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 study’s researchers, who expected to see an increase in cancer among people who smoked marijuana regularly in their youth.

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

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 isn’t smoking marijuana as dangerous as smoking cigarettes in terms of cancer risk?

The answer isn’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.

Tashkin presented the findings today at The American Thoracic Society’s 102nd International Conference, held in San Diego.

http://www.foxnews.com/story/0,2933,196678,00.html