ORIGINAL REPORT A Study of the Longitudinal Utilization and Switching- Patterns of Non-Steroidal Anti-In¯ammatory Drugs Using a Pharmacy Based Approach ROSEMARY SIFT PhD 1 , TJEERD-PIETER VAN STAA MD MSc 2 , LUCIEN ABENHAIM MD DSc 2 AND DANIEL EBNER BPhil 1 1 Medical Department, Ciba-Geigy, 4002 Basel, Switzerland 2 Center for Community Studies & Clinical Epidemiology, Jewish General Hospital, McGill University, Department of Epidemiology and Biostatistics, 3755 Chemin de la Co Ãte-St Catherine, Montreal, Canada SUMMARY The extent of the heterogeneity of drug utilization among NSAID users has not been extensively studied. We studied the longitudinal prescribing and switching patterns of NSAID users in a 1-year follow-up study in four German pharmacies. The study population consisted of 526 persons with an average age of 57 years. We observed that the legend duration of prescription increased with age; 14.3 days for patients aged 44 or younger to 25.1 days for persons 75 years or older, and was dependent on disease chronicity; 16.0 days for acutely ill persons compared to 23.9 days for chronic patients. The average legend duration also varied between dierent types of NSAIDs, from 18.0 days for ibuprofen to 29.1 days for tiaprofen. Switching from one type of NSAID to another proved to be related to the legend duration of prescription and patient characteristics such as compliance with NSAID therapy, duration of the disease and the eectiveness (poor tolerability or insucient eect) of the NSAID therapy reported by the patients 4 weeks after recruitment. We conclude that NSAID users cannot be viewed as an homo- geneous group of patients with respect to exposure time to the drug. This heterogeneity should be considered in the exposure de®nition, the `time-window' design of observational studies dealing with risk comparisons in particular. # 1997 John Wiley & Sons, Ltd. Pharmacoepidemiology and Drug Safety: 6: 263±268, 1997 No. of Figures: 1. No. of Tables: 3. No. of References: 20. KEY WORDS Ð NSAIDs; longitudinal prescribing pattern; switching; observational study INTRODUCTION Observational studies usually deal with a much greater heterogeneity in the way the drug is being used, as compared to clinical trials. However, many observational studies follow the clinical trial paradigm of contrasting exposed to non-exposed, assuming a homogeneity of drug use. For example, many observational studies use prescription information to classify exposure. In such studies, a number of exposure days termed here as a `time-window' must be assigned to each prescription as there is information only on the date and type of drug dispensing and not on the duration and timing of drug intake. Most of these studies assigned equal windows to prescrip- tions, 1±10 thus assuming a homogeneity of drug utilization among the members of the study population. Van Staa et al. have shown that the assignment of equal windows in the presence of utilization heterogeneity can lead to a substantive underestimation of the drug risk. 11;12 Another eect of spuriously assuming the homogeneity of drug utilization is that comparisons of drug risks can be biased. For example, comparisons of the drug risks between age, gender or type of NSAID CCC 1053±8569/97/040263±06$17.50 # 1997 John Wiley & Sons, Ltd. Accepted 5 February 1997 PHARMACOEPIDEMIOLOGY AND DRUG SAFETY, VOL. 6: 263±268 (1997)