Substance abuse can be defined as the repeated use of a substance even with the knowledge of its negative health consequences. Abused substances may be legal or illicit and thus include alcohol and nicotine as well as marijuana, cocaine, heroin, amphetamines, tranquilizers, hallucinogens, steroids, inhalants, and “club” drugs. Addiction plays a major role in substance abuse, and behavioral addictions, such as sex addiction, can also have important social, public health, and medical consequences. The prevalence of drug use in the United States is shown in Table 1. The stigma attached to illicit drug abuse obscures the impact of abuse of legal substances on society. Nicotine is a highly addictive drug and cigarette smoking is associated with staggering health care costs. The total cost of crime, accidents, and destruction of property associated with illicit drug abuse in the United States is approximately $50 billion over a 10-year period, which is one third less than costs for similar alcohol abuse-related damages over 2 years. The stigma attached to illicit drug abuse also prevents the recognition, or results from the failure to recognize, that underlying addiction is a chronic disease requiring treatment, rather than a social problem. Illicit drug use according to age, sex, and ethnicity is shown in Table 2; alcohol use is shown in Table 3. Men tend to use illicit drugs more than women do. Incidence of drug addiction tends to be highest among African Americans, followed by Hispanic Americans and white Americans. White Americans, however, have greater rates of alcohol use and addiction. Although these demographic patterns of substance abuse and addiction are not completely understood, they at least partly reflect accessibility and social practices. Important new drug abuse trends include the abuse of club drugs (see related article, page 25) and anabolic steroids among young people. Club drugs include methylenedioxymethamphetamine (MDMA, ecstasy), flunitrazepam (Rohypnol), gamma hydroxybutyrate (GHB), gamma butyrolactone (GBL), and ketamine. MDMA abuse increased from 5.8% to 8% among 12thgraders from 1998 to 1999. Methamphetamine addictions are replacing opiate addictions in several areas in the world and promise to become an even larger problem in the United States. Anabolic-androgenic steroid abuse increased from 2% to 2.7% among 10thgraders from 1998 to 1999. Because of increased pressure to perform in sports, abuse of steroids by young women rivals that of young men, and it is estimated that 175,000 high-school-aged women have used steroids at least once. Typical patterns of use include sharing needles for group injections with the aim of building muscle together, and these practices have been associated with substantial transmission of disease. Cessation of steroid use is associated with withdrawal symptoms. The question “Who are substance abusers?” is thus not an easy one to answer. It may not be surprising that 53% of the general population in the United States have used illicit drugs at one time or another. However, some aspects of the profile of the "average" injection drug user may be surprising: 60% are men, 45% are white, 43% have completed high school, and 53% are employed.