A study presented at a meeting of the American Thoracic Society on May 24 found that smoking marijuana, even heavily, does not increase the risk of cancer. The study was headed by Dr. Donald Tashkin of the David Geffen School of Medicine at UCLA.

Tashkin, who has studied the effects of marijuana on the lungs for years, had expected the study to reveal that heavy marijuana use results in elevated cancer risk.

Past studies have yielded varied results on this question, but most were conducted on a small scale and possibly affected by bias. The large-scale UCLA study focused on 2,200 people, about 1,200 of whom had lung, oral, laryngeal or esophageal cancer.

The study used personal interviews to collect information about lifetime marijuana, tobacco and other drug use, as well as information about family history of cancer, diet and other possible factors. The result was that people who smoked marijuana, even those who smoked heavily for years, were at no greater risk of developing cancer than those who did not smoke. In contrast, people who smoked more than two packs of cigarettes per day were 20 times more likely to develop cancer than those who smoked nothing.

Tashkin said that past studies have shown marijuana smoke to contain many of the same carcinogenic chemicals found in cigarette smoke. The findings of the study now have researchers considering the possibility that marijuana may have a protective effect against cancer, perhaps deterring tumor growth.