1- Depressants: These slow down the activity of the nervous system. They include alcohol, inhalants (including glue, nail polish remover, cleaning fluid, lighter fluid, antifreeze, aerosol from cans or household products and gasoline), minor tranquilizers (including Valium), and sedatives (including barbiturates, Quaaludes and PCP). All depressants except PCP can be obtained legally.

2- Stimulants: These stimulate activity, suppress the appetite, and ameliorate emotional depression. They include the legal drugs caffeine and nicotine as well as legal and illegal amphetamines and the illegal methadrine, ecstasy and cocaine.

3- Hallucinogens: These are mind-distorters. They have no medical use and create altered perceptions. They include marijuana, (only recently has marijuana been considered for medical purposes) LSD, and mescaline. All hallucinogens are illegal.

4- Narcotics: These have an analgesic effect. They relieve physical pain and make surgery possible, but they are highly addictive. They include morphine, codeine, and heroin.