Warfarin and rifampicin: why the dose of warfarin often needs to rise

Warfarin is a blood-thinning medicine taken to prevent clots. The right dose differs from person to person, which is why regular INR tests are used to tune it. The NHS explains that INR is a blood test that measures how quickly your blood clots, and that your dose may change based on your test results.

Rifampicin is an antibiotic. The BNF lists its uses as including tuberculosis in combination with other drugs, usually for a 6-month course, and certain serious infections such as brucellosis, Legionnaires disease and some staphylococcal infections, again in combination with other antibacterials.

The BNF states simply that rifampicin decreases the anticoagulant effect of warfarin, which means the same dose of warfarin does less. The outcome, if nothing is done, is a falling INR and a weaker defence against clots.

People take warfarin for reasons that can be serious: a past or current deep vein thrombosis or pulmonary embolism, an irregular heartbeat, a mechanical heart valve or a past mini-stroke, as the NHS describes. In each case the purpose of the treatment is to keep clotting in a target range. If rifampicin pushes the INR too low, a clot is more likely.

This is not just theoretical. A case reported in the Scottish Medical Journal by a Glasgow team describes a patient who developed a deep vein thrombosis after surgery while on rifampicin for itching in primary biliary cirrhosis, and who had no therapeutic INR after fourteen days of warfarin because of the interaction. The authors reviewed the mechanism and the use of rifampicin in that setting.

When rifampicin is prescribed to someone already taking warfarin, the usual approach is closer INR testing and dose adjustment through the anticoagulation service. The NHS notes that testing is frequent when you first start warfarin, and that some people need tests more regularly than every 12 weeks. Because rifampicin weakens warfarin, the dose may need to change, but the size of the change must be guided by the test results and not guessed.

If rifampicin is started in hospital, for example as part of a TB treatment plan, make sure the ward team and the community clinic share their records. Hand-over problems are a common reason for missed tests.

• Tell the prescriber about warfarin before rifampicin starts, even if the antibiotic is a short course.

• Tell your anticoagulation clinic the day you start and the day you finish.

• Keep your yellow anticoagulant record and alert card up to date.

• Do not change your own dose in response to a test result unless the clinic instructs you.

Stopping is as important as starting. If the warfarin dose was raised to compensate for rifampicin, the same dose may become too strong once rifampicin is no longer opposing it, which would increase bleeding risk. The sources I read do not set out how long the effect lasts after the last dose, so ask your clinic how long to keep testing.

The practical consequence is that INR monitoring is sensible after rifampicin ends, with any change made under supervision. The BNF classes the interaction as moderate, but unmanaged changes in either direction can matter.

Both directions of error carry symptoms. Signs of a clot, such as a painful swollen leg or sudden breathlessness or chest pain, need urgent medical help. Signs of too much anticoagulation are the bleeding symptoms the NHS lists for warfarin: blood in pee or poo, unexplained bruising or nosebleeds, coughing up or vomiting blood, and a sudden severe headache with confusion or weakness.

The NHS says to call NHS 111 if you have symptoms of bleeding or a head injury on warfarin. Call 999 for suspected stroke, severe breathlessness, chest pain or heavy bleeding.

Some people on long courses of rifampicin are considered for a different anticoagulant. NICE CKS lists warfarin alongside apixaban, dabigatran, edoxaban and rivaroxaban as licensed oral anticoagulants in the UK, and the choice depends on the condition, kidney function, other medicines and preference. Whether an alternative suits you is a specialist decision.

Whether your care is in the NHS or private, it is sensible to ask who is responsible for your INR plan during the antibiotic course.