They have their own image on the rules page,though the healing and death sticks are similar in appearance..healing stick points to the left and the death stick to the right.
I am skeptical on your seller`s claims but I do not own any of these items to see 100% either way
Death sticks are coded to appear exactly the same as healing sticks in the interface. You can only tell the difference by actually using them, that's the way it's supposed to be.
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.
"Drug" items work just like "Medical" items, the only difference is that "Drug" items can have a negative effect on your health. No other restrictions apply.
Continuing the subject - are these drugs really addictive? Is it possible for a character to get the "addicted" status, and - if yes - what happens when next doses aren't taken?
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.