Drugs that are used and abused by humans for nonmedical purposes can be grouped into several major categories. The drugs in each category have similar effects on the user, even though they may differ in the way they produce those effects. The major categories include:

1. alcohol (ethanol)
2. nicotine and tobacco
3. depressants (barbiturates, benzodiazepines)
4. stimulants (amphetamines, cocaine)
5. marijuana
6. opioids (morphine, heroin, methadone)
7. psychedelics (LSD, mescaline, ecstasy)
8. inhalants (glue, nitrous oxide)
9. phencyclidine (PCP)

This article describes and discusses drugs from each category. The discussion begins with alcohol and tobacco, which are both legal and relatively easy to obtain. The worldwide use and abuse of these drugs is far more widespread than all the other categories of abused drugs combined. Next in the discussion is depressants used as prescription drugs. Doctors write more prescriptions for diazepam (Valium) and the related benzodiazepines each year than for any other...