Despite popular belief, a new study shows that people who smoke marijuana do not appear to be at increased risk of developing lung cancer.

It seems even heavy, long-term marijuana users do not appear to increase the risk of head and neck cancers, such as cancer of the tongue, mouth, throat, or esophagus.

Senior researcher, Donald Tashkin, M.D., Professor of Medicine at the David Geffen School of Medicine at UCLA in Los Angeles says the findings were a surprise as they expected to find that a history of heavy marijuana use would increase the risk of cancer from several years to decades after exposure to marijuana.

The study looked at people in Los Angeles County - 611 who developed lung cancer, 601 who developed cancer of the head or neck regions, and 1,040 people without cancer who were matched on age, gender and neighborhood.

The researchers used the University of Southern California Tumor Registry, which is notified as soon as a patient in Los Angeles County receives a diagnosis of cancer.

The study was limited to people under age 60 as those born prior to 1940, were unlikely to be exposed to marijuana use during their teens and 20s - the time of peak marijuana use.

Dr. Tashkin says people who were exposed to marijuana use in their youth are only now getting to the age when cancer typically starts to develop.

The participants were questioned about lifetime use of marijuana, tobacco and alcohol, as well as other drugs, their diet, occupation, family history of cancer and socioeconomic status.

Dr. Tashkin says the subjects’ reported use of marijuana was similar to that found in other surveys.

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

Even these smokers did not have an increased risk of developing cancer and people who smoked more marijuana were not at any increased risk compared with those who smoked less marijuana or none at all.

The study found that 80% of lung cancer patients and 70% of patients with head and neck cancer had smoked tobacco, while only about half of patients with both types of cancer smoked marijuana.

A clear association was seen between smoking tobacco and cancer.

The study found a 20-fold increased risk of lung cancer in people who smoked two or more packs of cigarettes a day and the more tobacco a person smoked, the greater the risk of developing both lung cancer and head and neck cancers.

The findings support many previous studies.

Dr. Tashkin say the new findings are surprising for several reasons; previous studies have shown that marijuana tar contains about 50% higher concentrations of chemicals linked to lung cancer, compared with tobacco tar, and smoking marijuana cigarettes deposits four times more tar in the lungs than smoking an equivalent amount of tobacco.

Apparently marijuana is packed more loosely than tobacco, so less filtration takes place through the rod of the cigarette, so more particles are inhaled.

Dr. Tashkin says also that marijuana smokers hold their breath about four times longer than tobacco smokers, allowing more time for extra fine particles to be deposited in the lungs.

Tashkin does offer one possible explanation in that tetrahydrocannabinol (THC), a chemical in marijuana smoke, may encourage aging cells to die earlier and therefore be less likely to undergo cancerous transformation.

Dr. Tashkin says, the next step is to study the DNA samples of the subjects, to see whether there are some heavy marijuana users who may be at increased risk of developing cancer if they have a genetic susceptibility for cancer.

Other experts are warning that the study should not be viewed as a green light to smoke pot, as smoking marijuana has been associated with problems such as cognitive impairment and chronic bronchitis.