The American College of Chest Physicians (ACCP) recommends thromboprophylaxis for selected general medical patients using unfractionated heparin (UFH), low-molecular-weight heparin (LMWH) or fondaparinux (Arixtra).  The results of a new Cochrane review of 13 randomized trials suggest that LMWH may be safer than UFH with equal efficacy (level 2 [mid-level] evidence).  In analyses comparing the pooled effects of UFH or LMWH vs. controls (placebo or no treatment), the heparin treatments were associated with reduced rates of deep vein thrombosis (DVT) (2.9% vs. 7.3%, p < 0.00001) and pulmonary embolism (PE) (0.63% vs. 1.1%, p = 0.0073) with no significant difference in mortality (5.2% vs. 5.5%).  However, the heparin treatments were associated with increased risk of major bleeding (0.49% vs. 0.22%, p = 0.0042).  In a meta-analysis of 4 trials that directly compared UFH to LMWH, UFH was associated with a significantly higher risk of major bleeding (1.35% vs. 0.34%, p = 0.015), with no significant differences in DVT, PE or mortality (Cochrane Database Syst Rev 2009 July 8;(3):CD003747).