I have not bought drug by bitcoins but I think that it is possible,  if it is not here but in any other part of world,  as this is also a form of trading. Bitcoin is a crypto currency any one can use it for any of its purpose.  Either the person buy grocery or drug that will be their choice only.  As some supermarket starts accepting bitcoins as a payment there are possibilities that bitcoin also accept by the drugs dealer in exchange of drug.  Bitcoin is continously growing and it hasn't left any area,  where it can not be used as cash.