Heart medication warning: mixing pills with a common daily supplement could trigger fatal reaction, experts say

Millions of people on blood-thinning drugs may be putting themselves at risk of dangerous side-effects by taking certain supplements alongside their medication, experts have warned.

Warfarin is the most commonly prescribed anticoagulant in the UK, given to more than a million patients to prevent life-threatening blood clots that can trigger strokes.

The drug helps blood flow more freely through the veins and is routinely prescribed to people who have previously suffered clots or are at high risk.

This includes those with an irregular heartbeat, a replacementheart valve, a clotting disorder or patients recovering from surgery, when clots are more likely to form.

But Victoria Taylor, a dietician at the British Heart Foundation, cautioned that combining warfarin with supplements such as vitamin K could blunt its effects, leaving patients vulnerable to a fatal event.

She also urged patients to avoid omega-3 supplements and St John's wort, a popular herbal remedy, warning these can also interfere with blood-thinning drugs.

Instead, patients should speak to their doctor before adding any supplements to their diet, she said.

'Research shows that supplements do not help or improve heart and circulatory diseases,' Ms Taylor wrote in the charity's Heart Matters magazine. 'In some cases, they can even be harmful.'

She added: 'Some can also affect the medicines you are taking, causing side-effects.

'You may think supplements are healthier than food because they contain high levels of vitamins and minerals. But more is not always better – having too much of some [nutrients] can be bad for your health.'

The NHS issues similar guidance, warning patients not to combine herbal remedies with prescription drugs such as blood-thinners.

'Do not take St John's wort, the herbal remedy for depression, while you're taking warfarin,' its advice states.

'It can increase your risk of side-effects.'

Officials also caution that there is not enough evidence to say other herbal remedies or supplements are safe to take alongside warfarin, as they are not tested to the same strict standards as prescription medicines.

Ms Taylor reiterated that vitamin K and omega-3 can also interact with warfarin.

Health services advise anyone already taking vitamin K supplements to inform their doctor before starting the drug.

This is because vitamin K boosts the blood's clotting ability, which can make warfarin less effective.

Foods rich in the vitamin – including green leafy vegetables, egg yolks and olive oil – can also interfere with the drug. But rather than avoiding these foods altogether, the NHS advises patients to keep their intake steady.

By eating a consistent amount, the level of vitamin K in the blood stays balanced, making it more likely that INR readings – the measure of how long it takes blood to clot – remain stable.

Ms Taylor also warned against taking omega-3 supplements while on warfarin. Like the drug itself, omega-3 can thin the blood, potentially increasing the risk of bleeding.

According to the National Institute for Health and Care Excellence (NICE), using the two together 'might increase the risk of developing this effect'.

For the millions of Britons keen to improve their health, Ms Taylor advised caution: speak to a GP before spending money on supplements in what has become a multi-billion-pound industry.

She said: 'Talk to your doctor before spending money on supplements. If you need them they will prescribe or recommend them.'

The only exception to the rule is Vitamin D.

'It is recommended we all take 10 micrograms of vitamin D a day in autumn and winter, as the vitamin is mainly produced in the body with the help of sunlight,' Ms Taylor added.

'Only a small amount comes from food. Apart from this, a healthy, varied, balanced diet provides all the energy, vitamins and minerals you need.'

While warfarin has huge benefits for people at risk of blood clots – and in many cases can be life-saving – the downside is that it increases the risk of bleeding.

This can include bleeding for longer than usual after a minor cut, occasional nosebleeds that stop within 10 minutes, bleeding gums, or bruises that appear easily and take longer to fade.

According to NHS guidance, this type of bleeding is not dangerous and usually stops on its own. If it does not, patients are advised to contact their GP.

In rare cases, warfarin can trigger serious bleeding, sometimes without any obvious cut or injury. This can be life-threatening and requires urgent medical attention.

Warning signs include a nosebleed lasting more than 10 minutes, vomiting blood, or coughing up blood.

Very rarely, warfarin may cause bleeding in the brain. Symptoms include slurred speech, a sudden and severe headache, difficulty breathing or any heavy bleeding where blood is gushing or pooling.

In such instances, patients are advised to call 999 immediately.