We included nine trials enrolling 2,637 patients. Eight trials were of unclear risk of bias and one was classified as having low risk of bias. In trials comparing heparin with placebo or usual care, the risk ratio for death associated with heparin was 0.88 (95% CI = 0.77 to 1.00, I2 = 0%, 2477 patients, six trials). In trials comparing heparin with other anticoagulants, the risk ratio for death was 1.30 (95% CI = 0.78 to 2.18, I2 = 0%, 160 patients, three trials). In trials comparing heparin with placebo or usual care, major hemorrhage was not statistically significantly increased (risk ratio 0.79, 95% CI = 0.53 to 1.17, I2 = 0%, 2,392 patients, three trials). In one small trial of heparin compared with other anticoagulants, the risk of major hemorrhage was significantly increased (2.14, 95% CI = 1.07 to 4.30, 48 patients). Important secondary and safety outcomes, including minor bleeding, were sparsely reported.