Objective: To examine pharmacy costs of a Prior Authorization (PA) on lubiprostone for elderly chronic constipation (CC) patients in a Medicare Part-D plan. Methods: Cost impact of PA was calculated by estimating annual pharmacy cost differences with PA (PA administration costs + medication costs) and without PA (medication costs only). Model inputs included published estimates of CC prevalence; lubiprostone utilization from IMS Health, 2007; average PA approval rate, PA costs and co-payment from payer interviews; and lubiprostone wholesale acquisition costs. Annual medication costs in both scenarios included costs and utilization of lubiprostone less co-payment, assuming third-tier placement for lubiprostone. All previously rejected prescriptions were assumed to be approved after lifting PA, resulting in 21.24% increase in prescription volume. Sensitivity analyses were performed on PA cost, PA approval rate, and expected increase in prescription volume after lifting PA. Results: Results: CC prevalence was 14.7%, of which 1.14% were lubiprostone users. For a 1-million member plan, this resulted in 1,264 PA requests costing $27 each. Annual cost of PA administration was $34,130. PA approval rate for the elderly was 77.7% (or 982 approved users). Average number of fills per person per year was 3.8. A 30-day lubiprostone prescription costed $28.40 ($86.40 WAC-$60 co-payment+$2 dispensing fee). Drug costs were $105,997, resulting in total annual cost with PA of $140,127. Total annual costs without PA were $128,506, based on an additional 209 users, resulting in annual savings of $11,621. Sensitivity analyses indicated break even scenarios from removing PA on lubiprostone when cost per PA >$17.81 or PA approval rate >69.18%, or expected increase in prescriptions from lifting PA <32.20%. Conclusion: Conclusions: PA program for lubiprostone offers no financial savings to a Medicare plan based on current approval rates and annual utilization for elderly patients with CC in the base case as well as in sensitivity analyses.