The survey found that in 2003 an estimated 19.5 million Americans (8.2 percent of the population aged 12 or older) had used an illicit drug in the month prior to the survey interview. There was no change in the rate of illicit drug use between 2002 and 2003. The rate of current illicit drug use among juveniles ages 12 to 17 did not change significantly between 2002 (11.6 percent) and 2003 (11.2 percent), and there were no changes for any specific drug. Marijuana was the most commonly used illicit drug (rate of 6.2 percent in 2003). An estimated 2.3 million persons (1.0 percent) were current cocaine users, and hallucinogens were used by 1.0 million persons. A total of 119,000 persons were current heroin users. These 2003 estimates are similar to those for 2002. An estimated 119 million Americans aged 12 or older were current drinkers of alcohol in 2003 (50.1 percent). The highest rate of binge and heavy drinking was among young adults aged 18 to 25. An estimated 70.8 million Americans reported the use of a tobacco product in the past month. Data are also provided on trends in beginning substance use, youth drug prevention measures, substance dependence, drug treatment and treatment need, serious mental illness among adults, the co-occurrence of serious mental illness and substance-use disorders, and treatment for mental health problems. 15 figures, 7 tables, and 8 references