I don't know about heroin, but when a seedy guy on the street says:

"Opium?" or "Mescaline?" he is in essence, saying:

"I have opium for sale," or "I have mescaline for sale."

Whatever they are selling is neither of these substances.  If you stop and actually ask them if it really is mescaline, they will probably even tell you honestly that it's actually a mild hit of acid or just shrug and feign macho street ignorance.
          
Par- tay! Bottoms up!