led to a “modest but reliable” improvement in users’ memory test scores, which could be sufficient to raise students’ grades at school.

“We can strongly say that regular users will learn better when they abstain, and continuing to use the drug is likely to negatively impact the learning process,” said Randi Schuster at Massachusetts General Hospital’s Center for Addiction Medicine in Boston.

The scientists recruited 88 participants aged 16 to 25, all of whom used cannabis at least once a week. Each was then randomly assigned to one of two groups. The first continued their drug use as before, while the second agreed to quit for 28 days. All gave regular urine samples to ensure they stuck to the arrangement.

Over the next month, the participants took regular computer tests to assess their cognitive skills. While abstaining from cannabis had no measurable impact on attention skills, memory improved markedly, and particularly in the first week when much of the drug washed out of their system. The most notable effect was seen in a verbal memory test which recorded how well people could memorise and later recall a series of 18 words that flashed up on the computer screen.

Cannabis contains a cocktail of psychoactive ingredients, the most potent of which is THC, or delta-9 tetrahydrocannabinol, the substance largely responsible for the drug-related high. The compound works by binding to receptors on neurons throughout the brain, but which are concentrated in regions crucial for memory and cognitive function. The same brain areas are least developed in adolescents, potentially making younger people more vulnerable to THC-related harm.

“We see the most dramatic change in memory during the first week and then a slow taper through the rest of the month,” Schuster told the Guardian. “I think it’s likely that the degree of THC clearance is contributing to the degree of cognitive change.”

Those who took part in the study either smoked cannabis, ate it in some form, or used other preparations such as sticky dabs and oils. While some were daily users, others only took the drug at weekends. “I think the perception among parents and kids is that perhaps weekend use is not problematic, but we saw an improvement even among people who used cannabis just once a week,” Schuster said. “It’s a modest but reliable improvement.”

With cannabis increasingly being legalised around the world, the findings raise important questions about education and public health programmes, Schuster said. “We really need to be vocal about telling kids that their brains are still vulnerable, that they continue to develop until they are about 25, and that using cannabis during this time, particularly the high potency products that are prevalent today, is not likely to be good for them.”

“The primary job for these kids at this stage in their life is to learn. This could mean going from a C [grade] average to a B average, or a B average to an A average. We can’t say that with certainty, but modest improvements can certainly be significant,” Schuster said. Details of the work are published in the Journal of Clinical Psychiatry.

The scientists have now launched a follow-up trial of cannabis users aged 13 to 19 to investigate whether abstinence improves memory function to the performance level of non-users. A further six-month trial that is about to begin will look at whether memory improvements linked to cessation feed through to better performance at school.

Tom Freeman, a senior lecturer at the University of Bath, said the work was “extremely valuable” and provided new evidence that cannabis cessation might improve memory function. “These findings are also in line with evidence that neurobiological changes in regular cannabis users can be rapidly reversed following abstinence,” he said.

“The findings of this new study have strong clinical and public health relevance: cessation of cannabis use could produce a rapid and significant benefit in people’s ability to learn and remember information in their daily lives,” he added.