Various drug type classification systems have been compiled based on chemical structures, pharmacological action, and/or the observable behavioral effects of the drug action. Drugs of misuse enter the bloodstream and circulate through the system after being injected, inhaled, swallowed, or absorbed through the skin. Each drug has an “affinity” for specific receptor cell sites and may act as an antagonist or agonist. Antagonist drugs block or reduce cell response to natural agonists, usually one's own neurotransmitters (“endogenous ligands”). Conversely, agonist drugs stimulate specific receptors and may increase cellular activities. This chapter provides information on the various drug type classification schemes and an overview of the drugs within these classifications. These classifications help clinicians and researchers to understand observable effects and consequences of drug action and addiction potential. The chapter then addresses the case of caffeine and the case of tobacco. Next, the chapter addresses the history of drug use and abuse and concludes with current costs to society of drug abuse.

Types of Drugs

Around the world, approximately 15% of the population older than 18 years of age is considered to have serious drug use problems (other than nicotine addiction, which itself may involve up to 25% of the world's population), and this percentage has remained fairly constant since the early 1980s. Of these drug abusers, about two-thirds abuse alcohol and one-third abuse other drugs. Across the continents, the other major drugs of abuse are marijuana, amphetamines, cocaine, and heroin.