U.S. residents are responding to incentives to use generic drugs.

Researchers at Express Scripts Inc., St. Louis, a pharmacy benefit manager, has published figures supporting that finding in an analysis of the company’s 2007 drug trend experience.

The average total cost of a prescription rose to $55.93, from $55.01, but the average patient co-payment fell to $13.20, from $13.35, the researchers report.

The average co-payment fell because patients were more likely to try to hold down out-of-pocket costs by buying generic drugs rather than higher cost name-brand drugs, the researchers report.

Generics accounted for 67% of Express Scripts prescriptions filled in 2007, up from 42% in 2002.

Although insurers, employers and other plan payers paid a higher share of prescription costs, the shift to generics helped cut the increase in the cost of the average prescription to 1.6% in 2007, the Express Scripts researchers estimate.