Green Tea and Warfarin Interaction: What You Need to Know

When you take warfarin, a blood thinner used to prevent dangerous clots. Also known as Coumadin, it works by blocking vitamin K, which your body needs to form clots. But what you drink — like green tea, a popular beverage rich in antioxidants and vitamin K — can quietly change how well it works. This isn’t just a myth. Real patients have seen their INR levels spike or drop after starting or stopping green tea, leading to serious bleeding or clotting risks.

Green tea contains vitamin K, the same nutrient warfarin tries to block. If you suddenly drink more green tea, your body gets more vitamin K, which can make warfarin less effective. On the flip side, if you stop drinking it, your vitamin K levels drop, and warfarin can become too strong. It’s not about the caffeine — it’s about the vitamin K content. Even one cup a day, if it’s new to your routine, can throw off your dose. Studies show that consistent intake matters more than occasional sips. If you’ve been drinking green tea for years, keep doing it. But if you’re thinking of adding it in, talk to your doctor first. Your INR test doesn’t care if it’s ‘healthy’ — it only cares about numbers.

Other things can interfere too. Some supplements, antibiotics, or even changes in your diet can mess with warfarin. But green tea is one of the most common ones people overlook because it’s seen as harmless. You might think, ‘It’s just tea,’ but for someone on warfarin, it’s a medication modifier. The same goes for other teas like black or oolong — they have vitamin K too, though usually less than green. And don’t assume herbal teas are safe. Some, like ginseng or chamomile, can also affect blood thinning. What you need is consistency. Keep your green tea intake steady, know your INR numbers, and never make changes without checking in with your provider.

Below, you’ll find real-world stories and science-backed advice on how green tea interacts with warfarin, what other foods and supplements to watch, and how to keep your treatment stable without giving up your daily cup.