Although I wouldn't suggest being all up in your face about it, as that can just lead to angsty rebellion I HATE YOU!!! Especially from a 13 year old kid.

Just talk to him about it calmly, something along the lines of "I found your pipe...."
"What do you think of drugs?"
"Are you aware of the consequences?"

Just try to make sure that although he may be doing drugs (which you cannot stop) make him aware of the potential hazards, so that he can play it safe.  

Another very good question, but probably at this time unanswerable question would be "why are you doing drugs?"
Most likely at his age it could be to fit in, and although you cannot stop him now, hopefully he'll come to this realization later on in life, and then make a decision about it.

If he pushes you away and doesn't want to talk about it, I'd say bring it to someones attention that BOTH of you are able to trust to keep it on the down low.
I think it would be unfair to just thrust your brothers drug use into the public and get him embarrassed and  ridiculed unnecessarily, as that won't actually help him in any way.

Good luck mate, I hope your bro is open-minded, and willing to talk about this.

Post edited at 4:39 pm on Mar. 30, 2007 by Rastafarian

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"Who are you?" Sophie asked.
She received no response to this either, but felt a momentary
confusion as to whether it was she or her reflection who asked
the question.