“Cannabis and tobacco smoke are not equally carcinogenic… Cannabis smoke contains cannabinoids whereas tobacco smoke contains nicotine (plus added carcinogens). Available scientific data, that examines the carcinogenic properties of inhaling smoke and its biological consequences, suggests reasons why tobacco smoke, but not cannabis smoke, may result in lung cancer… Furthermore, compounds found in cannabis have been shown to kill numerous cancer types including: lung cancer, breast and prostate, leukemia and lymphoma, glioma, skin cancer, and pheochromocytoma…”-

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1277837/?tool=pubmed