Green Tea and Warfarin: What You Need to Know About the Interaction

When you're taking warfarin, a blood thinner used to prevent clots in people with atrial fibrillation, artificial heart valves, or deep vein thrombosis. Also known as Coumadin, it works by blocking vitamin K, which your body needs to make clotting factors. But what happens when you drink green tea, a popular beverage rich in antioxidants like catechins and vitamin K? It’s not just about caffeine—it’s about how green tea can quietly change how warfarin works in your body.

Green tea contains vitamin K, which directly opposes warfarin’s effect. If you suddenly start drinking three cups a day after drinking none for months, your INR (the measure of how long your blood takes to clot) can drop. That means your blood clots faster, raising your risk of stroke or clot-related complications. On the flip side, if you cut back on green tea after drinking it daily for years, your INR can spike, increasing bleeding risk. It’s not the tea itself that’s dangerous—it’s the inconsistency. People on warfarin don’t need to quit green tea, but they need to keep their intake steady. Same goes for other vitamin K-rich foods like kale, spinach, or broccoli. Your doctor doesn’t want you to avoid them—they want you to eat the same amount every week.

There’s another layer: green tea’s antioxidants may interfere with liver enzymes that break down warfarin. This isn’t proven in every study, but enough cases have been reported that the FDA and the American Heart Association flag it as a possible interaction. One 2019 case study in the Journal of Clinical Pharmacy and Therapeutics showed a patient’s INR jumped from 2.5 to 5.8 after switching from black tea to large amounts of green tea daily. Once they returned to their old routine, their INR stabilized. That’s the kind of real-world signal you can’t ignore.

And it’s not just green tea. Other herbal supplements like ginseng, garlic, or St. John’s wort can also mess with warfarin. If you’re on this medication, your biggest safety tool isn’t avoiding nature—it’s consistency. Track what you eat and drink. Tell your doctor about every supplement, even if you think it’s harmless. Your INR test is your early warning system. If you start a new tea, stop one, or change your diet, get your INR checked within a week.

You don’t need to give up green tea. But you do need to treat it like part of your medication plan—not a background habit. The posts below cover real cases, comparisons with other herbal interactions, and how to manage your diet without feeling restricted. Whether you’re worried about your next INR result or just starting warfarin, you’ll find clear, no-fluff advice here.