Health care professionals are increasingly confronted with incorporating dietary supplements—many of which are prescribed by clinicians for a wide range of conditions—into their care plans. As a result, it’s important to keep in mind that interactions between warfarin and supplements can either limit or exacerbate warfarin’s effectiveness and lead to increased risk of clotting and bleeding.

Vitamin K directly antagonizes warfarin’s inhibitory effect on vitamin K epoxide reductase and coagulation factors II, VII, IX, and X. The twin coagulation pathways, fibrinolysis, prostacyclin synthesis, thrombomodulin, and protein C maintain the careful hemostasis balance.

Age, gender, body weight, liver and kidney function, genetic factors, adherence, and diet all complicate warfarin management. Supplements of greatest concern in patients on warfarin are soybean, St John’s wort, and danshen, according to recent study results published in Biomedicine and Pharmacotherapy.

The researchers assessed 58 warfarin-dietary supplement interactions by conducting a broad search of English- and Portuguese-language primary and secondary literature in online databases. They extracted information on name and chemical composition of the medicinal plant, concomitant use with warfarin, and any interaction between the natural product and warfarin.

The herbs most commonly cited as potentially interacting with warfarin were ginseng, garlic, ginkgo, and St. John’s wort. Most interaction mechanisms were unknown or unstated.

Saw palmetto and kava inhibit the COX enzyme, while hawthorn, kava, garlic, and onion inhibit thromboxane production. Pomegranate, danshen, St. John’s wort, and red clover inhibit warfarin metabolism through hepatic CYP enzymes. Meanwhile, green tea, juniper, passion flower, soybean, and vervain contain high amounts of vitamin K.

Onion, garlic, fenugreek, green tea, and basil inhibit platelet aggregation. Aloe and psyllium reduce warfarin absorption in the gut, and alfalfa increases warfarin’s renal clearance.

Consideration of dietary supplements’ interactions with warfarin can aid in drug dosing and efficacy prediction. Most interactions potentiate warfarin’s effects, but certain herbs reduce the efficacy of warfarin (primarily by vitamin K).