Marijuana is the most frequently used illicit drug in the United States and the second most commonly smoked drug. Marijuana, or herbal cannabis, is the dried leaves, stalks, and flowers of Cannabis sativa. More than 50% of Americans have tried marijuana at least once. Studies consistently report that lifetime use of marijuana increases during adolescence, from about 7% of 13-year-olds to more than 40% of 17-year-olds (Rey et al. 2002). Of the 14.6 million past-month marijuana users in 2002, about one-third, or 4.8 million persons, reported that they used it on 20 or more days (Table 15B–1). In addition, regular or daily marijuana smoking and self-reported "marijuana addiction" have been increasing. Almost 17% of past-year marijuana users met criteria for dependence or abuse, and among the 3.1 million persons who received treatment for alcohol or drugs in the past year, an estimated 852,000 persons received treatment for marijuana abuse and dependence (Substance Abuse and Mental Health Services Administration 2003). People begin smoking marijuana to obtain the euphoria that it produces, and dependence can occur as they learn how to inhale the drug's vapors. About 4% of new cannabis users become dependent in the first 2 years of use. Individuals who make less than $20,000 per year, those who have used three or more other drugs prior to marijuana, and those who began using in earlier adolescence (Chen et al. 2005) have an even greater risk of developing dependence.