According to everything I've read, performance enhancing drugs and marijuana carry lesser penalties than the one he was hit with, which is why people are saying it had to be one of the "drugs of abuse"
Here's an excerpt from Hollinger's chat sesssion today:
John Hollinger: It wasn't marijuana, it was one of the more heavily penalized drugs as defined by the collective bargaining agreement (cocaine, LSD, methamphetamine), which is why he was banned immediately (Incidentally, no word yet on which it was, and due to confidentiality rules we may never know). I don't understand the distinction in penalties here -- last I checked marijuana and steroids were just as illegal as those other drugs, so the sanction should be the same.