OBJECTIVES: As a result of safety concerns, labeling for fentanyl patches was strengthened in June 2005 to limit use to opioid-tolerant patients only. We evaluated prior opioid use in MHS patients prescribed fentanyl patches to support the DoD Pharmacy & Therapeutics Committee decision-making process. METHODS: Study patients included all MHS patients newly started on fentanyl patch from Jan-Dec 05 (no fentanyl patch prescription =180 days prior to index date). Patients were assumed to be opioid-tolerant based on prescriptions for a defined set of opioids considered potentially equipotent to a starting dose of fentanyl patch (25 mcg/hr) filled during 45-60 days prior to their index date, or if hospitalized on or during 7-14 days prior to their index date (since opioids might have been started during hospitalization). We did not estimate cumulative dose or duration of opioids. Duration of "look-back" periods and the defined set of opioids were varied to provide information on prescribing patterns. Prescription data were obtained from DoD's Prescription Data Transaction Service Data Warehouse, hospitalization data from the MHS Management Analysis and Reporting Tool. RESULTS: The percentage of patients that could not be assumed to be opioid-tolerant prior to starting fentanyl patch ranged from 27% to 51%; it was most sensitive to changes in how potentially equipotent opioids were defined. Results from 3-month periods before (Jan-Mar 05) and after (Oct-Dec 05) labeling changes were similar. CONCLUSION: The number of MHS patients who are not opioid-tolerant prior to starting fentanyl patches is potentially large. Assessments of changes in prescribing behavior following educational efforts are underway. DoD decided in Jan 2007 to require prior authorization for fentanyl patches, based on prior opioid use.