AbstractIntroductionMedication errors are common causes of preventable adverse drug events and increase the risk of morbidity and mortality. Beginning in July 2014, Tan Tock Seng Hospital Pharmacy Department (Singapore) implemented the round‐the‐clock inpatient medication chart review service by pharmacists. We evaluated the impact of this service on medication errors and the consequences of these errors.MethodsPharmacy intervention data from 2013 to 2015 were accessed. Severity of medication errors was defined according to the National Coordination Council for Medication Error Reporting and Prevention (NCCMERP) categories. Interrupted time series analysis was used to assess the impact of the service on monthly error rates. Consequences of medication errors were investigated in a matched case‐control analysis comparing outcomes and costs between cases with “errors” (Category D to I) and controls who had “near‐misses” (Category B). A cost analysis was done in Singapore dollars (S$).ResultsThe mean number of “near‐misses” was 32.04 per 1000 patient‐days/month prior to the service. When the service was implemented, the interrupted time series analysis showed a level increase of 21.43 per 1000 patient‐days/month (95% confidence interval [CI], 14.21, 28.65, P < .01) with a positive trend change of 1.04 per 1000 patient‐days/month. Patients with “near misses” had lower median hospitalization costs (S$2673.60 interquartile range [IQR] [S$1365.70‐5336.20], vs S$6326.60 IQR [S$1926.20‐13 609.00], P = .002) and length of stay (10 days [4‐21] vs 17 days [5‐42], P = .025). No significant differences were observed for all other outcomes studied.ConclusionRound‐the‐clock pharmacist inpatient medication review service increased detection of “near misses” and prevented these errors from reaching patients, which may otherwise have resulted in increased hospitalization costs and length of stay.