Studies on brain function and mental illness cited above were conducted in adult marijuana users. How the drug affects adolescents is not completely resolved, but the data are more troubling.
A 2000 paper in the Journal of Addictive Diseases recruited 58 marijuana users and found structural changes in the brains of those who had starting smoking marijuana before age 17 but not in those who didn't start smoking until they were older.
"There's also a modest decrease in IQ if teens use heavily, though weekly users and folks who quit don't seem to show it," Earleywine says. Adolescence, he says, is a time when brain neurons are making oodles of new connections, and it's possible that a psychoactive drug such as marijuana may adversely influence that process.
Lungs
Before it has any effect on the brain, marijuana smoke enters the body through the lungs. Dr. Donald Tashkin, professor of medicine at the UCLA David Geffen School of Medicine, has studied the pulmonary consequences of marijuana use for 25 years, recruiting a group of 280 heavy habitual pot smokers in the early 1980s, including some who also smoked cigarettes. (Subjects averaged three joints per day for an average of 15 years.) For comparison, he also recruited cigarette smokers who didn't use marijuana and people who didn't smoke anything.
Tashkin has done a number of studies over the decades comparing these groups. "I began with the hypothesis that regular smoking of marijuana would have an impact on the lungs qualitatively similar to the impact of regular tobacco smoking," he says. That's because the smoke of both plants are more similar than different.
Tashkin and his colleagues did find symptoms of chronic bronchitis in his marijuana-smoking group. In a 1987 study in the American Review of Respiratory Diseases, they reported that incidence of chronic cough, sputum production and wheezing was similar to that in cigarette smokers.
In a second study in the same subjects published in the American Journal of Respiratory and Critical Care Medicine in 1998, examination of the airways and the cells lining the airways found swelling, redness and increased secretions in marijuana users. Biopsies showed "extensive, widespread damage to the mucosa," Tashkin says, similar to what was seen in tobacco users. "This is amazing, because the marijuana smokers average three joints a day, but the tobacco controls smoked 22 cigarettes, suggesting that on a cigarette-to-cigarette basis, marijuana may be more damaging."
