Low molecular weight heparins (LMWH) are a form of anticoagulation that is usually given for short-time period, e.g. as prophylaxis against venous thromboembolism. They are also used when urgent anticoagulation is needed before longer term anticoagulation is needed e.g. warfarin.

They are given subcutaneously.

Mechanism of action

Activates antithrombin III. Forms a complex that inhibits factor Xa

Adverse effects

  • Bleeding
  • Thrombocytopenia - lower risk than with standard heparin
  • Osteoporosis and an increased risk of fractures
  • Hyperkalaemia - this is thought to be caused by inhibition of aldosterone secretion


Routine monitoring is not required.

However, in certain situations such as renal impairment anti-Factor Xa may be used.