Co-pays are a blunt instrument designed to keep patients from over-using medications, says the study's lead author, but they create barriers to the use of essential and non-essential medications alike.

Specifically, researchers found a group of medicines called ACE inhibitors should be available at no cost to the 8 million Americans over age 65 who have diabetes.

The scientists said those drugs are so beneficial, even giving them away would save society large amounts of money by preventing heart attacks, strokes and kidney failure.

Although Medicare soon will begin partially covering prescription costs, seniors will still be responsible for premiums, deductibles and co-pays -- and the researchers say even small out-of-pocket costs keep many people from taking drugs.

Out-of-Pocket Costs Create Barriers to Use

Lead author Dr. Allison Rosen wrote, Patients' out-of-pocket costs such as co-pays are a blunt instrument designed to keep patients from over-using medications, but they create barriers to the use of essential and non-essential medications alike.

The same, she said, may apply to other drugs that have major preventive benefits.

The findings appear in the journal Annals of Internal Medicine.