Green Tea and Warfarin: What You Need to Know About INR Changes

If you're taking warfarin, you've probably heard to avoid cranberry juice or alcohol. But what about green tea? It’s healthy, calming, and full of antioxidants-so why would it matter? The truth is, drinking too much green tea can throw off your INR and make your blood thinner or thicker than it should be. This isn’t just a theory. Real people have seen their INR drop from 3.8 to 1.4 after drinking a gallon of green tea a day. That’s dangerous. And it’s not rare.

How Warfarin Works (And Why Vitamin K Matters)

Warfarin, sold under names like Coumadin and Jantoven, keeps your blood from clotting too easily. It does this by blocking vitamin K, which your body needs to make clotting factors. Without enough vitamin K, your blood takes longer to clot. That’s good if you have atrial fibrillation, a mechanical heart valve, or a history of blood clots. But if you suddenly get a lot of vitamin K-say, from a big bowl of spinach or a huge pot of green tea-your body can start making clotting factors again. That makes warfarin less effective. Your INR drops. And now you’re at risk for a stroke or clot.

The goal? Keep your INR steady. For most people, that’s between 2.0 and 3.0. Going below 2.0 means your blood is clotting too fast. Above 3.5, you risk bleeding. Neither is safe.

Green Tea Has Vitamin K-But Not Much in a Cup

Here’s where things get confusing. Dried green tea leaves are packed with vitamin K-over 1,400 micrograms per 100 grams. But you don’t eat the leaves. You brew them. And when you do, almost all of that vitamin K stays in the leaf. The brewed tea? Only about 0.03 micrograms per 100 milliliters. That’s tiny.

So if you drink one or two cups a day, you’re probably fine. A 2022 statement from the American Heart Association says it’s safe to have up to three 8-ounce cups daily without changing your warfarin dose. Many people have been drinking green tea for years with stable INRs. One Reddit user, ‘TeaLover87,’ has been drinking two cups a day for five years and never had an issue.

But here’s the catch: green tea isn’t one thing. There’s a big difference between regular brewed green tea and matcha.

Matcha Is a Different Story

Matcha isn’t brewed tea. It’s powdered green tea leaves. You’re swallowing the whole leaf. That means you’re getting all the vitamin K-up to 10 to 20 times more than in regular green tea. A 2023 study in the American Journal of Hematology found that matcha users were far more likely to see INR drops.

One patient on the Warfarin Support Group subreddit reported drinking four matcha lattes a day. His INR fell from 2.8 to 1.9 in two weeks. His doctor had to raise his warfarin dose by 15%. That’s not an outlier. Doctors at Mayo Clinic say 15% of matcha users on warfarin need dose adjustments.

If you’re on warfarin and love matcha, you can’t just assume it’s the same as green tea. It’s not. Treat it like a vitamin K bomb.

It’s Not Just Vitamin K-There’s More Going On

Green tea doesn’t just have vitamin K. It also has catechins-powerful antioxidants that can actually thin your blood by stopping platelets from sticking together. That’s the opposite of vitamin K. So why do some people still have INR drops?

The answer: vitamin K wins. In most cases, the vitamin K effect is stronger and faster than the catechin effect. But in rare cases, especially with very high intake or other medications, the two effects might fight each other. That’s why some people see weird swings in their INR-not just drops, but spikes too.

And then there’s caffeine. It doesn’t directly affect warfarin, but it can speed up how fast your body breaks down the drug. That’s another reason your INR might dip if you’re drinking five cups of strong green tea a day.

What Counts as Too Much?

There’s no magic number, but here’s what doctors actually use in practice:

• 1-3 cups (240-720 mL) per day: Safe for most people. No changes needed.
• 4-5 cups (720-1,200 mL) per day: Watch your INR. Check every two weeks.
• More than 1,000 mL (over 4 cups) per day: High risk. Talk to your doctor. You may need a dose increase.
• Matcha: Treat any daily intake over 1 teaspoon (about 2 grams) as high-risk. Even one cup can be enough to cause trouble.

Don’t be fooled by the idea that ‘natural’ means ‘safe.’ A 2023 survey found that 62% of warfarin users didn’t know green tea could affect their INR until they had a problem. That’s scary.

What About Other Teas?

Black tea? Similar to green tea. It has low vitamin K in brewed form. A 67-year-old woman in California stopped drinking black tea and her INR jumped from 1.7 to 5.0 in a week. That’s because her body was suddenly deprived of the vitamin K it had been getting daily. The same rule applies: consistency matters more than avoidance.

Herbal teas? Be careful. Ginkgo, ginger, and goji berry tea have been linked to bleeding risks on warfarin-not because of vitamin K, but because they interfere with platelets. Don’t assume herbal = harmless.

What Should You Do?

Here’s the simple, practical plan:

1. If you don’t drink green tea, don’t start.
2. If you already drink it, keep the amount the same. Don’t go from zero to five cups a day. Don’t go from five to zero. Consistency is your best defense.
3. Avoid matcha unless your doctor says it’s okay-and even then, track your intake.
4. Log your tea consumption. Use an app like WarfarinWise, or just write it down. You’ll see patterns.
5. If you drink more than 1,000 mL (about 4 cups) daily, ask your anticoagulation clinic to check your INR every two weeks instead of monthly.
6. If you suddenly stop drinking green tea or switch to matcha, get your INR checked within 72 hours.

Don’t quit green tea because you’re scared. Quit because you’re confused. You don’t need to give it up. You just need to be smart about it.

Why This Matters More Than You Think

Every year, about 18.7% of warfarin-related hospitalizations are tied to diet. That’s over 7,000 people in the U.S. alone. Green tea isn’t the biggest culprit-spinach, kale, and broccoli are higher in vitamin K. But because people think green tea is ‘safe,’ they don’t track it. And that’s where the problem happens.

It’s not about fear. It’s about control. You’re not powerless. You can drink green tea. You just need to know how much, how often, and when to check in with your doctor.

The FDA and European Medicines Agency now require warning labels on green tea products sold in large quantities. That’s because they know this isn’t just a myth. It’s a real, documented, preventable risk.

Bottom Line

You don’t have to give up green tea. But you do need to be consistent. Stick to 1-3 cups of regular brewed tea a day. Avoid matcha unless you’re under close supervision. Log your intake. Get your INR checked regularly. And if you change your tea habits-even a little-talk to your doctor before you assume everything’s fine.

Warfarin isn’t about strict rules. It’s about routine. Your body likes predictability. So does your INR.

If you’re unsure about your tea habits and your warfarin dose, talk to your anticoagulation clinic. A quick INR check and a short conversation can prevent a hospital visit. Your health isn’t about giving up what you love-it’s about managing it wisely.