Updated

In the United States, the world's largest market for illicit drugs, the number of teenagers abusing them fell between 2001 and 2007, the United Nations said on Thursday.

But the number of people abusing prescription drugs rose, the U.N. International Narcotics Control Board said in its annual report.

"The positive aspect," said Melvyn Levitsky, an INCB member and former U.S. diplomat, was that among American teenagers "it seems that drug abuse has gone down 24 percent in the last eight years."

There are an estimated 19.9 million Americans aged 12 years or older who are considered current illicit drug users, according to a 2007 U.S. government survey.

The INCB report said the drop in drug use among people aged 12-17 "is mostly attributed to a decrease in the abuse of cannabis; however there has also been a decrease in the abuse of almost all other drugs."

Among young adults, aged 18-25 years, abuse fell in all drug categories except pain relievers, it said.

After cannabis, prescription drugs are the most commonly abused substances.

The report said the non-medical use of prescription pain relievers in the last month by young adults rose from 4.1 percent in 2002 to 4.6 percent in 2007.

The INCB said the drop-off in cannabis use resulted from increased perceived risks of smoking the drug, perhaps due to anti-drug education campaigns.

"Despite that significant decline, the problem of substance abuse among United States teenagers continues to be widespread, as nearly half of United States students aged 17-18 (47 percent) have tried an illicit drug by the time they have finished secondary school," the INCB said.

According to the latest available data from the White House's Office of National Drug Control Policy, Americans spent $64 billion on illicit drugs in 2000.

An estimate from 2002 on the cost of drug abuse to the U.S. economy in lost productivity and health care was $181 billion.