There could be 3 types, one of the type that reduces the activity of your brain, other of the type that rises the activity on your brain, and the 3rd of the type that modifies the activity of your brain... Weed, Cocaine and Alchool maybe... So you could get realistic variety/effects (/fall, /look or HP loss/rise for the drugs).
Anyway, personally I won't mind if drug system stays like it is by now... Rusty has been doing a great work, and also you still have the ability to RP using other drug... After all you don't need scripts for everything you want to RP.