Systemic anticoagulants are parental agents used to prevent formation of thrombus or to stop extension of pre-existing thrombus. They do so by inhibiting elements of the clotting cascade, which is an integral part of haemostasis. Systemic anticoagulants include unfractionated heparin (UFH) and its derivative, low molecular weight heparins (LMWH). Both UFH and LMWHs have a multitude of uses in medical and surgical practice and exert their effects mainly via the inhibition of factor Xa, and to a lesser extent, antithrombin III. LMWH has largely replaced UFH in many clinical settings owing to its more convenient administration and the lack of need for routine laboratory monitoring.