Well, let's check some AMA research
"Marijuana smoking, like tobacco smoking,
may be associated with increased risk
of lung cancer. Marijuana smoke contains
cannabinoid compounds in addition to
many of the same components as tobacco
smoke. For instance, benzopyrene, a carcinogenic
polycyclic aromatic hydrocarbon,
is found in both tobacco and marijuana
smoke and has been implicated in mutations
related to lung cancer.4-7 Furthermore, experimental
studies support an association
between marijuana smoke exposure and
lung cancer, with lung cancer cell lines demonstrating
tetrahydrocannabinol (THC)-
induced malignant cell proliferation8,9 and
a murine model suggesting that THC promotes
tumor growth by inhibiting antitumor
immunity by a cannibinoid-2 receptor
mediated pathway.10 Although the
preponderance of in vitro data supports a
biologically plausible association, limited research
exists that suggests anticarcinogenic
cannabinoid effects.11-13 Given these
contrasting data, we chose to systematically
evaluate the association between smoking
marijuana and lung cancer."
http://archinte.ama-assn.org/cgi/reprint/166/13/1359.pdf
The study concludes that more study is required. But given how difficult it has been to convince people that smoking is bad for their health, I expect the results will eventually be the same: smoking kills whether it is tobacco or weed, and weed actually has more carcinogens.