Objectives. Lack of efficacy or tolerability of some non-steroidal anti-inflammatory drugs (NSAIDs) may lead to switching between non-selective NSAIDs (nsNSAIDs) and cyclooxygenase-2 (COX-2) selective inhibitors (coxibs), potentially increasing treatment costs due to additional physician visits and wastage of medication. This study assessed drug switching and associated costs among elderly chronic NSAID users. Methods. Data for patients who filled their first prescription for a coxib or nsNSAID

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... in 2001 were obtained from the Quebec Health Insurance Agency. Follow-up was terminated at the earliest of: 1 yr, the first day without NSAID exposure following the index filling date, or death. Patients could switch NSAIDs several times during follow-up. Person-days of exposure were categorized by the NSAID most recently dispensed: rofecoxib, celecoxib, Arthrotec Õ or non-Arthrotec (nA) nsNSAID. Cox regression models compared time to switch between groups, adjusting for patient baseline characteristics. Upon a switch, pills remaining from the previous prescription were considered wasted. The costs of wasted pills and switch-associated physician visits were estimated. Results. Throughout follow-up, patients filled 38 267 prescriptions for rofecoxib, 31 282 for celecoxib, 1108 for Arthrotec and 4388 for nA-nsNSAIDs. Adjusted hazard ratios (95% confidence interval) for switching versus nA-nsNSAIDs were: rofecoxib, 0.39 (0.35-0.44); celecoxib, 0.43 (0.38-0.48). Compared with nA-nsNSAID prescriptions, adjusted switching-related healthcare costs were 53 and 47% lower on average for rofecoxib and celecoxib prescriptions, respectively. These costs were 34% higher for Arthrotec prescriptions than for nA-nsNSAIDs. Conclusions. Compared with recipients of nsNSAIDs, coxib recipients were less likely to switch medications and had approximately half the adjusted costs for switching-related wasted resources per prescription. KEY WORDS: Cost analysis, Cyclooxygenase-2 inhibitors, Non-steroidal anti-inflammatory agents, Prescriptions. Non-steroidal anti-inflammatory drugs (NSAIDs) are among the most frequently prescribed medications worldwide [1]. These agents are effective in reducing the pain and inflammation associated with chronic musculoskeletal conditions, but it has long been recognized that non-selective NSAIDs (nsNSAIDs) also increase the risk for gastrointestinal (GI) events [2] [3] [4] . Clinical trials have demonstrated that cyclooxygenase-2 (COX-2) selective inhibitors (coxibs) provide similar arthritis pain relief to nsNSAIDs prescribed at recommended doses while reducing the risk for upper and lower GI events compared with nsNSAIDs [5-8], although unlike rofecoxib, the GI benefit observed for celecoxib in short-term treatment [9] appears to diminish after 6 months of follow up [10] . Recent evidence suggests that both coxibs and nsNSAIDs (henceforth, the term nsNSAID refers only to non-aspirin nsNSAIDs) may increase the risk for cardiovascular (CV) adverse events [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] . In this environment, an increased rate of switching between different anti-inflammatory medications may be predicted as physicians alter their prescribing practice in response to new information. However, medication switching due to dissatisfaction with different NSAIDs was observed before concerns over their CV safety were widely disseminated. Published studies assessing nsNSAID utilization suggest a high level of switching, commonly related to lack of efficacy or tolerability [21] [22] [23] . Comparative studies have found lower switching rates with coxibs than nsNSAIDs [24] [25] [26] [27] . Switching may result in wastage of medication and generate costs for additional physician visits. Therefore it is important to consider these costs when projecting the overall economic impact of a changing NSAID prescribing practice. The objective of this study was to identify determinants of switching among elderly new chronic users of coxibs or nsNSAIDs in 2001-2002 in Quebec, Canada, and to estimate the costs of healthcare resources associated with switching. The results of this study should be relevant to current circumstances, as the cost of a switch is expected to remain fairly stable over time.