Warfarin: INR tests, diet, alcohol and bleeding warning signs

Warfarin slows the blood's ability to clot. The NHS lists treatment and prevention of clots in the veins (deep vein thrombosis) and lungs (pulmonary embolism), and lowering the risk of clots in atrial fibrillation, artificial heart valves, clotting disorders and after a TIA.

Many people take it long term. It is available on prescription only, and you are normally given a treatment record that sets out your dose and test dates.

INR stands for international normalised ratio, a blood test that tells your clinic how quickly your blood clots. At the start, tests may be needed every day or every few days while the dose is found. Once you are stable the NHS says you usually only need a test every 12 weeks, although some people need them more often.

Sometimes you may be able to check your INR yourself at home. Your clinic sets a target range for you and changes the dose depending on each result, so always follow the latest instruction.

Take warfarin once a day at the same time. It can be taken with or without food. Foods rich in vitamin K, such as broccoli, spinach and other green leafy vegetables, can alter how well warfarin works. You can still eat them, but speak to your warfarin clinic for advice and tell them before any big change in diet.

The NHS says to avoid cranberry juice and cranberry products, grapefruit juice, and large amounts of alcohol. Herbal and supplement products matter too: avoid St John's wort and vitamin K supplements.

Most people can take warfarin. The NHS lists as reasons for caution an allergy to warfarin, liver or kidney problems, high blood pressure, a recent stroke, recent or upcoming surgery, pregnancy or planning one, and conditions that make bleeding easy such as a stomach ulcer.

The prescriber weighs these against the clot risk. If you are not sure which of your conditions matter, ask the clinic to go through them with you.

The main side effect is an increased risk of bleeding. Call NHS 111 for blood in urine or stools, unexplained bruising or nosebleeds, coughing or vomiting blood, or any head injury. A sudden severe headache with confusion, slurred speech, difficulty moving, nausea or vomiting can signal bleeding in the brain and needs urgent emergency help.

Less common serious problems include a painful skin rash, which can be a sign of calciphylaxis, and anaphylaxis with a swollen throat or tongue, an itchy rash and difficulty breathing. Warfarin does not usually affect driving.

Many medicines interact with warfarin, including anti-inflammatory painkillers such as ibuprofen and aspirin, antibiotics, antidepressants, heart medicines, stomach ulcer medicines, antifungals and statins. Always tell your warfarin clinic about any new medicine, even a short antibiotic course, and show the alert card to dentists and doctors.

If you miss a dose, take it as soon as you remember, but skip it if you only realise the next day, and never double up. Note any missed doses and tell the clinic if it happens often. Taking an extra dose can be dangerous so call NHS 111. Warfarin is not usually used in pregnancy because it can affect the baby's development, though it is sometimes used, so tell your doctor straight away if you are pregnant or planning to be. It can be taken while breastfeeding.