The findings are surprising because marijuana tar contains about 50 percent higher concentrations of chemicals linked to lung cancer compared with tobacco tar.

Marijuana smoking also did not appear to increase the risk of head and neck cancers, such as cancer of the tongue, mouth, throat or esophagus, the study found.

"We expected that we would find that a history of heavy marijuana use -- more than 500 to 1,000 uses -- would increase the risk of cancer from several years to decades after exposure to marijuana," said senior researcher Dr. Donald Tashkin of the David Geffen School of Medicine at the University of California-Los Angeles.

Lifetime Use Explored

More than 2,000 subjects who had developed cancer were asked about lifetime use of marijuana, tobacco and alcohol as well as other drugs, diet, occupation, family history of cancer and socioeconomic status.

People who smoked more marijuana were not at any increased risk compared with those who smoked less marijuana or none at all, Tashkin said.

Tashkin commented that the findings are surprising because marijuana tar contains about 50 percent higher concentrations of chemicals linked to lung cancer compared with tobacco tar.

The findings were presented at the American Thoracic Society International Conference in San Diego.