Smoking marijuana can cause changes in lung tissue that may promote cancer growth, according to a review of decades of research on marijuana smoking and lung cancer.

Still, it is not possible to directly link pot use to lung cancer based on existing evidence.

More than 40 percent of Americans 12 and older have tried marijuana at least once, Dr. Reena Mehra of Case Western Reserve University in Cleveland, Ohio and her colleagues point out.

“Given the widespread use of marijuana, its use for what are believed to be medicinal purposes, and the increasing abuse and dependence on this substance, it is important to examine potential adverse clinical consequences,” they write in the Archives of Internal Medicine.

To investigate whether marijuana smoking might lead to precancerous changes in the lungs or lung cancer, Mehra and her team reviewed 19 studies of the topic.

Analyses of sputum and lung tissue performed in some of these studies found more cancer-promoting changes in pot smokers than in cigarette smokers or non-smokers, including oxidative stress, dysfunction of tumor-fighting cells, changes in tissue structure and DNA alterations, the researchers report.

However, none of the studies they analyzed found evidence that marijuana smoking actually caused lung cancer, after factoring in the effects of tobacco use.

“We must conclude that no convincing evidence exists for an association between marijuana smoking and lung cancer based on existing data,” Mehra and her team write.

Nevertheless, they add, the precancerous changes seen in studies included in their analysis - as well as the fact that marijuana smokers generally inhale more deeply and hold smoke in their lungs longer than cigarette smokers, and that marijuana is smoked without a filter - do suggest that smoking pot could indeed boost lung cancer risk. It is known, they add, that marijuana smoking deposits more tar in the lungs than cigarette smoking does.

The failure to find a marijuana-lung cancer link may have been due to methodological flaws in existing research, rather than the absence of such a link, the researchers say. Doctors should advise their patients that marijuana does indeed have potential adverse effects, they conclude, including causing precancerous changes in the lungs.

SOURCE: Archives of Internal Medicine, July 10, 2006.