This study combined previously published error prevalence estimates in the United Kingdom to estimate the annual number and burden of medication errors to the National Health Service (NHS). The authors estimate that 237 million medication errors occur annually in the UK. The majority of errors occur during medication administration and prescribing and occur most frequently in long-term care and primary care settings. While the majority of errors have little or no potential for harm, 66 million errors were deemed potentially clinically significant; prescribing in primary care settings accounts for one-third of these potentially significant errors. These medication errors result in hospital admissions or longer length of stay, as well as death.