Heparin is a naturally occurring substance, usually made from pig intestines. It works by activating a protein in the blood (called antithrombin III) to prevent blood clots forming.

Heparin is used as an anticoagulant in atrial fibrillation, pulmonary embolism, deep vein thrombosis, in acute coronary syndrome (also called 'unstable angina' or a 'heart attack') and heart bypass surgery.

It is usually given by injection, usually in hospital as often many injections are required each day. It can also be given by a 'drip' into the veins, which also needs to be done in hospital.

There is a newer class of heparins, called Low Molecular Weight Heparins, which can be given once daily. The most commonly used example of these is enoxaparin (Clexane^®).

Enoxaparin is given both to patients in hospital, and also to people in their homes who can be given the injection by a community nurse, or may even be able to give it to themselves.

If long term anticoagulation is required, heparins are often only used until warfarin starts to work and the INR has become stable in the target range.