This study described medication omission errors occurring in inpatient mental health hospitals in the United Kingdom. Of over 18,000 scheduled medication doses, 2,700 omissions were identified (omission rate, 14.6%). One-third of all omissions were considered ‘preventable.’ The omission rate among ‘time critical’ medications (those that carry risk of harm if a single dose is omitted) was 19.3%, and half of these omissions were considered ‘preventable’. Predictors of omission errors are discussed, including administration route and medication type. Analyses found that administration route, medication class and time of medication administration rounds were significantly associated with omission errors. These findings should inform future research and quality improvement initiatives designed to reduce the burden of omission errors in psychiatric hospitals.