“Carcinogens found in tobacco are also concentrated in marijuana, but the latter also contains high levels of cannabinoids, bioactive compounds responsible for several effects such as euphoria and analgesia. However, Δ(9)-tetrahydrocannabinol (Δ(9)-THC), the major psychotropic cannabinoid found in plants, causes a reduction of cellular metabolism and induction of apoptosis (progarmmed cell death), both of which are anti-neoplastic properties…”

http://www.ncbi.nlm.nih.gov/pubmed/22727410