He's stated this before, and the league (or at least the teams) knew about it already, it's part of why he dropped to 29th in the draft despite being the ACC player of the year. I've heard interviews about this in the past.
Secondly, it's known but rarely mentioned that quite a few NBA players smoke pot, even though they test for it now; it's easier to play the next day relative to drinking and doesn't affect the wind of an athlete like cigarettes do. It also apparently produces a pleasant, semi-euphoric state known on the streets as "being high".
Not necessarily smart of him to speak up about it, with Stern as obsessed with image (vs actually fixing things) as he is. Still, it's been going on for awhile (hell, most of the 86 team did it to some degree at some point in their careers), it's going on now, and it'll be going on for the foreseeable future. Oddly enough, we've yet to see the devastating life and health consequences that we all know marijuana use has anywhere in this high-profile sample
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Now, if you want to call him stupid, here's a better tack: from his Wiki page "In order to get into Wake Forest, Howard needed an SAT score of at least 950. Howard did not get a 950, saying his score was "somewhere in the 500s"" Remember, you get a 400 just for signing your name...