Damaging habit?

But marijuana smokers differ from tobacco smokers in other, potentially more important ways, Tashkin adds. They do not seem to develop more serious consequences of cigarette smoking, namely chronic obstructive pulmonary disease (COPD) -- the fourth leading cause of death in the U.S., killing 130,000 people each year -- or lung cancer, the most common cancer in Americans and responsible for an additional 160,000 annual deaths, according to 2005 statistics from the Centers for Disease Control and Prevention.

To study lung cancer, Tashkin looked at more than 600 lung cancer patients and more than 1,000 control patients matched for age, socioeconomic class, family history and other alcohol and drug use (along with many other potential influences).

The results, published in a 2006 paper in Cancer Epidemiology Biomarkers and Prevention, found a large number of regular marijuana smokers were present in both groups, but statistically there were no more in the cancer group than control group, suggesting no association between marijuana use and lung cancer. Tobacco smokers, on the other hand, showed a dose-dependent increase in risk: with a 30%, 800% and 2,100% increased risk of lung cancer in those who smoked less than a pack, one to two packs or more than two packs per day, respectively.

Other studies have found increased cancer risk. A study of 79 lung cancer patients and 300 controls published in the European Respiratory Journal this year found a fivefold increased risk in the heaviest marijuana users (daily use for 10 years) and no effect in less heavy users.

But Tashkin says this conflicting report was much smaller in scale, having fewer than 20 subjects in the group of heaviest marijuana users. "My critique would be: It's a small study. I think that their small sample size is responsible for vastly inflated estimates," he says. --

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Vapor versus smoke

Smoking anything is perceived as bad these days, says Dr. Donald Abrams, chief of hematology and oncology at San Francisco General Hospital and professor of clinical medicine at UC San Francisco. And so he devised a pilot study to evaluate a novel inhalation method conducted in 18 otherwise-healthy subjects. "We used a device that heated cannabis below the point of combustion -- basically, a heating element and a fan. The fan filled up a balloon from which the patients could inhale," Abrams says.

The findings, published in Clinical Pharmacology and Therapeutics in 2007, showed that levels of THC were "virtually identical," as were patients' reports of subjective "high." No increase in exhaled carbon monoxide was observed with vaporized marijuana, as was the case with smoked marijuana, and patients preferred vaporization to smoking.

"The fact is that whole marijuana, particularly when vaporized and not smoked, is a safe and effective delivery system," says psychiatrist Dr. Igor Grant, director of the UC Center for Medicinal Cannabis Research in San Diego.

-- Jill U. Adams