The NORML organization. That is more than three people who deny any harmful effects of marijuana, right there. In fact, not only does NORML have a history of denying harmful effects, it has pushed the narrative that marijuana is a wonder drug that has been suppressed, for...reasons.
Plus, I have come across many FReepers who make the same claim. Sorry, I'm not keeping a list of their names, but it seems there are a few posting in this very thread.
More fake facts from prohibiitonists - here's the truth: "Use of cannabis, to the extent that it impairs health, personal development or achievement, is abuse, to be resisted by responsible cannabis users." - http://norml.org/marijuana/personal/item/principles-of-responsible-use