A "little bit" of heroin is good for pain control. It is prescribed as such to pregnant women giving birth in the UK (it's called "diamorphine" but it's chemically the same). Amphetamines are prescribed in the army to keep soldiers and pilots awake.
Amphetamines are also the basis of most ADD medications.
... I think OP was using those examples because they're the stereotypical horror story drugs, but their analogues are everywhere.