Smoking marijuana does not increase the risk of developing lung cancer, US researchers claim.

The scientists, who presented their results at the American Thoracic Society meeting in San Diego, California, this week, found that even heavy marijuana use was not associated with the disease.

The results are surprising, as marijuana tar has previously been demonstrated to contain about 50% higher concentrations of lung-cancer causing chemicals than tobacco tar.

Furthermore, smoking a marijuana cigarette, or "joint", is known to deposit four times as much tar in the lungs as an ordinary cigarette.

For their study, Donald Taskin (University of California, Los Angeles) and colleagues recruited 611 lung cancer patients, 601 patients with head and neck cancer, and 1040 age-, gender-, and neighbourhood-matched controls without cancer, and analyzed their lifetime use of marijuana, measured in joint years (1 joint year = smoking 365 joints).

The participants, who lived in Los Angeles County, were all under the age of 60 years.

Using logistic regression analysis, and taking into account age, gender, and ethnicity, as well as cumulative alcohol and tobacco use, Taskin et al found no association between joint years and lung cancer status.

They emphasize, however, that marijuana does not appear to have any protective effect against cancer, as no dose-dependent effects were evident.

The study confirmed the association between tobacco smoking and lung cancer, with people who smoked two packets of cigarettes a day at a 21-fold increased risk for the disease.

The authors speculated that tetrahydrocannabinol, the psychotropically active ingredient of cannabis, may induce apoptosis of damaged lung cells, causing them to die before becoming cancerous.

"We did not observe a positive association of marijuana use – even heavy long-term use – with lung cancer, controlling for tobacco smoking and other potential confounder," they summarized.

Meeting website