TragicMonkey
Poisoned Waffles
I wouldn't be at all surprised that smoking some MJ causes cancer, but from most of the articles I've read, the jury is still out. This has a lot to do with the fact that it's difficult to study something that was illegal for so long. Self-reporting isn't very reliable, many people who smoked weed also smoked tobacco, etc. I think, now, with it being legal in many places, the research will be much more sound over the next 10-15 years. And just for measure, while it would seem like a no-brainer that smoking ANYTHING could cause cancer, animal research suggests that some chemicals in marijuana work against tumor growth, which could explain why lung cancer isn’t showing up as often as scientists might expect in people who smoke it.
But yes, at the end of the day, I wouldn't be surprised at all to find out that it can cause cancer. While you smoke less of it than you would tobacco, you also hold it in, and you usually smoke the "tarry" part as you don't let any go to waste. As I understand it, the tarry part (resin?) is more dangerous. Tobacco smokers usually throw out a cigarette before smoking this portion.
Breathing anything that isn't just air is probably some degree of bad for your health. The smoke from ordinary wood fireplaces, that smells so nice? Yeah, that's not good for you. Everything is bad for us, and even if we manage to avoid absolutely everything we still eventually implode or rot from within.