I agree this isn't great to have the President contradicting official messages. But will it do him any harm? The young instinctively recognize the hypocrisy of we elders about the drugs they prefer compared to the ones we prefer, so Obama earns points for being straight with them. Hard conservatives tend not to vote for him anyway, but also are amenable to reason- such as the fact that pot is prevalent in many social circles and if laws are supposed to reflect society's intentions then refusing to recognize the tide leads to Bumble's perception that the law is an ass (donkey.)

Also, re the negative health aspects of pot: there's no point in a carcinogen contest with tobacco and pot's effect on thought processes anecdotally was known for decades including to those currently in their 50s and 60s. Numbers pointing to reduction in IQ? What a surprise that pot heads seem a bit thick. But pot heads know how pot affects them and it's one of the reasons they take it- to dull their perceptions on purpose. To those so inclined, tell them that pot will dull their sensibilities and they'll ask where to sign up, especially if it carries the slight frisson of lawlessness. Take that away and you also take away one of the channels to harder drugs.