WASHINGTON (AFP) - The use of illegal drugs among Americans in general is holding steady, but it's surging among middle-aged baby boomers, according to report released on Wednesday.

The National Survey on Drug Use and Health indicated that 9.2 per cent of Americans aged 12 and over, or 23.9 million, were current consumers of illicit substances.

That's down slightly from rates of 8.7 to 8.9 per cent in 2009 to 2011, said the report, sponsored by the United States government's Substance Abuse and Mental Health Services Administration.

But among adults aged 50 to 64, illicit drug use has grown substantially - a trend that the report attributed in part to "the aging ... of members of the baby boom cohort" born between 1946 and 1964.

For adults aged 50 to 54, the rate more than doubled from 3.4 per cent in 2002 to 7.2 per cent last year.

For those aged 55 to 59, it more than tripled from 1.9 per cent to 6.6 per cent.

"Among those aged 60 to 64, the rate increased from 1.1 percent in 2003 to 3.6 percent in 2012," the report added.

Marijuana - ranked on par with heroin under federal law, despite a trend towards legalisation at the state level - was the most commonly used illegal drug.

Current use between 2007 to 2012 grew from 5.8 per cent to 7.3 per cent of the overall population, and the number of Americans who used marijuana daily or almost daily grew from 5.1 million in 2007 to 7.6 million in 2012.

Current use is defined as consumption at least once during the month prior to participating in the survey.

The report also identified a significant increase in the number of Americans who had used heroin in the past year, from 373,000 in 2007 to 669,000 in 2012.

Among younger Americans, the survey found that past-month non-medical use of prescription drugs among young adults aged 18 to 25 was 5.3 per cent - "significantly lower" than 6.4 per cent in 2009.

And binge or heavy drinking among adolescents aged 12 to 17 - in a nation where the legal drinking age is 21 - "remained lower than their levels in 2002 and 2009."

Commenting on the report, White House drug czar Gil Kerlikowske said reducing the impact of drug use called for "a robust public health response," as well as smart law enforcement.

"For the first time in a decade, we are seeing real and significant reductions in the abuse of prescription drugs in America," he said.

"Expanding prevention, treatment and support for people in recovery for substance use disorders will be our guide as we work to address other emerging challenges," he said.