ObjectiveTo determine the prevalence, nature and predictors of patients having medication administration omissions in hospitals.MethodsAll medication administration omissions data collected using the standardised methodology of the Medication Safety Thermometer in January 2015 were examined. Hospital inpatients prescribed at least one medication were included in the analysis. Multilevel logistic regression models ascertained the effects of patients’ gender, age, number of prescribed medicines, ward specialty and medicines reconciliation initiation status on the likelihood of having omissions. Valid clinical reasons (VCRs) were excluded from regression models. A sensitivity analysis, excluding patient refusal (PR) omissions, was also conducted.ResultsThe final study sample included 5708 patients from 320 wards in 37 hospitals. Excluding VCRs, 30% of patients had medication administration omissions (95% CI 29 to 30). Approximately half of patients with omissions had refused medicines (51%, 95% CI 49 to 53). Univariable analysis suggested that all variables were significantly associated with omissions. However, in the multivariable model, significant differences were only observed regarding the numbers of medicines patients were prescribed and their ward specialty. Patients prescribed more than 20 medications were approximately five times more likely to have had omissions than patients prescribed one to four medications (OR 4.99, 95% CI 3.22 to 7.73). Patients on surgical wards were also more likely to have had omissions than those on medical wards (OR 1.58, 95% CI 1.14 to 2.18, p=0.006), but there was no significant difference when PRs were excluded (OR 0.5, 95% CI 0.27 to 1.22, p=0.473).ConclusionMedication administration omissions are a substantial problem that affect many hospital patients, and certain patient groups are at higher risk. Specific interventions are required targeting the underlying reasons for medication omissions for different patient subgroups.