Warfarin is an anticoagulant that thins the blood.

As a rat poison it is used to induce fatal haemorrhages. In humans it's used to reduce the risk of blood clots.

Why it's prescribed

Warfarin is often prescribed to those with heart problems to reduce the risk of clots in the heart, which could travel to the brain and cause a stroke or to the lungs and cause a pulmonary embolism.

In particular, it's usually used after:

• Development of an abnormal heart rhythm, particularly atrial fibrillation, where the smaller chambers of the heart quiver rather than pump regularly, allowing blood flow to stagnate and clots to develop
• Insertion of artificial heart valves, which are prone to develop clots on their surface

Warfarin is also given when people form clots elsewhere in the body, such as a deep vein thrombosis (DVT) in the calf or pelvis following surgery.

Extra care needed

People on warfarin need to take particular care that their doctor monitors the dose. The blood needs to be at the correct viscosity to balance the risk of clotting and with the risk of haemorrhage.

If the dose is too high, severe bleeding can occur even after a tiny knock or no obvious trauma at all. Even on a normal dose, a minor fall can lead to extensive bruising.

Ideally, your mother should carry a warning device so in case of an accident the emergency staff know to administer the antidote to warfarin (vitamin K). She should also tell her dentist and mention it to the pharmacist when buying any over-the-counter medication, as some interfere with warfarin.

Simple aspirin can greatly increase the effects of warfarin and shouldn't be taken without consulting the doctor. Alcohol can also interfere with the effects and should be avoided. In addition, a number of complementary therapies and even foods can interfere with warfarin. There have been reports that St John’s Wort, garlic, dong quai, devil’s claw, broccoli and Brussels sprouts can all increase the anti-coagulant effect of warfarin, while Korean ginseng and green tea may decrease the effect – always check with your pharmacist before taking herbal and other remedies.

This article was last medically reviewed by Dr Trisha Macnair in September 2008