This document is one of seven publications contained in a series of materials for physicians on recognizing, intervening with, and treating adolescent alcoholism. The materials in this unit of study are designed to help the physician know the different classes of drugs, recognize common presenting symptoms of drug overdose, and place use and abuse in context. To do this, drug characteristics and pathophysiological and psychological effects of drugs are examined as they relate to administration, distribution, special concerns, drug-drug interactions, and tolerance. This unit of study will enable the physician to: (1) state the general principles of clinical pharmacology and pharmacokinetics as they relate to substance abuse; (2) describe the major neurophysiological effects of drugs belonging to the five major drug categories (sedatives, stimulants, narcotics, hallucinogens, and marijuana); (3) associate pathophysiological deleterious effects on systems other than the neurological system with the five major types of drugs; (4) indicate several factors which modify drug actions; (5) evaluate the additive, agonistic, and antagonistic interactions of drugs in combination with alcohol; (6) describe the common presenting symptoms of the five major drug groups in the acute/toxic, chronic, and withdrawal states; (7) list special concerns associated with each drug group; and (8) describe how, when, and where different classes of drugs are used. (NB)