Which supplements are best avoided when taking blood thinners?
Several commonly used supplements, particularly ginkgo biloba, cranberry, and St. John's wort, can dangerously amplify or reduce the effect of blood thinners. If you take warfarin, discuss every supplement with your doctor or pharmacist beforehand.
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Three supplements have the strongest evidence for a dangerous interaction with warfarin and similar blood thinners. Ginkgo biloba has been linked to serious bleeding complications, including bleeding in or around the brain. Cranberry amplifies the blood-thinning effect of warfarin, with serious cases reported in the literature. St. John's wort works differently: it actually increases the risk of a blood clot, because it activates an enzyme that breaks down warfarin more quickly, leaving too little active substance in the body.
Cases in which the blood-thinning effect was enhanced have also been reported for ginger, green tea (in large amounts), goji berries, chamomile, and high doses of garlic supplements. These are isolated case reports rather than large controlled studies, but combining any of these with warfarin calls for caution.
Milk thistle (also known as mariadistel in Dutch) is described in one case report in which a person who was stably maintained on warfarin developed dangerously high blood thinning after adding a liver-cleansing supplement containing milk thistle. After stopping it, the value returned to normal. Milk thistle is thought to interfere with the same breakdown enzyme as St. John's wort.
Ginseng presents a mixed picture: some cases show less blood thinning, others show more. There is insufficient evidence for a clear recommendation, but unpredictable fluctuations in the degree of blood thinning are themselves dangerous. Glucosamine warrants caution but has no hard evidence of a genuine interaction.
If you are taking a blood thinner such as warfarin, always discuss any new supplement with your doctor or pharmacist first. Ginkgo biloba, cranberry, and St. John's wort in particular are best avoided. The others have not been proven safe, but are also not as strongly supported as risky, unless you are taking large amounts.
The evidence consists largely of case reports and small clinical observations, not large randomised studies. Strength of evidence is 'moderate' for ginkgo, cranberry, and St. John's wort; 'limited' for the remaining supplements. No evidence outside of warfarin has been examined; other blood thinners may have similar interactions but have not been directly studied here.