Supplements and Blood Thinners: Warfarin, Aspirin and Newer Anticoagulants

Bleeding warning signs. The NHS lists blood in urine or stool, unexplained bruising or nosebleeds, coughing up or vomiting blood, and a sudden bad headache with confusion, slurred speech or difficulty moving the arms or legs (possible bleeding in the brain) as symptoms of serious bleeding with warfarin. [1] Get emergency care (911 in the US) for these.

The question

Can an herb, vitamin or other supplement change how a blood thinner works, or add to the bleeding risk? For some, sources say yes. For many, the evidence is thin, mixed or old. This page sorts what we read. It does not tell anyone which supplement to use or avoid. Anyone taking a blood thinner should ask the prescriber or pharmacist before adding anything.

Terms

• Warfarin is a prescription blood thinner (anticoagulant). The NHS says people on it need regular blood tests called INR tests to check how quickly the blood clots, and that the dose may change based on the results. [1] Reading those results is the clinic's job.
• Aspirin can also thin the blood, according to the FDA. [2] Apixaban and rivaroxaban are newer prescription anticoagulants. [3][4]
• Narrow range. NCCIH lists warfarin among medicines with a "narrow therapeutic range," where a little too much or too little can cause serious reactions. [5] See Narrow-range medicines and supplements.

What sources document, by supplement

Labels are explained in How interactions work.

Older case reports and reviews

A 2005 review of 43 case reports and 8 trials found warfarin the cardiovascular drug most often involved in herb interactions. It reported over-anticoagulation with garlic, ginkgo, danshen, dong quai, fenugreek and others, and a weaker anticoagulant effect with ginseng, green tea and St. John's wort. [15] These are reported interactions from a 2005 search, not proof of cause, and the review concluded that patients taking anticoagulants are at the highest risk. [15]

A 2022 review of bleeding and supplements found that in people on anticoagulants, ginkgo, chondroitin-glucosamine, melatonin, turmeric, bilberry, chamomile, fenugreek, milk thistle and peppermint were associated with bleeding risk, with no evidence of bleeding for these without an anticoagulant. [14] It rated the evidence for St. John's wort, ginger, ginkgo and cranberry as conflicting. [14]

When official advice and trials differ

The NHS says not to have cranberry juice or cranberry products while taking warfarin. [1] The two reviews above call the evidence on cranberry conflicting or unclear. [8][14] When a leaflet is more cautious than the trials, follow the leaflet's instruction to ask your clinic.

Foods and drink

The NHS says large amounts of alcohol, cranberry and grapefruit juice should be avoided on warfarin, and that foods high in vitamin K such as broccoli, spinach and green leafy vegetables can affect it; you can still eat them, but ask your warfarin clinic. [1] MedlinePlus advises eating consistent amounts of vitamin K foods from week to week. [11] See Food and drink interactions with medicines.

Limits

Most trials with warfarin were small and of low quality. [8] Absence of an effect in one small trial is not proof of safety. Quality problems in products are a separate risk; see Quality testing and certifications.

Before surgery or dental work

Tell the surgical, anesthesia and dental teams about every supplement and blood thinner. NCCIH says some supplements may increase the risk of bleeding. [5] See Supplements before surgery. People with a bleeding condition should also see Bleeding disorders and supplements.