OBJECTIVE--To examine changes in the prescribing of analgesics after the market entry and subsequent withdrawal of zomepirac sodium, a nonsteroidal anti-inflammatory drug (NSAID), following repeated reports of zomepirac-related deaths. DESIGN--To evaluate this natural quasi experiment, we conducted time-series analyses to compare prescribing in two cohorts of primary care physicians from July 1980 through September 1983. SETTING--Study physicians provided outpatient pharmaceutical care to patients enrolled in the New Jersey Medicaid program. PARTICIPANTS--We identified 260 primary care physicians who provided 10 or more prescriptions for zomepirac (zomepirac prescribers) and 308 who provided 10 or more prescriptions for NSAIDs other than zomepirac (other-NSAID prescribers) in Medicaid during the study period. MAIN OUTCOME MEASURES--Monthly rates of prescribing for zomepirac and several categories of substitute analgesics among Medicaid patients seen by study physicians. MAIN RESULTS--Zomepirac accounted for a stable 11.0% of analgesic prescribing among the zomepirac-prescriber cohort; label changes and manufacturer product-risk warnings 11 months before the product's withdrawal from the market had no impact on use. After market entry, zomepirac prescribers reduced use of other NSAIDs and propoxyphene (hydrochloride or napsylate) in comparison with other-NSAID prescribers (-8.1% and -2.8% of total analgesic prescribing, respectively; P < .001). After the product's withdrawal from the market, zomepirac prescribers showed significant increases in relative prescribing of other NSAIDs (+6.8%; P < .001), propoxyphene (+2.1%; P < .05), and analgesics containing barbiturates (+2.7%; P < .001). CONCLUSIONS--The sudden withdrawal of zomepirac from the market resulted in substitutions not only of other NSAIDs, but also of alternative analgesics that carry risks of habituation and adverse effects. Apparent gains in patient safety resulting from market withdrawal of medications must be evaluated in comparison with risks of medications likely to be substituted.