Background Adverse drug events from preventable medication errors can result in patient morbidity and mortality, and in cost to the healthcare system. Medication reconciliation can improve communication and reduce medication errors at transitions in care. Objective Evaluate the impact of medication reconciliation and counselling intervention delivered by a pharmacist for medical patients on clinical outcomes 30 days after discharge. Setting Sultan Qaboos University Hospital, Muscat, Oman. Methods A randomized controlled study comparing standard care with an intervention delivered by a pharmacist and comprising medication reconciliation on admission and discharge, a medication review, a bedside medication counselling, and a take-home medication list. Medication discrepancies during hospitalization were identified and reconciled. Clinical outcomes were evaluated by reviewing electronic health records and telephone interviews. Main outcome measures Rates of preventable adverse drug events as primary outcome and healthcare resource utilization as secondary outcome at 30 days post discharge. Results A total of 587 patients were recruited (56 ± 17 years, 57% female); 286 randomized to intervention; 301 in the standard care group. In intervention arm, 74 (26%) patients had at least one discrepancy on admission and 100 (35%) on discharge. Rates of preventable adverse drug events were significantly lower in intervention arm compared to standard care arm (9.1 vs. 16%, p = 0.009). No significant difference was found in healthcare resource use. Conclusion The implementation of an intervention comprising medication reconciliation and counselling by a pharmacist has significantly reduced the rate of preventable ADEs 30 days post discharge, compared to the standard care. The effect of the intervention on healthcare resource use was insignificant. Pharmacists should be included in decentralized, patient-centred roles. The findings should be interpreted in the context of the study's limitations.