June 26 (HealthDay News) -- Adding small co-payments to Medicaid prescription drug plans reduces the use of medications by patients with chronic diseases, says a U.S. study.

Daniel M. Hartung, of Oregon Health & Science University, and his colleagues analyzed the effect of small co-payments -- $2 for generic and $3 for brand-name -- for prescription drugs introduced for Oregon Medicaid enrollees in 2003. The co-pay fees weren't required for patients who were unable to pay.

The researchers examined pharmacy claims data on about 117,000 Medicare enrollees with depression, schizophrenia, respiratory disease, cardiovascular disease and diabetes.

The patients' overall use of prescription drugs decreased by about 17 percent after introduction of the co-pay policy.

"Subjects with chronic diseases were less likely to reduce the use of drugs used for that disease compared with drugs not for that disease," the study authors wrote.

Most state Medicaid programs now use co-payments to help control prescription drug costs, but few studies have examined how these co-payments affect medication use, the researchers noted.

"This study suggests that in response to cost-sharing, patients discriminate what therapies they reduce based on the diseases they have," they wrote.

The study was published in the June issue of Medical Care.

In Oregon, co-payments for some outpatient services were introduced at the same time as the drug co-payments. But there were no significant changes in the use of those outpatient services, the study found.

SOURCE: Medical Care, news release, June 24, 2008