NEW YORK (Reuters Health) - You’ve heard about using marijuana and drugs derived from it to keep some of the side effects of toxic cancer chemotherapy in check. But what if smoking marijuana for 10 to 20 years could actually protect against certain tumors?

A young man smokes marijuana before the 10th annual "Marijuana March" in downtown Toronto May 3, 2008. REUTERS/Mark Blinch

In a study, researchers have found that long-term pot smokers were roughly 62 percent less likely to develop head and neck cancers than people who did not smoke pot.

The new study featured 434 patients with head and neck cancers, which include tumors in the mouth, tongue, nose, sinuses, throat and lymph nodes in the neck, and 547 individuals without these cancers seen in the Greater Boston area from December 1999 to December 2003.

After factoring out the impact of smoking, drinking, and other factors that might influence the results, smoking marijuana from once every two weeks to three times every two weeks, on average, was associated with about half the risk of head and neck cancer, compared with less frequent use.

Those who took up pot smoking at an older age appeared to have less risk of these cancers than those who started it at a younger age.

Compared to people who never smoked pot, those who began smoking marijuana between the ages of 15 and 19 years were 47 percent less likely to develop head and neck cancer, while users who began at age 20 or older had a 61 percent reduced risk, Kelsey and colleagues found.

It’s unclear why marijuana would prevent cancer, if in fact the study is borne out by others, but the authors note that chemicals in pot called cannabinoids have been shown to have potential antitumor effects. Other studies have linked marijuana use to a reduced risk of some cancers, such as cancer of the prostate, and now head and neck cancer.

It’s also been suggested that smoking pot may help stave off Alzheimer’s disease and help combat weight loss associated with AIDS, and nausea and vomiting associated with chemotherapy in cancer patients.

Overall, however, research on the effects of marijuana on human health is mixed. Some studies have suggested the drug can increase a person’s risk of heart attack or stroke and cause some cancers such as lung cancer.

In the journal Cancer Prevention Research, the researchers emphasize that further research from larger studies is needed to verify this link. Moreover, even if marijuana use were found to protect against these cancers, the risks of use may still outweigh this benefit, they say.

“Marijuana is an entry-level drug and can be associated with later use of more serious addictive drugs, as well as other risk behaviors,” warn Dr. Karl T. Kelsey, from Brown University, Providence, Rhode Island, and colleagues.

Any policy regarding marijuana use should take this into consideration “and should not be made based on one study’s results,” they note.

SOURCE: Cancer Prevention Research, August 2009.