(Reuters Health) - - Not using cannabis for a month could improve learning ability for adolescents and young adults who used the drug at least weekly, a U.S. study suggests.

The study from Massachusetts General Hospital in Boston assessed 88 participants, ages 16 to 25, and found that not using cannabis for a month resulted in measurable improvement in memory functions important for learning.

“We saw much of the improvement in the first week of the abstinence, which was pretty surprising. We thought it would take longer,” Randi Melissa Schuster, lead author of the study, told Reuters Health by phone.

Cannabis use in adolescence is widespread, and rates of use are likely to increase further as more states move toward legalization. The authors note that rates of daily use double between 8th and 12th grades.

The participants in the study were randomly split into two groups. One group abstained from cannabis use, and one continued. Urine samples were tested weekly for levels of tetrahydrocannabinol (THC), the psychoactive element in cannabis.

Those abstaining were incentivized with monetary rewards at the end of each week, the authors reported in the Journal of Clinical Psychiatry.

Memory, but not attention, improved more among adolescents and young adults who abstained from cannabis compared to those who continued to use, researchers found.

Declarative memory, particularly encoding of new information, was the aspect of memory most impacted by cannabis abstinence, the authors found, adding those who maintained abstinence learned more words than those who continued to use cannabis.

The study also showed that cannabis abstinence is associated with improvements in verbal learning that appear to occur largely in the first week following last use.

This study provides convincing evidence that adolescents and young adults may experience improvements in their ability to learn new information when they stop using cannabis, the researchers said - although attention does not appear to be impacted by a month of abstinence.

Dr. Salomeh Keyhani, professor of medicine at the University of California, San Francisco, said this is one more small study that shows cannabis use is associated with adverse neurocognitive effects and may affect learning.

“This study suggests that use of cannabis during adolescence may have lifelong implications in terms of educational attainment,” Keyhani, who was not involved in the study, told Reuters Health in an email.

The study’s main limitation was the absence of a control group of non-users, the authors wrote, with an additional limitation being the inability to determine a more precise time point when memory improvement occurred during the first week of abstinence.

Still, the authors believe their findings have the potential to make an impact on physicians’ advice to adolescents and their parents and on local, statewide, and national policymaking.

Schuster noted another caveat: it is not known whether the improvement has been normalized within the first week. “So yes, we see improvement and some of the cognitive deficit was abated by abstinence ... what we need to know is if they continue to improve.”

SOURCE: bit.ly/2FRL9pt Journal of Clinical Psychiatry, online October 30, 2018.