Definitions are provided for drug-related terms (e.g., tolerance, dependence, withdrawal), and the Medicines Act 1968 and the Misuses of Drugs Act 1971 are described. The risks of illicit drug use are discussed, including overdose, interactions among drugs, effects of adulteration, motor and perceptual impairment, negative effects on development, and severe criminal sanctions. Special hazards posed by injection of illicit drugs and of drug use during pregnancy also are noted. Drugs of misuse then are considered separately in terms of their production, legal status, prevalence and availability and short- and long-term effects. Drugs considered include heroin and other opiates, cocaine, solvents, amphetamines and other stimulants, cannabis, lysergic acid diethylamide, hallucinogenic mushrooms, hypnosedatives, minor tranquilizers, alcohol, and tobacco. It is noted that most psychoactive drugs impair motor control, reaction time, and attention; may amplify negative moods; and may distort perception and response to the environment. As tolerance and dependence develop, problems of financing the purchase of drugs can add to the deterioration of diet, housing, and lifestyle. It also is noted that drug effects are strongly influenced by the amount of drug taken, the surroundings, the reactions of others, and social and cultural attitudes and beliefs. Information and treatment resources are listed.