Cannabis, whether as leaves/flowers (marijuana) or resin (hashish), is the most commonly used illicit drug worldwide (United Nations Office on Drugs and Crime 2005). Overall, use in the United States decreased from the 1970s to the 1990s and then increased into the 2000s, although not to the levels of the 1970s. In 2004, 6.1% of subjects in the National Household Survey on Drug Use and Health reported past-month use of marijuana, with about 35% of those using other illicit drugs as well (Substance Abuse and Mental Health Services Administration 2005). Between 1994 and 2004, the percent of patients admitted to substance abuse treatment in the United States who identified cannabis as their primary substance of abuse increased from 9% to 16% (Substance Abuse and Mental Health Services Administration, Office of Applied Studies 2006). In 2004, that percentage was secondary only to alcohol and opioids; the average age of treatment admissions was about 10 years younger than for alcohol, opiates, or stimulants (mid- to late 20s versus mid- to late 30s).