BACKGROUND: Clinical pharmacists can help prevent medication errors. However, data are scarce on their role in preventing medication prescription errors in the post-operative period, a high-risk period, as at least two prescribers can intervene, the surgeon and the anesthetist. We aimed to describe and quantify clinical pharmacist' intervention (PIs) during validation of drug prescriptions on a computerized physician order entry system in a post-surgical and post-transplantation ward. We illustrate these interventions, focusing on one clearly identified recurrent problem. METHODS: In a prospective study lasting 4 years, we recorded drug-related problems (DRPs) detected by pharmacists and whether the physician accepted the PI when prescription modification was suggested. RESULTS: Among 7005 orders, 1975 DRPs were detected. The frequency of PIs remained constant throughout the study period, with 921 PIs (47%) accepted, 383 (19%) refused and 671 (34%) not assessable. The most frequent DRP concerned improper administration mode (26%), drug interactions (21%) and overdosage (20%). These resulted in a change in the method of administration (25%), dose adjustment (24%) and drug discontinuation (23%) with 307 drugs being concerned by at least one PI. Paracetamol was involved in 26% of overdosage PIs. Erythromycin as prokinetic agent, presented a recurrent risk of potentially severe drug-drug interactions especially with other QT interval-prolonging drugs. Following an educational seminar targeting this problem, the rate of acceptation of PI concerning this DRP increased. CONCLUSION: Pharmacists detected many prescription errors that may have clinical implications and could be the basis for educational measures.