Warfarin and aspirin: why the combination needs care

Warfarin is an anticoagulant, sometimes called a blood thinner. The NHS says it is used to treat and prevent blood clots, including clots in the veins and lungs, and to reduce clot risk in people with an irregular heartbeat such as atrial fibrillation, a mechanical heart valve, a clotting disorder or a previous transient ischaemic attack.

Aspirin works differently. The NHS describes it as a non-steroidal anti-inflammatory drug that can be taken at a low, regular dose to help prevent blood clots, heart attack and stroke, or at a higher dose, only when needed, to ease pain. The NHS anticoagulant page makes the distinction explicit: anticoagulants are different from antiplatelet medicines such as low-dose aspirin and clopidogrel, even though they are used for similar purposes.

The shared risk is bleeding. Neither medicine is harmless when it comes to bleeding. A person who takes both is exposed to the bleeding effect of each, which is why this pairing appears on medicine-safety lists and why prescribers are careful about it.

The NHS warfarin page lists several groups of medicines that may not mix well with warfarin, including non-steroidal anti-inflammatory drugs such as ibuprofen or aspirin, antibiotics, antidepressants, medicines for heart problems, stomach ulcers and fungal infections, and cholesterol-lowering statins. It asks you to tell your doctor about any other medicines, herbal remedies, vitamins or supplements before you start warfarin, and to speak to your doctor before starting any new medicine while you take it.

The NHS aspirin page approaches the same pairing from the other side. In its list of medicines to mention before taking aspirin, anticoagulants such as warfarin or rivaroxaban come first, followed by some steroid medicines, some antidepressants, some blood pressure medicines, methotrexate and other anti-inflammatory painkillers.

Notice what these pages do not say. They do not say that nobody may ever take both. Some people are given aspirin and an anticoagulant on purpose, for reasons their specialist has weighed. The message is that the combination should be a decision that someone has made and is monitoring, not something that happens by accident because a packet of aspirin was in the bathroom cabinet.

The NHS warfarin page lists the signs of serious bleeding that people should know, and the aspirin page lists stomach bleeding as a rare but serious effect. Because the two medicines can add to each other, it is worth knowing both lists.

People on warfarin have regular blood tests to measure how quickly their blood clots, called the international normalised ratio or INR. The NHS says that when you first start warfarin you need tests every day or every few days, and that once the dose is stable a test about every 12 weeks is usual, although some people need them more often. The dose is adjusted according to the results.

Anything that changes how warfarin behaves can shift the INR, and anything that adds its own bleeding tendency, as aspirin does, changes the safety picture even when the INR looks acceptable. That is why the warfarin or anticoagulation clinic should be told about every new medicine, including those bought over the counter and including short courses. The NHS also notes that you can be given a treatment record and an alert card; keeping the record up to date is part of staying safe.

Most of the protection comes from habits rather than from any special test.

• Before buying any painkiller, cold remedy or indigestion product, ask a pharmacist and say that you take warfarin. Some cough and cold products contain aspirin or another anti-inflammatory ingredient.

• Carry your anticoagulant alert card and show it to doctors, dentists and pharmacists, as the NHS advises.

• Do not start or stop aspirin on your own. The NHS aspirin page says that if you regularly take low-dose aspirin you should keep taking it unless you are advised to stop, so any change should come from the clinician who prescribed it.

• Keep one up-to-date list of every medicine, including supplements and herbal products, and bring it to appointments.

• If you are told to take aspirin for a short period, for example after a procedure, check that the person advising you knows you take warfarin.

In the NHS, warfarin treatment is usually supervised through a warfarin or anticoagulant clinic, and the NHS page tells people to contact that clinic if they often miss doses or have any other problem. Your GP surgery and community pharmacist can also review your medicines. Pharmacists can usually tell you quickly whether a product on the shelf contains an ingredient to avoid.

If you see a private clinician for another problem, the same principle applies: they need to know about warfarin and the alert card, and your NHS clinic should hear about any new prescription. A private prescription does not remove the interaction; it simply means the information has to be passed on deliberately.

Most people who take warfarin go on to live ordinary lives. The aim of this page is not to make aspirin sound forbidden but to make the check automatic.