They had expected to find that a history of heavy marijuana use, like cigarette smoking, would increase the risk of cancer.

Instead, the study, which compared the lifestyles of 611 Los Angeles County lung cancer patients and 601 patients with head and neck cancers with those of 1,040 people without cancer, found no elevated cancer risk for even the heaviest pot smokers. It did find a 20-fold increased risk of lung cancer in people who smoked two or more packs of cigarettes a day.

The study results were presented in San Diego Tuesday at a meeting of the American Thoracic Society.

The study was confined to people under age 60 since baby boomers were the most likely age group to have long-term exposure to marijuana, said Dr Donald Tashkin, senior researcher and professor at the UCLA School of Medicine.

The results should not be taken as a blank check to smoke pot, which has been associated with problems like cognitive impairment and chronic bronchitis, said Dr John Hansen-Flaschen, chief of pulmonary and critical care at the University of Pennsylvania Health System in Philadelphia. He was not involved in the study.

Previous studies showed marijuana tar contained about 50 percent more of the chemicals linked to lung cancer, compared with tobacco tar, Tashkin said. In addition, smoking a marijuana joint deposits four times more tar in the lungs than smoking an equivalent amount of tobacco.

"Marijuana is packed more loosely than tobacco, so there's less filtration through the rod of the cigarette, so more particles will be inhaled," Tashkin said in a statement. "And marijuana smokers typically smoke differently than tobacco smokers - they hold their breath about four times longer, allowing more time for extra fine particles to deposit in the lung."

He theorized that tetrahydrocannabinol, or THC, a chemical in marijuana smoke that produces its psychotropic effect, may encourage aging, damaged cells to die off before they become cancerous.

Hansen-Flaschen also cautioned a cancer-marijuana link could emerge as baby boomers age and there may be smaller population groups, based on genetics or other factors, still at risk for marijuana-related cancers.

Source: Reuters