Errors in the management of regular medications at the time of hospital admission are common. This randomised controlled three-arm parallel-group trial examined the impact of pharmacist medication history taking and pharmacist supplementary prescribing on unintentional omissions of postoperative medications in a large perioperative service. Participants included elective surgical patients taking regular medications with a postoperative hospital stay of one night or more. Patients were randomly assigned, on admission, to usual care (n=120), a pharmacist medication history only (n=120) or pharmacist medication history and supplementary prescribing (n=120). A medication history involved the pharmacist interviewing the patient preoperatively and documenting a medication history in the medical record. In the supplementary prescribing group the patients' regular medicines were also prescribed on the inpatient medication chart by the pharmacist, so that dosing could proceed as soon as possible after surgery without the need to wait for medical review. The estimate marginal mean number of missed doses during a patients hospital stay was 1.07 in the pharmacist supplementary prescribing group, which was significantly less than both the pharmacist history group (3.30) and the control group (3.21) (P < 0.001). The number of medications charted at an incorrect dose or frequency was significantly reduced in the pharmacist history group and further reduced in the prescribing group (P < 0.001). We conclude that many patients miss doses of regular medication during their hospital stay and preoperative medication history taking and supplementary prescribing by a pharmacist can reduce this.