Based on WHO | Is it true that I should avoid eating spinach while taking warfarin because the vitamin K can reduce the effectiveness of the medication?

You do not have to completely avoid spinach while taking warfarin, but you should keep your vitamin K intake consistent from day to day because large or sudden increases in vitamin K (found in leafy greens like spinach) can lower warfarin’s effect and change your INR. ^{[1] [2]}

Why vitamin K matters on warfarin

Warfarin works by blocking the action of vitamin K, which your body uses to make clotting factors; when you suddenly eat much more vitamin K, warfarin’s blood‑thinning effect can be reduced. ^{[1] [3]}
Because of this interaction, official guidance advises a normal, balanced diet with a consistent amount of vitamin K rather than cutting it out entirely. ^{[1] [4]}

What “consistent” looks like

Health instructions for warfarin specifically recommend avoiding drastic diet changes and not eating large amounts of green leafy vegetables all at once, since these are high in vitamin K. ^{[5] [6]}
Keeping a steady pattern similar portions of greens each day or each week helps maintain a stable INR and reduces the need for dose changes. ^{[7] [8]}

Common high–vitamin K foods

Many green vegetables are vitamin K–rich, including spinach, kale, collard greens, Swiss chard, Brussels sprouts, broccoli, and certain lettuces. ^{[9] [10]}
Some vegetable oils (such as soybean and canola) also contain substantial vitamin K, so large, sudden increases of these oils may affect your INR. ^{[2] [11]}

What the research shows

Clinical studies have shown that single servings of spinach or broccoli are unlikely to push anticoagulation outside the therapeutic range, but repeated daily large portions over a week can raise clotting activity and may require warfarin dose adjustments. ^{[12] [13]}
Dietary counseling that targets consistent vitamin K intake has been shown to improve the percentage of people who achieve their INR target over time compared with usual care. ^{[14] [15]}

Practical eating tips

• Aim for consistency: if you enjoy spinach, you can keep eating it, but try to have similar amounts regularly rather than big swings in portion size. ^{[1] [7]}
• Avoid “loading” on greens: don’t suddenly add large salads, smoothies, or green juices on top of your usual intake without discussing it, as this can lower warfarin’s effect. ^{[5] [4]}
• Coordinate with monitoring: if you plan a dietary change (e.g., starting a new diet rich in greens), let your healthcare team know so INR can be checked and warfarin adjusted if needed. ^{[11] [16]}

Other beverages and supplements to note

Certain drinks and supplements can interact with warfarin; large or frequent changes should be avoided or discussed with your clinician. ^{[2] [17]}
Alcohol and some herbal products can affect bleeding risk, so use caution and keep your care team informed about any changes. ^{[18] [1]}

Bottom line

You do not need to cut out spinach, but you should keep your intake of vitamin K–rich foods consistent to help your warfarin work reliably and keep your INR in range. ^{[1] [9]}