Phenindione

How it works

Like coumarins it antagonises vitamin K, inhibiting hepatic synthesis of the vitamin K-dependent clotting factors II, VII, IX and X to produce an anticoagulant effect.

Prescribing in practice

• Haemorrhage is the principal risk, and dosing must be guided by the INR; the effect is potentiated or reduced by numerous drug and dietary interactions.
• It can cause hypersensitivity reactions and may colour the urine pink or orange, which is harmless but should be distinguished from haematuria.
• It is generally used only when warfarin is unsuitable, with dosing titrated to INR per current prescribing references.

Monitoring

Monitor the INR regularly to keep anticoagulation within the target range and review for bleeding and hypersensitivity reactions.

Counselling the patient

• Warn that the medicine can turn urine pink or orange, which is not dangerous.
• Advise reporting unusual bleeding or bruising, rash, sore throat or fever, and the importance of regular blood tests.

Evidence & guidelines

Phenindione's role as a reserve oral anticoagulant for warfarin-intolerant patients is established in UK practice and the SPC.