The weeds should make you get dizzy and nauseous, maybe black out if it was a bad overdose.
Endo and Dezraldin should make you pass out and maybe stop breathing and start dying if it was a bad overdose.
mRc and Nevadone should cause chest pains and psychosis, possibly leading into cardiac arrest if you took way too much.
LnA-3z and nA-3z should make you trip so hard you can't walk properly (as if drunk) and possibly make you black out and go into a passed out/dissociative state if you took too much.
The combat stims should drive you into a rage where you start attacking random characters in the room, and they should definitely kill you if you took too much.
In all cases the auto-SICing should probably go away, it doesn't quite make sense given what SIC is. This might seem tricky to code given how ODs seem to work, but you could have the system just look at whatever the last drug you took was if you wanted to implement this without doing a bunch of math.
I also think that the scaling for ODing could be looked at. It seems like it's way too easy to OD early on, and then as your stats improve it becomes way too difficult very quickly.