The differential impact of copayment on drug use in a Medicaid population.

In this study of the influence of copayment on drug utilization, we examined the prescription experience of 10 categories of drugs used by Medicaid beneficiaries in South Carolina over a four-year period. We observed an immediate and significant effect of copayment on the level of expenditures for all drug categories except analgesics and sedative/hypnotic drugs. The impact of copayment on the changes in the slopes of the series was not as dramatic. Only expenditures for cardiovascular, cholinergic, diuretic, and psychotherapeutic agents demonstrated a significant change in the long-term trend. We conclude that the imposition of a copayment for prescription services exerted a differential effect among therapeutic categories.