Neuropsychologist Krista Lisdahl recently completed a six-year study of brain functioning in teens and young adults who regularly smoke marijuana. It showed that smoking pot at least once a week changes a teenager’s thinking abilities.

Moreover, the research confirmed findings of previous studies showing an association between chronic pot-smoking and poorer working memory and slower processing speed.

“The more joints they smoked in the past year, the worse they did on the cognitive performance,” says Lisdahl, an associate professor of psychology. “These areas of cognition were still worse in the marijuana users compared to controls, even after they stopped using for three weeks.”

The study stopped short of answering definitively whether people who smoked pot in their teens and early 20s permanently harmed their cognitive abilities. But it did show that aerobic fitness may protect against some of the cognitive damage that young marijuana users are inflicting on their still-developing brains.

Lisdahl separated the study’s marijuana smokers by their levels of fitness and then compared their performance on neuropsychological tests. Participants abstained from marijuana smoking for three weeks before taking a test called VO2 max, which measured how efficiently they use oxygen during intense exercise.

Lisdahl found that high aerobic fitness, indicated by the VO2 score, was related to better performance of visual memory, verbal fluency and sequencing abilities.

Most interestingly, aerobically fit marijuana users did better on the cognitive tasks such as processing speed, visual memory and sequencing ability compared to users who weren’t fit.

Lisdahl notes that the study offers some health intervention possibilities. “We could take people who are trying to quit and offer a method to improve brain function while they are scaling back use,” she says. “It would be an inexpensive treatment option.

“This could boost several other areas besides cognition,” Lisdahl continues, “because brain receptors for cannabis, called CB1 receptors, are involved in a lot of other functions besides enabling pot smokers to experience that high. These include emotional control, mood, cognition and pain tolerance.”

Because most research on exercise involves older adults, Lisdahl says, the study also adds insight into the health effects of aerobic fitness in young people. That’s important when you consider that activity levels of teens and young adults drop dramatically after high school.