[…] Psychotropics can be classified in several different ways- for example, according to their chemical structure, or the mechanisms by which they act, or whether they are used recreationally or medically, or whether they are legal or illegal. But in addiction research, psychotropics are usually classified according to their pharmacological effects on people’s nervous systems, because these effects refer to a reality that is immediately perceptible.

Snatch from: „HOW DRUGS AFFECT NEUROTRANSMITTERS“
http://www.thebrain.mcgill.ca/