DES MOINES - When Dena Flynn lost her husband to cancer last November, she had no idea how to get rid of his piles of pills, including narcotics, other painkillers and anti-nausea medication.

She locked them away until she heard Ames was collecting and disposing of unused prescriptions in an effort to keep them out of the water supply and the wrong hands. Flynn was one of more than 200 people who drove to City Hall on Saturday and dropped off medicine.

''I felt safe and confident that it would be disposed of properly,'' the 47-year-old Boone resident said.

Drug drop-offs have become more common as concerns grow about the abuse of prescription painkillers and contamination of the nation's water supply.

An Associated Press investigation found trace pharmaceuticals in drinking water supplies of at least 51 million Americans. Scientists and the Environmental Protection Agency say the minute concentrations of drugs pose no confirmed human risks.

The city of Ames collected 331 pounds of pills, lotions, creams and powders in just three hours, said Erin Kennedy, a drug-free coordinator with Youth and Shelter Services in Ames, which helped coordinate the program.

The pills filled six 5-gallon paint buckets, she said. All the medicines were taken to the city's incinerator and destroyed.

Drug drop-offs also were held recently in the Iowa cities of Clinton and Nevada. Others have been held in New York and Michigan.

''The main reason why people were bringing in the drugs is because they didn't know what to do with them otherwise,'' Kennedy said. ''They had been piling up in the back of their medicine cabinets.''

Organizers didn't keep track of specific drugs because of confidentiality issues with prescriptions, but people brought in ''a huge variety of stuff'' from narcotics and anti-anxiety drugs to vitamins and pet medicines,'' she said.

''The oldest medication that we got that was still in the packaging was a Tylenol sinus medication that had expired in 1987,'' Kennedy said.

New drug users are more likely to abuse prescription pain relievers than marijuana or cocaine, according to the U.S. Drug Enforcement Administration. Nearly 1 in 10 high school seniors admits to abusing prescription painkillers, while 40 percent of teens and an almost equal number of parents think abusing painkillers is safer than abusing ''street'' drugs, the agency says.

Flynn, who has a teenager at home, said she just wanted the medicine out of her house.

''Safety reasons, definitely,'' she said.

The pills, ointments and lotions she dropped off filled about three grocery bags and a box.

Barbi Greenlaw, 55, of Ames, also dropped off drugs. She said she had old antibiotics, and she didn't want to flush them down the toilet.

''I had read in the newspaper that the water supply was being contaminated with prescription drugs,'' she said.

Kennedy said the program was so successful, they hope to have another drug drop-off this fall in Ames.

If a drug drop-off program is not available, the federal government recommends mixing drugs with cat litter or used coffee grounds and putting the mixture into sealed containers for disposal in the trash.