Although most discussions of mood-altering drugs and patterns of use typically focus on a single drug or particular drug class, it is a widely acknowledged fact that the majority of drug users, from the junior high school experimenter to the hard-core narcotic addict, employ more than one legal or illegal substance to alter their subjective states. Multiple drug use has been documented by epidemiological investigations among several major population groups including: students, housewives, military personnel, doctors, alcoholics, and narcotic addicts. Patterns of multiple drug use are invariably complex but can generally be broken doen into three major categories: (1) the use of combinations of drugs; (2) the concomitant use of separate drugs; and (3) the consecutive or sequential use of two or more substances in an alternating fashion. Multiple drug-use patterns can be considered in light of the pharmacological classes of the drugs involved. Some users, for example, restrict their drug taking to particular types of substances, such as depressant drugs (alcohol, sedative-hypnotics, minor tranquilizers, and narcotics), or stimulant drugs (amphetamines and cocaine), or hallucinogens (including cannabis, LSD, mescaline, and related substances). Other users mix not only individual drugs but pharmacological classes of drugs as well. Because an overwhelming number of articles written on particular drug-using patterns mention, at least in passing, the predominance of multiple drug use, items were selected for inclusion in this guide only if they treated the phenomenon of polydrug use exclusively or as a major point of focus. (Author)