Experts warn common supplements can alter warfarin’s effects and raise risk of harm

Millions of people prescribed the anticoagulant warfarin may be at increased risk of dangerous side effects if they take certain over‑the‑counter supplements alongside their medication, health experts warn.

Victoria Taylor, a dietician at the British Heart Foundation, said combining warfarin with supplements that affect blood clotting can either blunt the drug’s protective effect or increase bleeding risk. She highlighted vitamin K, omega‑3 oils and the herbal remedy St John’s wort as products that can interact with warfarin and urged patients to speak to their doctor before adding supplements to their routine.

Warfarin, prescribed to more than a million patients in the U.K., reduces the likelihood of life‑threatening blood clots that can cause strokes. Patients taking warfarin normally undergo regular blood tests to measure the international normalized ratio, or INR, which indicates how long it takes their blood to clot. NHS guidance says those tests should happen at least every 12 weeks while on the drug to ensure the dose remains correct.

Vitamin K plays a direct role in blood clotting and can counteract warfarin’s effect, potentially leaving patients more susceptible to clots. Foods high in vitamin K include green leafy vegetables, egg yolks and some oils. Health services advise patients not to eliminate such foods but to keep their intake consistent so INR readings remain more stable. People already taking vitamin K supplements are advised to inform their doctor before starting warfarin.

Omega‑3 fatty acids, commonly taken as fish oil capsules, have properties that can also thin the blood. The National Institute for Health and Care Excellence has warned that combining omega‑3 supplements with anticoagulant drugs "might increase the risk" of bleeding. St John’s wort, marketed for low‑grade depression, is also singled out by NHS guidance because it can interact with a range of prescription medicines and increase the risk of side effects.

Health experts note that many supplements are not subject to the same regulatory testing as prescription medicines, leaving uncertainties about safety and interactions. "Research shows that supplements do not help or improve heart and circulatory diseases," Taylor wrote in the British Heart Foundation’s Heart Matters magazine. "In some cases, they can even be harmful."

The NHS makes a specific exception for vitamin D, recommending that people take 10 micrograms daily in autumn and winter because the body produces less of it in months with limited sunlight. Aside from that recommendation, health services say a varied, balanced diet should supply most nutrients for the majority of people.

While warfarin can be life‑saving for patients at risk of clots, it also raises the risk of bleeding. Common, usually nonurgent signs include cuts that bleed longer than normal, brief nosebleeds and easily formed bruises. More serious bleeding may occur without an obvious injury and requires immediate medical attention. Warning signs of severe bleeding include a nosebleed lasting more than 10 minutes, vomiting or coughing up blood, sudden severe headache, slurred speech or any heavy or uncontrolled bleeding.

Health organisations advise patients not to start or stop supplements without medical advice and to tell their GP or anticoagulation clinic about any over‑the‑counter remedies they are taking. Doctors can advise on safe choices, monitor INR more closely if needed, or prescribe alternatives. Given the size of the vitamins and supplements market, clinicians say clear communication is essential to avoid unintended, potentially fatal interactions.