Warfarin has been used an anticoagulant to thin the blood for over 60 years. When used safely it has been shown to effectively prevent stroke when an individual is diagnosed with AF and is also used to treat a number of other conditions (for example deep vein thrombosis and pulmonary embolus or clot in the leg and lung).

Mechanism of action

Warfarin inhibits a number of clotting factors (II, VII,IX and X), which cause the formation of thrombus or clot. Individuals metabolise Warfarin differently and a number of medications and food-stuffs, for example alcohol, can interfere with the drug levels. As a result regular monitoring, to ensure that a patient’s blood is appropriately anticoagulated (or thinned), is required. On most occasions this is to ensure a correct target INR (International Normalised Ratio), usually aiming between 2.0 - 3.0.

Potential side-effects

The main side-effects from Warfarin are as a result of a limited ‘therapeutic window’. This means that it is effective over a narrow range. When the INR is less than 2 the action of Warfarin is less effective and when higher than 3 there is a tendency for bleeding to occur. It has been shown that many individuals have erratic INR readings (roughly 50% of the time in the UK).  The main signs of bleeding are: bruising, a rash, pink or red urine, black or red stool, dizziness or an altered conscious level. Other less common side-effects include rash, nausea and vomiting, diarrhoea and hair loss.

The future

With the advent of new ‘novel oral anticoagulants’, which have had considerable publicity in the media recently, monitoring INR levels with Warfarin may become a thing of the past, particularly in cases of individuals who have not tolerated Warfarin well.

Despite these advances there remains a case for continuation of Warfarin in some individuals. An argument in favour of Warfarin is that the medical profession has considerable experience of the drug and its potential side effects. One solution to this may be INR home monitoring to ensure that INR levels are correct so that Warfarin is both effective and safe.

For related articles please see 'Warfarin: friend or foe?' or search our Blog section.