Which herbal supplements most commonly interact with anticoagulant medications like warfarin or DOACs?
Executive summary
Herbal and dietary supplements commonly reported to interact with warfarin include garlic, ginkgo, ginseng, green tea, St. John’s wort, cranberry, chamomile and others; a systematic review found 78 herbs/foods/supplements reported to interact with warfarin, with 45 (57.7%) potentiating anticoagulation and 23 (29.5%) inhibiting it [1]. DOACs have fewer proven herb–drug interactions, but case reports and pharmacology warn that P‑glycoprotein and CYP3A4 modulators (for example St. John’s wort) and platelet‑affecting herbs (for example ginger, garlic) can alter DOAC safety [2] [3] [4].
1. The headline list: which herbs show up most often in reports
Clinical reviews and systematic analyses repeatedly name garlic, ginkgo biloba, ginseng, green tea, St. John’s wort, cranberry, chamomile, danshen and certain Chinese herbal products among the herbs most often implicated with warfarin or oral anticoagulant problems [1] [5] [6]. The 2020 systematic review identified 78 distinct herbs/foods/supplements reported to interact with warfarin and documented both potentiation (more bleeding) and inhibition (less anticoagulation) in many cases [1].
2. Two different mechanisms: pharmacodynamic vs pharmacokinetic
Herb–anticoagulant risks arise by two basic mechanisms. Pharmacodynamic effects include added antiplatelet or anticoagulant activity (garlic, ginger, ginkgo, fish oil), which can increase bleeding risk when combined with warfarin or DOACs [7] [8]. Pharmacokinetic effects come from herbs that induce or inhibit metabolizing enzymes or transporters—most importantly CYP450 enzymes and P‑glycoprotein—altering drug levels (St. John’s wort is a classic inducer; many herbs can affect CYPs relevant to warfarin and to DOACs) [7] [9] [10].
3. Warfarin is the most vulnerable, but DOACs are not risk‑free
Warfarin’s narrow therapeutic index and dependence on CYP metabolism make it highly susceptible: reviews and case reports show many herb interactions and even fatal bleeding linked to herbal co‑use [1] [5]. DOACs were believed to have fewer food/herb interactions, but modern reviews caution that DOACs can be affected by strong CYP3A4 or P‑gp modulators and by herbs that alter bleeding risk—case reports include fatal bleeding after combining herbal products (ginger and cinnamon) with dabigatran [2] [4] [3].
4. Which specific herbs reduce anticoagulant effect (risking clotting)?
Herbs and supplements reported to reduce warfarin effect (lower INR) include products containing vitamin K, St. John’s wort (induces metabolism), green tea and possibly coenzyme Q10 and certain ginseng preparations; reviews flag these as agents that can decrease INR or DOAC exposure theoretically through enzyme induction or vitamin K content [6] [1] [9].
5. Which herbs increase anticoagulant effect (bleeding risk)?
Garlic, ginger, ginkgo, danshen, dong quai, papain (papaya enzyme), and some Chinese herbal mixtures have been associated with increased bleeding or higher warfarin activity in case reports and reviews [6] [5] [11]. Systematic evidence varies in quality; many signals come from case reports, small trials or in vitro data [1] [5].
6. Evidence strength: many signals, few definitive trials
Multiple high‑quality reviews repeatedly note that for most herb–anticoagulant pairs the supporting data are limited, often case reports or small, heterogeneous studies; systematic reviewers warn that content variability in herbal products and small sample sizes prevent firm causal conclusions for many interactions [1] [2]. Authoritative sources (NHS, hospital anticoagulation centers) therefore advise caution and clinician notification because safety cannot be guaranteed [12] [13].
7. Practical guidance: what clinicians and patients should do
Experts recommend that patients on warfarin or DOACs always tell clinicians about any supplements and avoid unsupervised addition of herbs known to affect platelets, CYP enzymes or P‑gp (examples: garlic, ginger, ginkgo, St. John’s wort, green tea extracts) because of documented reports and plausible mechanisms [7] [10] [12]. For warfarin the evidence base is larger and regular INR monitoring should accompany any change in diet or supplement use [1] [14].
8. Competing viewpoints and hidden limits in the literature
Some authoritative reviews emphasize many plausible interactions but conclude there is little direct, high‑quality evidence for DOACs specifically; others point to case reports of severe outcomes, creating a tension between theoretical/mechanistic concern and limited clinical trial evidence [15] [4] [2]. Systematic reviewers explicitly cite heterogeneity of products and low study quality as major limitations [1] [2].
Limitations: available sources do not provide a single universally accepted ranked list of "most common" herb–DOAC interactions; many conclusions rely on warfarin data and case reports rather than randomized trials [1] [2].