A retrospective analysis of reports of medication administration errors over a period of three and a half years was carried out in a UK psychiatric hospital. A total of 112 errors and "near misses" were studied. The reporting rate increased over time. Psychotropic, intramuscular, and as-needed medications were overrepresented in the error reports. Fifteen percent of the errors had the potential to cause moderate or severe harm to patients. The two most common factors cited by nurses as contributing to error causation were a busy, noisy environment and personal factors, such as feeling tired or unsupported. Physicians were cited as having contributed to some errors.