April 17, 2007 (Los Angeles) -- Cannabis may be bad for the lungs, but the active ingredient in marijuana may help combat lung cancer, new research suggests.

In lab and mouse studies, the compound, known as THC, cut lung tumor growth in half and helped prevent the cancer from spreading, says Anju Preet, PhD, a Harvard University researcher in Boston who tested the chemical.

While a lot more work needs to be done, “the results suggest THC has therapeutic potential,” she tells WebMD.

Moreover, other early research suggests the cannabis compound could help fight brain, prostate, and skin cancers as well, Preet says.

The findings were presented at the annual meeting of the American Association for Cancer Research.

The finding builds on the recent discovery of the body’s own cannabinoid system, Preet says. Known as endocannabinoids, the natural cannabinoids stimulate appetite and control pain and inflammation.

THC seeks out, attaches to, and activates two specific endocannabinoids that are present in high amounts on lung cancer cells, Preet says. This revs up their natural anti-inflammatory properties. Inflammation can promote the growth and spread of cancer.

In the new study, the researchers first demonstrated that THC inhibited the growth and spread of cells from two different lung cancer cell lines and from patient lung tumors. Then, they injected THC into mice that had been implanted with human lung cancer cells. After three weeks, tumors shrank by about 50%, compared with tumors in untreated mice.

Preet notes that animals injected with THC seem to get “high,” showing signs of clumsiness and getting the munchies. “You would expect to see the same thing in humans, so if this work does pan out, getting the dose right is going to be all important,” she says.

Paul B. Fisher, PhD, a professor of clinical pathology at Columbia University, says that though the work is “interesting,” it’s still very early.

“The issue with using a drug of this type becomes the window of concentration that will be effective. Can you physiologically achieve what you want without causing unwanted effects?” he tells WebMD.