Marijuana use among high school students has increased over the last decade, according to a 2017 National Institutes of Health Survey.

And that has researchers concerned about the long-term effects on the developing brain.

On Tuesday, the Journal of Clinical Psychiatry published a study that found verbal learning and memory begins to improve in as little as a week when young people stop using marijuana.

In 2016, a Health and Human Services survey showed that 5 percent of eighth-graders and 23 percent of 12th-graders had reported using marijuana at least once in the last month.

The number of teens who believe the drug poses a great risk to their health has decreased significantly since the mid-2000s, according to a long-term national survey on drug use.

Another study looked at the historical shifts in marijuana laws and found that the perceived harmfulness may be related to the legalization of the drug.

How was the study conducted?

The cross-sectional study published in the JCP, led by Randi Schuster, director of Neuropsychology at Massachusetts General Hospital's Center for Addiction Medicine, recruited 88 pot-using participants, ages 16 to 25, for the research project. Some were heavy users and others used once a week.

Volunteers were randomly assigned to abstaining and non-abstaining groups for the four-week study. The abstainers received increasing amounts of money for each week they abstained, which helped keep them motivated to stick with the program.

Participants in the abstaining group were urine tested weekly to make sure their THC levels were going down. And non-abstainers were urine tested to make sure their levels remained consistent.

During the weekly visit with researchers, all participants' memory and attention were tested using a validated cognitive assessment tool through the Cambridge Neuropsychological Test Automated Battery.

The verbal memory test, which is especially important for students, analyzed the participants' ability to learn and recall new words.

The test allowed scientists to "look both at their ability to learn information the first time the words were presented, as well as the number of words that they're able to retrieve from long-term memory storage after a delay," Schuster said, according to National Public Radio.

"The adolescent brain is undergoing significant neurodevelopment well into the 20s, and the regions that are last to develop are those regions that are most populated by cannabis receptors, and are also very critical to cognitive functioning."

What were the results?

Researchers were surprised to find that the greatest improvements in verbal memory occurred during the first week of the study for the group who abstained.

"We were pleasantly surprised to see that at least some of the deficits that we think may be caused by cannabis appear to be reversible, and at least some of them are quickly reversible, which is good news," Schuster said.

The results contribute to the growing evidence that marijuana use among young people is associated with reduced neurocognitive functioning.

"For an adolescent sitting in their history class learning new facts for the first time, we're suspecting that active cannabis users might have a difficult time putting that new information into their long-term memory," Schuster said.

However, this study did not prove that abstaining from marijuana use improves attention in adolescents. Other studies have shown that pot users do worse on attention tests than non-users.

Schuster hypothesized that it could take longer than a month of abstinence to improve a person's attention levels.

What were the study's weaknesses?

The study lacked a non-marijuana-using control group, according to Krista Lisdahl, an associate professor of psychology at the University of Wisconsin Milwaukee. Lisdhal, who is a researcher in the neuroscience of addiction, was not involved with the study.

Lisdhal told NPR that without the control group, it's difficult to conclude whether the participants' memories improved to their baseline levels before they started using marijuana.

Also, the four-week study didn't last long enough to determine the long-term effects including sleep, academic performance, and depression for users.

But the study provides hope for improvements in adolescents who abstain.

"I remain optimistic that we can show recovery of function with sustained abstinence," Lisdhal told NPR.

What else?

The study's authors wrote that more "studies are needed to determine whether the improvement in cognition with abstinence is associated with improvement in academic and other functional outcomes."

The National Institutes of Health has enrolled more than 11,000 children, ages 9 and 10, for the largest long-term study in the U.S. on brain development. The study will assess everything from screen time to concussions.

Researchers will follow the participants for 10 years into young adulthood.