As Meph said - I don't know why the person that sold them to you said they showed up that way for the time being, unless you are paraphrasing them. If you find any way to differentiate between healing and drug items, bug base it so it can be fixed.
You should go read the Medical rules page. It will tell you everything you need to know about how drugs work, and how they differ from normal medical items.
Thought there was a lot of interesting discussion about different drugs, and effects on the player and withdrawl symptoms etc, just keeping it the simple way we have now was deemed the best way to fit drug use into the PG-13 rating of this game.