A new report says that Marijuana is not a cancer risk since the THC acts to counter the cancer-causing chemicals produced by it's smoke unlike tobacco where the nicotine does the opposite. The report is by Dr Robert Melamede of the Univ. of Colorado.
"THC turns down the carcinogenic potential," he said stating that in studies on mice THC seems to block the enzyme that turns chemicals in smoke to their cancer promoting form. Also cannabinoids help to reduce immune regulated responses to smoke.
Cells in the respiratory tract have cells with receptors where nicotine docks but none have been found for THC or other cannabinoids. Cannabinoid receptors do appear in the brain where they may protect cells from death due to injuries like a stroke. Source