Whereas nicotine has several effects that promote lung and other types of cancer, THC acts in ways that counter the cancer-causing chemicals in marijuana smoke, Melamede explained in an interview with Reuters Health.

"THC turns down the carcinogenic potential," he said.

For example, lab research indicates that nicotine activates a body enzyme that converts certain chemicals in both tobacco and marijuana smoke into cancer-promoting form. In contrast, studies in mice suggest that THC blocks this enzyme activity.

Another key difference, Melamede said, is in the immune system effects of tobacco and marijuana. Smoke sends irritants into the respiratory system that trigger an immune-regulated inflammatory response, which involves the generation of potentially cell-damaging substances called free radicals. These particles are believed to contribute to a range of diseases, including cancer.

But cannabinoids -- both those found in marijuana and the versions found naturally in the body -- have been shown to dial down this inflammatory response, Melamede explained.

So smoke up, boys and girls! But remember: don't smoke pot and cigarettes if you still want to avoid most of the nasties. Nicotine's powers to harm are still far greater than THC's powers to help.