There are many ways to classify drugs, depending on the purposes for the classification. For example, a classification can be based on the chemical properties of drugs and may actually disregard the effects the drugs have on the body, or it may be based on legal principles, such as legal versus illegal, or prescription versus over-the-counter (prescription not needed). For purposes of discussion and teaching, the various drugs that are used and abused by humans for nonmedical purposes are usually grouped into several major categories, each based on their pharmacological actions and their subjective effects. Although the mechanisms of action may vary among drugs within a single category, the general subjective effects of the drugs are similar.

The major categories include: (1) ethanol (ALCOHOL); (2) NICOTINE and tobacco: (3) central nervous system depressants (BARBITURATES, BENZODIAZEPINES); (4) central nervous system stimulants (AMPHETAMINES, COCAINE); (5) cannabinoids; (6) OPIODS (MORPHINE, HEROIN, METHADONE); (7) psychedelics (LSD, MESCALINE...