There are two main types of heparin - unfractionated, 'standard' heparin or low molecular weight heparin (LMWH).

Mechanism of action

Heparins generally act by activating antithrombin III. Unfractionated heparin forms a complex which inhibits thrombin, factors Xa, IXa, XIa and XIIa.

Adverse effects

Adverse effects of heparins include:
  • bleeding
  • thrombocytopenia - see below
  • osteoporosis and an increased risk of fractures
  • hyperkalaemia - this is thought to be caused by inhibition of aldosterone secretion


The anticoagulant effect of heparin is monitored by the activated partial thromboplastin time (APTT)