Background Medication errors are an important cause of morbidity and mortality and adversely affect clinical outcomes. Prescribing errors constitute one type of medication error and occur particularly on admission to hospital; little is known about how they arise.

Aim This study investigated how doctors obtain the information necessary to prescribe on admission to hospital, and the number and potential impact of any errors.

Setting English teaching hospital—acute medical unit.

Methods Ethics approval was granted. Data were collected over four 1-week periods; November 2009, January 2010, April 2010 and April 2011. The patient admission process was directly observed, field notes were recorded using a standard form. Doctors participated in a structured interview; case notes of all patients admitted during study periods were reviewed.

Results There were differences between perceived practice stated in interviews and actual practice observed. All 19 doctors interviewed indicated that they would sometimes or always use more than one source of information for a medication history; a single source was used in 31/68 observed cases. 7/12 doctors both observed and interviewed indicated that they would confirm medication with patients; observations showed they did so for only 2/12 patients. In 66/68 cases, the patient/carer was able to discuss medication, 14 were asked no medication-related questions. Of 688 medication charts reviewed, 318 (46.2%) had errors. A total of 851 errors were identified; 737/851 (86.6%) involved omission of a medicine; 94/737 (12.8%) of these were potentially significant.

Conclusions Although doctors know the importance of obtaining an accurate medication history and checking prescriptions with patients, they often fail to put this into practice, resulting in prescribing errors.