Young marijuana users suffer most cognitive damage: Study

By Dr. Ananya Mandal, MDNov 15 2010

Many marijuana smokers begin early. A new study shows that these people did much worse on tests of executive brain function than those who started smoking when they were older. Researchers highlight the danger posed by marijuana and the need to develop effective strategies to reduce marijuana use, especially among young people in their study.

Adolescence is a critical time in brain development. Brain executive functions include planning, flexibility, abstract thinking and inhibition of inappropriate responses. The study involved 33 chronic marijuana smokers and 26 non-smokers. Twenty had started smoking marijuana regularly before age 16, while 15 started smoking regularly at age 16 or later. All had similar levels of education and income.

Results show that habitual marijuana users appeared less able to maintain focus and were more likely to make errors. The test involved the participants to sort cards with different shapes, numbers and colours. This is a measure of cognitive flexibility, the ability to stay focused, stick to rules and control impulsive responses. Of the group those who started the habit before they were 16 made twice as many mistakes on tests of executive function as those who started smoking the drug after age 16. These early users also used three times as much marijuana per week and twice as often as the late-onset users.

Study author Staci A. Gruber, director of the Cognitive and Clinical Neuroimaging Core at McLean Hospital in Belmont, said, “Our data suggest that the earlier you begin smoking, the more marijuana you smoke and the more frequently you smoke. That’s an important finding…We have to be clear about getting the message out that marijuana isn’t really a benign substance… It has a direct effect on executive function. The earlier you begin using it, and the more you use of it, the more significant the effect.” She added that the portion of the brain that “modulates executive function is the last part to develop,” and that per the data, age restrictions on legal marijuana should be considered.

The study was to be presented Nov. 15 at the annual meeting of the Society for Neuroscience in San Diego.