OBJECTIVES: To determine the economical and clinical impact of pharmacists’ interventions in ambulatory care within the context of a prescribing error of Nonsteroidal Anti-Inflammatory Drugs (NSAIDs). METHOD: A national survey was carried out in 900 town pharmacies during 12 weeks to record all prescriptions with an anomaly like contraindication, interaction which could be dangerous for the patient. We used a decision analysis to compare two strategies, with or without a systematic pharmacist’s intervention before a prescription of NSAIDs. The outcome was upper gastrointestinal side effects of NSAID therapy (peptic ulcer and complications) and we used a prescribing errors rate varying between 0,5% to 2% to estimate the costs savings and to measure the occurrence of peptic ulcer avoided. Computer simulation was performed with Tree Age 3.0. RESULTS: 446 cases of NSAIDs prescription errors were notified including combination of NSAIDs, NSAIDs overdose and NSAIDs prescription with risk factors like peptic ulcer history or age>65. Study gave an estimation of 49.390 prescriptions involving NSAIDs prescription at risk of upper gastrointestinal side effects in France for one year. The pharmacist’s intervention before a NSAID prescription with an error lead to save 1.037.190 FF (158.119 Euros) to 3.951.200 FF (602.356 Euros) per year for the Social Security and lead to avoid 58 to 232 peptic ulcers per year (30 to 120 complicated ulcers). CONCLUSION: This study confirmed that pharmacist intervention is cost saving and has a clinical impact as regard prescribing errors. Moreover this effect may be larger as long as we only took into account the upper gastrointestinal NSAIDs side effects.