Despite a lot of effort to suppress it, pot's medicinal qualities are widely recognized. A lot of people in the western world have been smoking a lot of pot since at least the 1960s. I other parts of the world people have been toking away for a lot longer. I don't know of any studies or literature that attribute pot smoking to large scale health issues.
This is anecdotal, but I've known quite a few people who have smoked pot for many many years. None of these people have out of the ordinary health problems (I do know some straight people who have some). Nor have any of them had children with birth defects.
On the other hand, nobody seriously argues that putting a whole lot of heavy smoke in your lungs and holding it in there as long as possible is good for your lungs. Just can't be. For the people who are going to smoke pot in any case (and there are a bunch of people like that), it seems to me it might be more healthy if they could openly grow pot organically. I imagine current growers really sock the chemicals to those plants in the typical grow-op. If pot were legal, I doubt that people would smoke more pot, but I'm certain they would smoke better pot, and maybe even less pot. Seems like that would be a good thing for everyone.
Well maybe not. It's an easy bust, with a bunch of money involved. Cops would have to move on to more gritty things. It would also raise hell with growers, dealers, and transporters, not to mention corrupt officials. We might have to come up with a retraining program for these folks.