Big differences in co-payments for brand-name drugs and generic drugs lead to noticeable changes in prescription plan members’ purchases.

Researchers at Express Scripts Inc., St. Louis, a pharmacy benefit manager, have published that conclusion in an analysis of 2005 claims data from 3,900 commercial plan sponsors of drug plans with at least 100 insured members.

The researchers found members of plans that charged $0 to $5 more per prescription co-payment for brand-name drugs, than for generic drugs, filled only 47.6% of their prescriptions with generic drugs.

Members of plans with brand-name-to-generic co-payment differences of $16 to $20 filled 52.6% of their prescriptions with generic drugs, and differences of $21 or greater led to a “generic fill rate” of 55%, the researchers report.