Changes in market shares for internal analgesic products after a Medicaid formulary restriction

The effects of a Medicaid formulary restriction, adding propoxyphene napsylate to a Negative Drug List, were examined. The market shares of products in subgroups of the internal analgesic therapeutic category were compared for periods before and after the restriction. Data from Wisconsin Medicaid Management Information System claims records were analyzed to determine changes in the percent of recipients, prescriptions, and expenditures. For both noninstitutional patients, the market shares held by propoxyphene napsylate primarily were shifted to propoxyphene hydrochloride products. Secondarily, market shifts to nonsteroidal anti-inflammatory drugs (NSAIDs) occurred, and this effect was more pronounced for noninstitutional patients. The formulary restriction most affected the firm that solely marketed propoxyphene napsylate products prior to the change, since that entire market share was redistributed to other products.