Why do warfarin levels fluctuate?

Q: My husband takes warfarin because he has an irregular heartbeat.  He has to have regular blood tests  and the doctor is forever altering the dose . Why do warfarin levels fluctuate so much?

A: Warfarin prevents blood clots by inhibiting the action of vitamin K, an essential component of the clotting system. The blood tests are to check he is on the right dose and once a stable level is achieved he should be able to stick to the same amount and be re-tested every 4 – 6 weeks.  However  some people struggle to reach this steady state and drugs and foods that interfere with warfarin may be to blame – lots of medicines, including aspirin, simvastatin and antibiotics, can influence warfarin levels, as can heavy drinking and the herb St John’s wort.

There have also been problems in people eating lots of leafy green veg and brussel sprouts (they contain vitamin K and compounds that interfere with warfarin metabolism) and cranberry juice (cause unknown).

The key is consistency – if your husband likes his sprouts and broccoli then he should try and eat some every day so any effect is constant– and always ask your pharmacist about interactions before taking any remedy or supplement, prescribed or otherwise.

If he is really struggling ask his GP about the newer anticoagulants (eg rivaroxaban) that don’t require monitoring.

Subscribe today for just £34.95 for 12 issues...

Subscribe today and save 51%