NEW YORK, Jan 24 (Reuters) -- Heavy pot smoking does not cause a decline in lung function that could lead to chronic-obstructive pulmonary diseases (COPD), such as emphysema, a new study suggests.
However, the results do not mean that smoking marijuana is safe for the lungs, according to Dr. Donald Tashkin, professor of medicine at the University of California, Los Angeles School of Medicine.
"Yes, marijuana does injure the airways, but the injury seems to be more in the larger airways and the major risk down the line is lung cancer," he said. "Emphysema (which mainly affects the small air sacs in the lungs) does not appear to be a risk."
In the new study, Tashkin and colleagues looked at almost 400 healthy young men and women, 131 who were heavy marijuana smokers (over three joints a day), 112 who smoked both pot and cigarettes, 65 smokers of cigarettes only, and 86 smoke-free individuals.
They found that tobacco smokers had a decline in lung function -- as measured by ability to expel air -- over a period of 1 to 8 years, while marijuana smokers did not have such a decline. Nor were the marijuana and tobacco smokers more likely to have a decline in lung function compared with cigarette smokers alone, according to the report in the American Journal of Respiratory and Critical Care Medicine.
"What our study implies is that smoking pot on a regular basis -- and even heavily -- does not appear to lead to chronic airflow obstruction," Tashkin said. "Now, at the same time, if you actually look at tissue obtained from the airways of these subjects... you find extensive damage, the same kind of damage and to the same extent that you see in heavy, regular tobacco cigarette smokers."
Such changes are not necessarily related to emphysema, but they may be related to the development of lung cancer, according to the California researcher. Indeed, one marijuana cigarette deposits four times as much carcinogen-containing tar in the lungs as a single tobacco cigarette of the same weight.
One reason the two different types of smoke can have different effects on the body may be because marijuana contains THC, said Tashkin. THC, the psychoactive component of marijuana, appears to suppress the function of immune system cells, called alveolar macrophages.
"The cells protect the body against infection and against the development of lung cancer," Tashkin said. Suppressing their function may, therefore, assist the development of malignancy.
SOURCE: American Journal of Respiratory and Critical Care Medicine (1997;155:141-148)
