The study, based on data from the National Survey on Drug Use and Health, underscores the public education challenge that law enforcement officials face in persuading legitimate prescription drug users to dispose of their medications properly before they fall into the wrong hands.

The U.S. Drug Enforcement Administration has cracked down pharmaceutical abuse by targeting distributors and pharmacies. The agency is due to begin an administrative hearing on Wednesday in the case of two CVS Caremark Corp. stores suspected of selling the painkiller oxycodone outside legitimate channels in Florida, the center of the growing epidemic.

"At the end of the day, our strongest tool is vigilance among everyone with access to a medicine cabinet," said Gil Kerlikowske, chief of the White House's anti-drug campaign, in a statement accompanying the study's results.

The administration is working on new regulations to make it easier for people and institutions to dispose of unused prescription drugs under legislation signed into law by President Barack Obama in October 2010.

The illicit use of legal medications poses a huge public health and safety problem in the United States, where overdose deaths from their use has surpassed older street narcotics such as heroine and cocaine.

The DEA estimates that 7 million Americans abuse pharmaceuticals, leading to a 346 percent spike in overdose deaths from oxycodone alone from 2005 to 2010.

The trend is responsible for 11 deaths per day, on average, from oxycodone, methadone, hydrocodone, benzodiazepines and morphine, federal officials say. Prescription drug abusers include an estimated one in seven teenagers.

The latest study shows that 55 percent of prescription pain killer abusers got the drugs from a family or friends for free, while 11 percent bought them from the same sources and 5 percent took them without asking. Reliance on friends and relatives is highest among new and occasional abusers.

About 25 percent of those who rank as chronic abusers of pain relievers get their drugs from doctors, while a slightly larger number buy them from dealers, over the Internet or from friends and relatives.

Among the chronic abusers, 41 percent obtain pills for free or without asking from friends and relatives.

All told, the study showed that about 2 percent of the U.S. population aged 12 and older has abused a prescription pain killer within the past 30 days.

The data, from 2009 and 2010, was released ahead of DEA-sponsored public education events scheduled for Saturday at more than 5,000 collection sites under the banner, "National Take Back Day."

The National Survey on Drug Use and Health, one of the federal government's largest, is conducted by the Substance Abuse and Mental Health Services Administration.

It gathers information from about 67,000 randomly selected people aged 12 and above who are asked about their mental health and their use of drugs, alcohol and tobacco.

(Editing by Michele Gershberg and Philip Barbara)

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