According to the Tashkin study - the largest and longest running study of marijuana smoking ever conducted in the US - marijuana smokers are LESS LIKELY to contract lung cancer than NON-SMOKERS by a rate of 4:3. This indicates a protective function in marijuana smoke. Tashkin, BTW, was the NIDA doctor who promulgated the theory that smoking marijuana causes cancer. His study was intended to prove his theory, and instead disproved it.