Which supplements are best avoided in combination with blood thinners?

Vitamin K is the most clear-cut interaction: because warfarin works by blocking vitamin K, a vitamin K supplement can directly and significantly reduce the blood-thinning effect. Fluctuating intake makes dosing harder to regulate. This is a well-understood, causal relationship.

St. John's wort falls into a different category from most herbs: it speeds up the breakdown of blood thinners in the liver. That applies to warfarin, but also to newer agents such as dabigatran and rivaroxaban. Blood levels therefore fall, and the risk of thrombosis increases. This is the best-documented herbal risk in people who use blood thinners.

Ginkgo biloba has been on the radar for some time because of case reports of bleeding complications. These sometimes occurred even without a blood thinner being involved; the combination is therefore considered especially risky. Similar case reports exist for dong quai (an herb with oestrogen-like activity that appears to amplify the effect of warfarin) and for horse chestnut extract. Ginseng has reports pointing in both directions: it sometimes appears to strengthen the anticoagulant effect, and sometimes to weaken it.

Vitamin E in supplement form is also flagged by experts as a point of attention when using blood thinners, although the evidence for this is less well developed. Cranberry juice appeared suspicious on the basis of case reports, but formal studies found no effect; its clinical significance is therefore unclear.

What most supplements have in common is that the evidence comes largely from case reports rather than controlled studies in humans. But that does not make the risk smaller, because case reports are precisely about people for whom things went wrong. An estimated one in three people who take blood thinners also uses supplements or herbal preparations. Always inform your pharmacist or doctor, even about 'natural' products.