Unfractioned heparin (UHF) is the most commonly used anticoagulant in hemodialysis, as it is generally well tolerated and can be quickly reversed with protamine sulfate. Low-molecular weight heparin (LMWH) is however, becoming increasingly popular and is now the norm in western Europe. Compared to UHF, LMWH has the advantage of an easier mode of administration and reduced bleeding but the effect cannot be easily reversed. Heparin can infrequently cause a low platelet count due to a reaction called heparin-induced thrombocytopenia (HIT). In such patients, alternative anticoagulants may be used. The risk of HIT is lower with LMWH compared to UHF. Even though HIT causes a low platelet count it can paradoxically predispose thrombosis. In patients at high risk of bleeding, dialysis can be done without anticoagulation.