One cig is enough for nicotine effect.
One meth for the meth effect.
Rest is stim. You can’t actually overdose on “regular” consumable stims - 120 stim from 3 cracks is the highest you can go and it should give you 8 int. But crack gives you “high”, which is -1 int.
So expect to have +10 int from drugs, if you take them all.
There is a hidden “exploit” that is very hard to abuse: Prozac gives you +3 stim without cap. Every ~4.3 Prozacs should give you 1 int. Again - no cap. Though you’ll need to beat the 25% multiplier on morale if you still want that +2 int from morale.
You overdose at 251 stim, but only if the game time is a multiple of 5 minutes.