WASHINGTON — Cocaine and methamphetamine use among young adults declined significantly last year as supplies dried up, leading to higher prices and reduced purity, the government reports. Overall use of illicit drugs showed little change.

About one in five young adults last year acknowledged illicit drug use within the previous month, a rate similar to previous years. But cocaine use declined by one-quarter and methamphetamine use by one-third.

Drug use increased among the 50-59 age group as more baby boomers joined that category. Their past month drug use rose from 4.3 percent in 2006 to 5 percent in 2007.

“The baby boomers have much higher rates of self-destructive behavior than any parallel age group we have data from,” said John Walters, director of the White House Office of National Drug Control Policy. Walters, 55, is a boomer himself.

The National Survey on Drug Use and Health, being released Thursday by the Substance Abuse and Mental Health Services Administration, is based on interviews with about 67,500 people.

Overall, about 20 million people 12 or older reported using illicit drugs within the past month. Marijuana was the most popular by far, with 14.4 million acknowledging use of marijuana in the past month.

Among adolescents, age 12 to 17, drug use dipped from 9.8 percent in 2006 to 9.5 percent last year, continuing a five-year trend. Their use of alcohol and cigarettes also fell during the same period.

“The earlier you use drugs, alcohol and cigarettes, the more likely you are to have a lifelong problem,” Walters said.

Much of the progress in curbing drug use occurred between 2002 and 2005. Critics of the nation’s drug policies warned not to read too much into the latest numbers.

“Use of marijuana and other drugs naturally fluctuates and if you look at long-term trends, current rates are smack in the middle of the range they’ve been in for decades,” said Bruce Mirken of the Marijuana Policy Project, which advocates the decriminalization of marijuana. “There is simply no evidence that current policies … have made any difference.”

A World Health Organization survey of 17 countries this year showed that people in the U.S. were more likely than people elsewhere to have tried illicit drugs. The U.S. tied New Zealand for the highest rate of marijuana use and far outpaced other countries on cocaine use, the survey found.

The U.S. report measured drug use over the past month, while the WHO’s looked at drug use over a lifetime.

The WHO survey concluded: “The use of drugs seems to be a feature of more affluent countries. The U.S., which has been driving much of the world’s drug research and drug policy agenda, stands out with higher levels of alcohol, cocaine, and cannabis, despite punitive illegal drug policies as well as a higher minimum legal alcohol drinking age than many comparable developed countries.”

More than half the people who tried drugs for the first time in 2007 used marijuana, according to the U.S. survey. The rate of new marijuana users came to about 6,000 people a day.

The overall rate of illicit drug use dropped from 8.3 percent of those 12 and older to 8.0 percent in 2007.

Walters also acknowledged concern about nonmedical use of prescription pain relievers among young adults. He urged parents to have more awareness of where they keep their prescriptions and to throw them away when the drugs are no longer needed.

The survey, which also examined mental health, indicated that 24.3 million people 18 or older experienced “serious psychological distress over the past year.” It stressed the link between mental health and substance abuse, noting that adults experiencing depression within the past year were more than twice as like to have tried illicit drugs during that time than other adults.