This study was published in the Harm Reduction Journal in October 2005. It studied the effects of cannabis smoke and tobacco smoke in terms of potential carcinogenic effects.
The paper was titled "Cannabis and tobacco smoke are not equally carcinogenic", so the results are pretty obvious.
They observed a big difference in the pharmacological and chemical effects of smoked cannabis and smoked tobacco. So much so that they were comfortable concluding that cannabis smoke actually inhibits the mechanisms in the human body that create carcinogenic conditions.
This was a review study, so the best way to summarize a summary is probably the authors' words:
THC inhibits the enzyme necessary to activate some of the carcinogens found in smoke
nicotine promotes tumor angiogenesis whereas cannabis inhibits it
while both tobacco and cannabis smoke have similar properties chemically, their pharmacological activities differ greatly.
Components of cannabis smoke minimize some carcinogenic pathways whereas tobacco smoke enhances some.
Available scientific data, that examines the carcinogenic properties of inhaling smoke and its biological consequences, suggests reasons why tobacco smoke, but not cannabis smoke, may result in lung cancer.
The full text article is here at PubMed Central.