A new study found that while cigarette smoking greatly increased the risk of lung cancer, smoking marijuana did not seem to have any effect, Fox News reported May 23.

Researchers from the UCLA David Geffen School of Medicine said they were surprised by the findings. "We know that there are as many or more carcinogens and co-carcinogens in marijuana smoke as in cigarettes," said researcher Donald Tashkin. "But we did not find any evidence for an increase in cancer risk for even heavy marijuana smoking."

The study involved participants aged 60 and younger, including 611 lung-cancer patients and 601 patients with other cancers of the head and neck. The cancer patients were compared to a control group of 1,040 people without cancer.

Heavy marijuana smokers were those who self-reported smoking more than 22,000 joints during their lifetime. Researchers found that people who smoked two packs of cigarettes per day increased their cancer risk twentyfold, but even the heaviest marijuana smokers had no elevated risk. They also said that cancer risk among cigarette smokers rose in proportion to how much they smoked.

Previous studies have found that marijuana smoke has high levels of carcinogens, and that marijuana smokers inhale deeply, which would be expected to increase their risk of cancer. But some observers say that the THC in marijuana may have some protective properties that prevent cancers from forming or thriving.

The findings were presented at the American Thoracic Society's 102nd International Conference in San Diego.