Cannabis Cleared of Causing Cancer

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People who smoke marijuana do not appear to be at increased risk for developing lung cancer, new research suggests.

While a clear increase in cancer risk was seen among cigarette smokers in the study, no such association was seen for regular cannabis users.

Even very heavy, long-term marijuana users who had smoked more than 22,000 joints over a lifetime seemed to have no greater risk than infrequent marijuana users or nonusers.

The findings surprised the studyâ??s researchers, who expected to see an increase in cancer among people who smoked marijuana regularly in their youth.

â??We know that there are as many or more carcinogens and co-carcinogens in marijuana smoke as in cigarettes,â? researcher Donald Tashkin, MD, of UCLAâ??s David Geffen School of Medicine tells WebMD. â??But we did not find any evidence for an increase in cancer risk for even heavy marijuana smoking.â? Carcinogens are substances that cause cancer.

Tashkin presented the findings today at The American Thoracic Societyâ??s 102nd International Conference, held in San Diego.
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Boomers Reaching Cancer Age

The study population was limited to people who were younger than 60 because people older than that would probably not have used marijuana in their teens and early adult years.

â??People who may have smoked marijuana in their youth are just now getting to the age when cancers are being seen,â? Tashkin says.

A total of 611 lung cancer patients living in Los Angeles County, and 601 patients with other cancers of the head and neck were compared with 1,040 people without cancer matched for age, sex, and the neighborhood they lived in.

All the participants were asked about lifetime use of marijuana, tobacco, and alcohol, as well as other drugs, their diets, occupation, family history of lung cancer, and socioeconomic status.

The heaviest marijuana users in the study had smoked more than 22,000 joints, while moderately heavy smokers had smoked between 11,000 and 22,000 joints.

While two-pack-a-day or more cigarette smokers were found to have a 20-fold increase in lung cancer risk, no elevation in risk was seen for even the very heaviest marijuana smokers.

The more tobacco a person smoked, the greater their risk of developing lung cancer and other cancers of the head and neck. But people who smoked more marijuana were not at increased risk compared with people who smoked less and people who didnâ??t smoke at all.

The THC Connection

Studies suggest that marijuana smoke contains 50 percent higher concentrations of chemicals linked to lung cancer than cigarette smoke. Marijuana smokers also tend to inhale deeper than cigarette smokers and hold the inhaled smoke in their lungs longer.

So why isnâ??t smoking marijuana as dangerous as smoking cigarettes in terms of cancer risk?

The answer isnâ??t clear, but the experts say it might have something to do with tetrahydrocannabinol, or THC, which is a chemical found in marijuana smoke.

Cellular studies and even some studies in animal models suggest that THC has antitumor properties, either by encouraging the death of genetically damaged cells that can become cancerous or by restricting the development of the blood supply that feeds tumors, Tashkin tells WebMD.

In a review of the research published last fall, University of Colorado molecular biologist Robert Melamede, PhD, concluded that the THC in cannabis seems to lessen the tumor-promoting properties of marijuana smoke.

The nicotine in tobacco has been shown to inhibit the destruction of cancer-causing cells, Melamede tells WebMD. THC does not appear to do this and may even do the opposite.

While there was a suggestion in the newly reported study that smoking marijuana is weakly protective against lung cancer, Tashkin says the very weak association was probably due to chance.

Cancer risk among cigarette smokers was not influenced by whether or not they also smoked marijuana.

â??We saw no interaction between marijuana and tobacco, and we certainly would not recommend that people smoke marijuana to protect themselves against cancer,â? he says.

By Salynn Boyles, reviewed by Louise Chang, MD

SOURCES: American Thoracic Society 102nd International Conference, San Diego, May 23, 2006. Donald Tashkin, MD, professor of medicine, David Geffen School of Medicine, University of California at Los Angeles. Robert Melamede, PhD, molecular biologist, University of Colorado at Colorado Springs. WebMD Medical News: â??Pot Smoke: Less Carcinogenic Than Tobacco?â?