Even if they could discover some relationship between long term smoking of marijuana and increased risk of lung cancer, to quote Bill Hicks:
This for all you non-smokers out there, you and you only, because I know for a fact that you don't know this. Ready? Non-smokers die every day.Sleep tight! I know you've employed some kind of eternal life fantasy because you've chosen not to smoke. Let me be the first to pop that bubble and send you hurtling back to reality - because you're dead too. And you know what doctors say? "Shit, if only you'd smoked - we'd have the technology to help you. It's you people dying from nothing who are screwed". I've got all sorts of neat shit waiting for me - oxygen tent, iron lung...it's like going to Tandy.