Nearly 20 million Americans used illicit drugs in the month before responding to an annual national survey last year, according to the Substance Abuse and Mental Health Services Administration (SAMHSA). That figure includes not only illegal drugs, but also prescription drugs used for non-medical purposes. The numbers come from the National Survey on Drug Use and Health, which interviewed 67,500 people for its annual report.

The numbers for overall drug use are similar to those for recent years, although the survey reported marginal declines in cocaine and methamphetamine use among young people. Among 18-to-25-year-olds, cocaine use dropped to 1.7%, down 23% from 2006, while meth use dropped to 0.4%, down about a third from 2006.

Drug control officials attributed the decline to increased interdiction and enforcement leading to higher prices. But the decline could reflect the generational learning curve typically observed in drug use patterns over time.

The declines in illegal stimulant use were countered by an increase in the non-medical use of prescription pain pills. According to the survey, 4.6% of young adults reported using pain pills for non-medical reasons last year, a 12% increase over 2006.

Marijuana remains by far the most commonly used illicit drug, with an estimated 14.4 million people reporting use in the previous month. That is about 5.8% of the population, down slightly from 6% in 2006.

Baby boomers moving into their fifties are taking their drug habits with them, according to the survey. Illicit drug use among those 55 to 59 more than doubled to 4.1% last year.

Despite millions of drug arrests and hundreds of billions of dollars spent enforcing drug prohibition in the past three decades, drug use levels remain roughly where they have been for the entire period.