1) Tobacco is cancer causing largely because it delivers specific carcinogens such as NNK and NNAL that are not present in cannabis. Not all "tar" is created equal, and tobacco has some of the most carcinogenic types of tar known to science, whereas cannabis does not.

jnci.oxfordjournals.org/.../1194

2) Cannabis (marijuana) use is associated with a DECREASE in several types of cancer... potentially even providing a protective effect against tobacco and alcohol related cancer development.

Donald Tashkin, a UCLA researcher whose work is funded by NIDA, did a case-control study comparing 1,200 patients with lung, head and neck cancers to a matched group with no cancer. Even the heaviest marijuana smokers had no increased risk of cancer, and had somewhat lower cancer risk than non-smokers (tobacco smokers had a 20-fold increased lung cancer risk). Tashkin D. Marijuana Use and Lung Cancer: Results of a Case-Control Study. American Thoracic Society International Conference. May 23, 2006.

Researchers at the Kaiser-Permanente HMO, funded by NIDA, followed 65,000 patients for nearly a decade, comparing cancer rates among non-smokers, tobacco smokers, and marijuana smokers. Tobacco smokers had massively higher rates of lung cancer and other cancers. Marijuana smokers who didn't also use tobacco had no increase in risk of tobacco-related cancers or of cancer risk overall. In fact their rates of lung and most other cancers were slightly lower than non-smokers. Sidney, S. et al. Marijuana Use and Cancer Incidence (California, United States). Cancer Causes and Control. Vol. 8. Sept. 1997, p. 722-728.

In a 1994 study the government tried to suppress, federal researchers gave mice and rats massive doses of THC, looking for cancers or other signs of toxicity. The rodents given THC lived longer and had fewer cancers, "in a dose-dependent manner" (i.e. the more THC they got, the fewer tumors). NTP Technical Report On The Toxicology And Carcinogenesis Studies Of 1-Trans- Delta-9-Tetrahydrocannabinol, CAS No. 1972-08-3, In F344/N Rats And B6C3F Mice, Gavage Studies. See also, "Medical Marijuana: Unpublished Federal Study Found THC-Treated Rats Lived Longer, Had Less Cancer," AIDS Treatment News no. 263, Jan. 17, 1997.