Warfarin and simvastatin: what the BNF and NHS say about statins and INR

The NHS describes warfarin as an anticoagulant used to treat and prevent blood clots, including in the veins and lungs, and to reduce risk in conditions such as atrial fibrillation or a mechanical heart valve. Doses are individual, and the NHS says regular INR blood tests check that you are getting the right dose.

The NHS adds that when you first start warfarin you need tests every day or every few days, and that once on a stable dose you usually need a test every 12 weeks, although some people need them more often. A new medicine is one of the things that can unsettle a stable INR, which is why clinics ask about changes.

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

The NHS atorvastatin page names warfarin in its list of medicines that may not mix well with atorvastatin. The NHS simvastatin page lists different examples: antifungals, other cholesterol medicines, antibiotics such as erythromycin and clarithromycin, HIV and hepatitis C medicines, ciclosporin, nefazodone, blood pressure and heart medicines and danazol. Warfarin does not appear in the simvastatin list.

The BNF warfarin interaction list includes statin entries: fluvastatin increases the anticoagulant effect of warfarin, and rosuvastatin does too, each with manufacturer advice to monitor INR and adjust the dose and each rated severe with study evidence. In the list read for this guide there is no separate entry for simvastatin or atorvastatin.

This is a useful reminder that references differ in how they group and rate interactions. The NHS gives a broad warning about statins, and the BNF gives named entries. When your anticoagulant clinic is told about a new statin they decide how closely to check, and the practical approach is the same whichever statin it is: tell them, and expect an extra INR check around the start.

The NHS says the main side effect of warfarin is an increased risk of bleeding. Symptoms of serious bleeding can include blood in your pee or poo, unexplained bruising or nosebleeds, coughing up or vomiting blood, and a sudden bad headache with confusion, slurred speech, difficulty moving your arms or legs, or feeling or being sick, which can be signs of bleeding in the brain.

It advises calling NHS 111 if you are taking warfarin and have symptoms of bleeding, have had a head injury or think you might be having any other serious side effect.

Mention warfarin whenever a statin is prescribed, and mention the statin to the anticoagulant clinic. Ask when the next INR check should be. Keep taking both medicines as directed, and never double up on a missed dose of either.

The NHS also says not to drink grapefruit juice with warfarin, and, separately, simvastatin's own page says to avoid grapefruit juice. Both points are about the wider medicine list rather than about the pair together.

• Tell the anticoagulant clinic about any new statin or dose change.

• Ask when the next INR test should be.

• Keep a written list of all medicines and supplements.

• Report bleeding or bruising promptly.

In the NHS INR checks are arranged by an anticoagulant clinic, the GP practice or, for some people, through self-testing at home as the NHS warfarin page describes. Private prescribers use the same references, so the same rule applies in every setting: whoever prescribes a new medicine needs to know about the warfarin.