Warfarin Interactions

Danshen (Salvia miltiorrhiza), widely used in traditional Chinese medicine for cardiovascular indications, has both pharmacokinetic (decreased clearance of R- and S-warfarin) and pharmacodynamic (antiplatelet, antithrombotic) interactions with warfarin. Multiple published case reports describe massive over-anticoagulation with INRs above 8 and serious bleeds including haemothorax.

Alcohol affects warfarin in two opposing ways: acute heavy drinking inhibits hepatic CYP2C9 metabolism of warfarin, raising INR and bleeding risk, while chronic heavy drinking induces enzymes that lower INR and increase clot risk. Alcohol also damages the liver and platelets, compounding bleeding hazards.

Vitamin K directly antagonizes the anticoagulant effect of warfarin by serving as a cofactor for the gamma-carboxylation of clotting factors II, VII, IX, and X, thereby reducing warfarin's efficacy.

Ginkgo biloba inhibits platelet-activating factor and can prolong bleeding time, adding an antiplatelet effect on top of warfarin's vitamin-K-antagonist anticoagulation. A 2025 PLOS One analysis of 2,647 prescriptions found ginkgo co-prescription was associated with a significantly higher rate of bleeding adverse events (hazard ratio ~1.38) and abnormal coagulation profiles.

Dong quai (Angelica sinensis) contains coumarin derivatives (ferulic acid, osthole) and has documented antiplatelet activity. A widely cited case report (Page & Lawrence, Pharmacotherapy 1999, PMID 10417036) described a woman whose INR rose to 4.9 within four weeks of adding dong quai 565 mg once to twice daily to stable warfarin.

Curcumin, the main active in turmeric, has antiplatelet activity and may also inhibit CYP2C9 metabolism of warfarin, raising warfarin levels. New Zealand Medsafe issued an alert in 2018 after a patient's INR rose above 10 within weeks of starting a turmeric/curcumin product on previously stable warfarin therapy.

Fluconazole inhibits CYP2C9 and CYP3A4, the enzymes that clear warfarin, and can rapidly raise INR by 50 to 100 percent or more within two to three days of starting, with documented cases of major bleeding and death.

Cranberry juice contains flavonoids that may inhibit CYP2C9, the primary enzyme that metabolizes the active S-enantiomer of warfarin. Multiple case reports describe elevated INR and major bleeding (including fatal hemorrhage) in patients who drank cranberry juice while stably anticoagulated, though randomized trials with smaller doses have not consistently reproduced the effect.

Spinach is one of the highest dietary sources of vitamin K1 (phylloquinone), with roughly 145 mcg per cup raw and 889 mcg per cup cooked. Because warfarin works by blocking vitamin K-dependent clotting factor synthesis, large or fluctuating spinach intake antagonizes warfarin and can drop the INR into a sub-therapeutic range, raising clot risk.

Collard greens are one of the highest-vitamin-K vegetables available, with roughly 836 mcg of vitamin K1 per cup cooked - more than seven times the daily adequate intake for adults. Sudden increases or decreases in consumption directly antagonize warfarin and can push the INR out of its therapeutic range.

Swiss chard delivers approximately 299 mcg of vitamin K1 per cup raw and over 570 mcg per cup cooked - several times the adult adequate intake. As a direct vitamin K antagonist, warfarin's effect is reduced when dietary phylloquinone rises, so sudden increases in swiss chard intake can lower the INR into a sub-therapeutic, clot-prone range.

Turnip greens contain about 529 mcg of vitamin K1 per cup cooked - roughly five times the daily adequate intake. Because warfarin works by blocking vitamin K-dependent clotting factor synthesis, large or fluctuating servings can drop the INR into a sub-therapeutic range and raise clot risk.

Mustard greens are a dark leafy green packed with vitamin K1, providing roughly 419 mcg per cup cooked - several times the adult daily adequate intake. Because warfarin works by blocking vitamin K-dependent clotting factor synthesis, large or fluctuating intake can lower the INR and reduce clot protection.

Cassia cinnamon contains coumarin, a natural compound with anticoagulant activity that may potentiate warfarin and increase bleeding risk. Case reports describe elevated INR and bleeding when cinnamon supplements were added to stable warfarin therapy.

Curcumin, the principal active compound in turmeric, has antiplatelet and anticoagulant activity in vitro (inhibiting thrombin and factor Xa) and inhibits CYP enzymes involved in warfarin metabolism. Case reports describe INR rising above 10 within weeks of starting turmeric in patients on stable warfarin.

CBD inhibits CYP2C9 (and CYP3A4), the enzymes responsible for metabolizing the more potent S-enantiomer of warfarin. Co-use raises plasma warfarin concentrations, elevates INR, and increases bleeding risk; a published case report required a roughly 30% warfarin dose reduction after the patient started CBD.

Chamomile contains coumarin-like compounds that may potentiate warfarin's anticoagulant effect. A published case report described a 70-year-old woman on stable warfarin who developed retroperitoneal hemorrhage with an INR of 7.9 after using chamomile tea and lotion for upper respiratory symptoms.

Concentrated garlic supplements have antiplatelet activity (mainly via ajoene and allicin) and have been associated with elevated INR and bleeding when added to warfarin in case reports and herbal-interaction reviews. The Vaes & Chyka review in Annals of Pharmacotherapy classified garlic as a potential potentiator of warfarin via additive antithrombotic effect.

Fish oil may increase bleeding risk when combined with warfarin

Fresh parsley is extraordinarily dense in vitamin K1 - about 1,640 mcg per 100 grams, or roughly 62 mcg per tablespoon - so although typical garnish-sized servings are small, large culinary uses (tabbouleh, chimichurri, parsley smoothies, juicing) can deliver enough vitamin K to oppose warfarin and lower the INR.

Feverfew (Tanacetum parthenium) inhibits platelet aggregation in vitro via its parthenolide sesquiterpene lactones. There are no robust human case reports of bleeding with warfarin specifically, but standard herbal-interaction references (StatPearls, Australian Prescriber) recommend avoidance based on the pharmacologic plausibility of additive bleeding risk.

Ginger inhibits thromboxane synthase and reduces platelet aggregation; case reports describe elevated INR after addition of oral ginger to stable warfarin therapy. A 2019 case report (Rubin et al., Case Reports in Medicine) and the Tan 2021 BJCP systematic review document the signal, though controlled trials in healthy volunteers have been mixed.

Noni juice (Morinda citrifolia) products vary substantially in vitamin K content - one published case of warfarin resistance was attributed to a high-vitamin K noni preparation. Noni has also been linked to drug-induced liver injury and may induce CYP2C9, both of which can destabilize warfarin in unpredictable directions.

Matcha is powdered whole green tea leaf, so each serving delivers far more vitamin K than a normal brewed cup. Vitamin K is the cofactor warfarin antagonises, so large or fluctuating matcha intake can lower INR and reduce the anticoagulant effect, similar to the documented green tea-warfarin case report.

Green tea leaves contain vitamin K, which is a cofactor for hepatic synthesis of the clotting factors that warfarin inhibits. Large or fluctuating intake of green tea can lower INR and reduce the anticoagulant effect of warfarin, as documented in a published case report.

Broccoli is rich in vitamin K1 (phylloquinone), the very cofactor warfarin blocks to suppress clotting factor synthesis. Large or wildly inconsistent intake can lower the INR and reduce warfarin's anticoagulant effect, raising clot risk.

Romaine lettuce contains roughly 48 to 60 mcg of vitamin K1 per cup shredded - lower than dark cooking greens but still meaningful at multi-cup salad servings. Large daily salads or romaine-heavy juices can supply enough phylloquinone to oppose warfarin and drift the INR downward.

Microgreens - the immature seedlings of vegetables and herbs harvested 7 to 21 days after germination - concentrate nutrients on a per-gram basis. Phylloquinone content varies widely by species, from roughly 0.6 to 4.1 mcg per gram, with brassica and amaranth microgreens highest. Daily handfuls or smoothie additions can supply enough vitamin K to oppose warfarin and shift the INR.

Fenugreek (Trigonella foenum-graecum) contains coumarin-related compounds and has documented in vitro anticoagulant activity. A published case report describes elevated INR when boldo-fenugreek was added to stable warfarin therapy, with INR normalizing on discontinuation and rising again on rechallenge.

Psyllium is a soluble fiber that forms a viscous gel in the gut, which can trap warfarin and slow or reduce its absorption when taken at the same time. Significant changes in fiber intake may also alter gut flora vitamin K production, indirectly destabilizing INR.

Chia seeds are high in alpha-linolenic acid (an omega-3 fatty acid) and soluble fiber. Omega-3 intake at high doses may mildly inhibit platelet aggregation and could theoretically enhance warfarin's anticoagulant effect, while the soluble fiber may also blunt warfarin absorption when taken at the same time.

Flax seeds are high in alpha-linolenic acid (an omega-3 fatty acid) that can mildly inhibit platelet aggregation, and their soluble fiber and lignans can slow warfarin absorption when taken at the same time. Combined, these effects can shift INR in either direction, with case-level reports of altered anticoagulation.

Kimchi is fermented Napa cabbage and contains roughly 65 micrograms of vitamin K1 per cup, which provides the cofactor for the very clotting factors warfarin blocks. Inconsistent kimchi intake can lower the INR and reduce warfarin's anticoagulant effect, increasing clot risk.

Sauerkraut is fermented green cabbage and contains roughly 18 to 70 micrograms of vitamin K1 per cup, depending on whether the brine is consumed and how the cabbage was prepared. Vitamin K1 supplies the cofactor for the very clotting factors warfarin blocks, so changing sauerkraut intake can shift INR and weaken anticoagulation control.

Flaxseed contains alpha-linolenic acid that can mildly reduce platelet aggregation and may add to warfarin's bleeding risk, particularly at high supplemental doses. Flaxseed is also very high in soluble fiber, which can bind warfarin in the gut and erratically reduce its absorption.

Cocoa flavanols inhibit platelet adhesion and aggregation and can mildly prolong bleeding time. Layered on top of warfarin's anticoagulant effect, large or fluctuating intakes of cocoa can increase bleeding risk without necessarily moving the INR.

Pomegranate juice contains punicalagins and other polyphenols that inhibit CYP2C9 and CYP3A4 in vitro, which would slow warfarin metabolism. Case reports describe both elevated INR (one patient reached INR 14 after heavy consumption) and subtherapeutic INR after stopping habitual juice intake.

Acai berries contain polyphenols, salicylate-like compounds, and unsaturated fatty acids that may have mild antiplatelet activity, but there are no published case reports of clinically significant INR changes. The theoretical concern is additive bleeding risk at high doses or with concentrated extracts, not enzymatic CYP interference.

Capsaicin, the active constituent in cayenne (Capsicum), has been reported to potentiate the anticoagulant effect of warfarin, theoretically through additive effects on platelet aggregation and possible enhancement of warfarin absorption or activity, increasing bleeding risk.

Walnuts contain only trace amounts of vitamin K (about 2.7 mcg per 100 g) and small quantities of plant-based omega-3 (alpha-linolenic acid). Typical servings are unlikely to meaningfully shift INR, but dramatic changes in intake or very large daily portions could theoretically nudge anticoagulation in either direction.

Brazil nuts contain no detectable vitamin K (phylloquinone) per USDA analysis, so they do not antagonize warfarin's mechanism. Their main concern is the very high selenium content, which has no established direct interaction with warfarin metabolism.

Culinary hemp seeds (hulled or whole) contain only trace levels of CBD and very low vitamin K, so they are unlikely to meaningfully shift warfarin's effect. Hemp seed oil products vary widely, and a minority of products have been measured to contain detectable CBD that could theoretically inhibit warfarin metabolism via CYP2C9 at high enough doses.

Pumpkin seeds (pepitas) contain a moderate amount of vitamin K, with about 54 mcg per half-cup of dried roasted seeds. Large or fluctuating intakes can shift INR through the standard vitamin K mechanism, while typical snack portions are unlikely to cause problems.

Sesame seeds contain lignans (sesamin, sesamolin) that have been shown in vitro to inhibit CYP-mediated 7-hydroxylation of warfarin, which could theoretically slow warfarin clearance. Vitamin K content is low and the clinical relevance of the CYP effect at culinary intake levels has not been demonstrated.