Here’s a link to a graph that documents Past Month Use of Selected Illicit Drugs among Persons Aged 12 or Older in the US: 2002- 2007. It is based on data from the 2007 National Survey on Drug Use and Health: National Findings [pdf] which is produced by the Department of Health and Human Services.  Among other things, they find that:

In 2007, an estimated 19.9 million Americans aged 12 or older were current (past month) illicit drug users, meaning they had used an illicit drug during the month prior to the survey interview. This estimate represents 8.0 percent of the population aged 12 years old or older. Illicit drugs include marijuana/hashish, cocaine (including crack), heroin, hallucinogens, inhalants, or prescription-type psychotherapeutics used nonmedically.

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