- Objective We compared adherence to unfractionated heparin (UFH) 2 or 3 times daily prophylaxis orders versus low-molecular-weight heparin (LMWH) once daily orders. Our goals were to determine which strategy demonstrated the best adherence in terms of timing and frequency of dose administration, and to determine reasons for ordered heparin not being administered. Methods We queried our electronic medication administration record where nurses document reasons for delayed administration or omitted doses. We identified 250 consecutive patients who were prescribed prophylaxis with UFH 2 or 3 times daily or LMWH once daily. We followed patients for their hospitalization to determine adherence to physicians' prophylaxis orders. Results Adherence, defined as the ratio of prophylaxis doses given to doses ordered, was greater with LMWH (94.9%) than UFH 3 times daily (87.8%) or UFH twice daily (86.8%) regimens (P <.001). Patients receiving LMWH more often received all of their scheduled prophylaxis doses (77%) versus UFH 3 times daily (54%) or UFH twice daily (45%) (P <.001). There were no differences between regimens regarding reasons for omitted doses. The most common reason for late or omitted doses was patient refusal, which explained 44% of the UFH and 39% of the LMWH orders that were not administered. Conclusions LMWH once a day had better adherence than UFH 2 or 3 times daily. For both LMWH and UFH, patient refusal was the most common reason for not administering prophylaxis as prescribed. These findings require consideration when evaluating pharmacological prophylaxis strategies. Educational programs, explaining the rationale, may motivate patients to improve adherence during hospitalization.