To read my 2 cents on the matter, check out the following link:
Cannabis & Cancer: On the carcinogenicity of marijuana smoke.
Here's an excerpt (I didn't want to take up too much space):
"To date, no animal study has demonstrated the carcinogenicity of marijuana smoke. Future studies would, ideally, be conducted with animal models that reflect the stages of both initiation and progression observed in human cancer, but no such model currently exists (Khanna et al. 2005). In vivo studies on the neoplastic properties of cannabinoids have typically been conducted with BALB/cJ, B6C3F1, and C57BL/6J murine [mouse] strains, which are not susceptible to chemically-induced SCCL [Squamous-Cell Carcinoma of the Lungs]. Even the most susceptible strains do not develop SCCL as the result of exposure to tobacco smoke (Wang et al. 2004), the definitive benchmark for human carcinogens....
In evaluating the carcinogenicity of any type of smoke, it might help to remember that it was epidemiology, rather than animal research, that first incriminated tobacco smoke as a carcinogen (Proctor 2004). While there have been lurid case-reports of tobacco-related cancers among young and middle-aged marijuana smokers (Sridhar et al. 1994), a large cohort study found no evidence of precocious tobacco-related cancers among middle-aged marijuana smokers (Sidney et al. 1997).
As tobacco-related cancers develop increasingly with age and exposure, the cohort study did not follow its participants for long enough to ascertain the relationship between marijuana smoking and tobacco-related cancers. Small hospital-based case-control studies have inconsistently found an association (Chacko et al. 2006, Hsairi et al. 1993, Llewellyn et al. 2004a, Llewellyn et al. 2004b, Sasco et al. 2002, Voirin et al. 2006, Zhang et al. 1999), whereas population-based case-control studies -- especially those with a large number of cases and/or randomly selected controls -- tend to suggest, if anything, an inverse association (Ford et al. 2001, Hashibe et al. 2006, Rosenblatt et al. 2004, Zhu et al. 2002).
The question as to whether heavy prolonged marijuana smoking causes tobacco-related cancer has recently been addressed by a large case-control study that included significant numbers of heavy long-term marijuana smokers (Hashibe et al. 2006). Evidence from this study -- the largest case-control study to date -- suggests that even heavy prolonged marijuana smoking does not appreciably increase the risk of tobacco-related cancers. One way or the other, if marijuana smoking affects the risk of cancer, it may do so to an extent revealed only by large-scale epidemiologic consortia, such as those under the auspices of the International Association for Research on Cancer (q.v., The International Head and Neck Cancer Epidemiology Consortium)."
Peace