I read an article/study about the dangers of cannabis a couple of weeks ago. (Not a "pro-cannabis article" btw.) A study trying to find out how tobacco and cannabis was related to lung cancer said someting like this:
every subject tested started with a probability of 1 to get lung cancer. Users of tobacco had something like 32 times bigger chance to get lung cancer. Cannabis users (as long as the cannabis wasn't mixed with tobacco naturally) actually ended up with a 0,8 chance. So the conclusion was that smoking weed actually seemed to make it slightly less likely to get lung cancer then people that did neither tobacco or weed. Badly explained, but hopefully you get the point..
Other studies have shown similar results, at least none of the studies has shown that cannabis smokers are more likely to get lung cancer then people that didn't smoke anything at all.