coumarin
5 interactions related to coumarin
warfarin + dong quai
Dong quai (Angelica sinensis) contains coumarin-family compounds (ferulic acid, osthole) and has antiplatelet activity in laboratory studies. A published case report described a previously stable warfarin patient whose INR climbed well above her target range within weeks of adding dong quai, then returned to normal after she stopped it. The signal rests on a single human case plus animal data, so it is taken seriously but is not extensively documented.
metformin + cinnamon
Cinnamon has a mild glucose-lowering effect that can add modestly to metformin's. In pooled human trial data the effect on fasting glucose is small and there are no reports of serious low blood sugar from the combination, so the practical concern is minor for most people. The main extra consideration is choosing the lower-coumarin Ceylon variety for long-term daily supplement use.
cinnamon + warfarin
Concentrated cassia cinnamon supplements are a major source of coumarin, a compound that can be hard on the liver and may interfere with how the body clears warfarin. Because warfarin has a very narrow safety margin, even small shifts can raise bleeding risk, and case reports describe elevated INR when cinnamon-containing products were added to stable warfarin therapy.
fenugreek + warfarin
Fenugreek (Trigonella foenum-graecum) contains coumarin-related compounds and has shown anticoagulant activity in laboratory studies. A published case report describes a rise in INR when a boldo-fenugreek combination product was added to stable warfarin therapy, with INR normalizing on discontinuation and rising again on rechallenge. The evidence is limited to that single case plus mechanistic plausibility, so the interaction is treated as plausible rather than firmly established.
chamomile tea + warfarin
Chamomile contains naturally occurring coumarin-type compounds and may slow the liver enzymes that clear warfarin, so heavy or sudden chamomile use could add to warfarin's blood-thinning effect. A published case report linked frequent chamomile tea and lotion use to a dangerously high INR and severe internal bleeding in an older woman whose warfarin had previously been stable.