PurposeHealthcare risk epidemiology identifies medication error as the commonest cause of adverse effects on patients. Medication error can occur at any phase of the complex medication process so prevalence rates need to be estimated at each drug treatment phase: prescription, transcription and administration along with their clinical repercussions. This paper aims to investigate this issue.Design/methodology/approachMedication errors were recorded on an ad hoc sheet and staff were observed handling medications. Recorded errors were later classified and their clinical repercussions determined by experts.FindingsIn total 757 inpatients and 5,466 drug prescriptions were studied. The prescription error rate was 4.79 percent (95 percent CI 4.21‐5.36). The most frequent error in this phase was failing to observe international prescribing standards. The highest error rate was found in transcription (14.61 percent, 95 percent CI 13.67‐15.54). Almost 1,900 dose administrations were observed. There was a 9.32 percent error rate (95 percent CI 7.98‐10.67). The commonest error in this phase was omission. Most were transcription errors, which were detected before harm was done.Research limitations/implicationsThe dispensation phase is absent.Practical implicationsErrors can be reduced if they are understood. Education and training based on the study's findings can reduce medication errors.Originality/valueThe paper highlights ways to reduce errors in the medication process.