Warfarin is an anticoagulant drug you swallow to prevent and treat blood clots. You take it if you’re at risk of getting harmful blood clots in your body. Blood clots can keep blood from reaching your organs, which is how a heart attack or stroke happens. Your dose of warfarin will change based on results from frequent blood tests.
Warfarin is an anticoagulant medication you take to prevent and treat blood clots that can hurt you. You may know warfarin by its brand names, Coumadin® and Jantoven®. Warfarin slows down how quickly your blood clots. This helps keep unwanted clots from forming inside your blood vessels or heart when you have certain medical conditions or don’t walk or move around for a long time.
If you have a blood clot, warfarin keeps the clot from getting larger. It also may prevent a piece of the clot from breaking off and traveling to your lungs, brain or heart.
Warfarin helps prevent new clots from forming, but it doesn’t dissolve blood clots. With time, a clot may dissolve on its own. A clot that doesn’t dissolve gets in the way of blood flowing through a blood vessel.
Warfarin uses include the prevention and/or treatment of:
You may take warfarin if you’ve had the above conditions before or if you’re at risk now because:
About 2 million people in the United States take warfarin.
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Warfarin interferes with part of your body’s clot-making process. It stops an enzyme from activating vitamin K. Some of your body’s clotting factors need vitamin K to do their part in the process of making a clot.
The dose usually ranges from 1 mg (milligram) to 10 mg. Your healthcare provider will prescribe a specific dosage strength. But you may take a different dose based on the results of frequent blood tests.
You can buy warfarin as a tablet in many different shapes, sizes and colors. Each tablet color represents a different strength in milligrams. All manufacturers follow the same color code for the different strengths of their tablets, but the size and shape of the tablets may differ from one company to the next.
Each tablet has its strength stamped on one side. It also has an indent across the tablet so you can break it in half if your provider adjusts your dose. For example: if your provider prescribes a 5 mg tablet and then changes your dose to 2.5 mg, you should break one of the 5 mg tablets in half and take the half tablet. If you have any questions about your dose of warfarin, contact your provider.
A provider will need to check your blood with a prothrombin time (PT or protime) test. They can use these results to calculate the international normalized ratio (INR), which they use to standardize the results across different labs and testing centers. The PT and INR help a provider monitor how well warfarin is working to prevent blood clots. The results tell them if they need to adjust your dose.
You can get the blood tests at a lab or anticoagulation clinic, in a medical office or at home. You usually need to do the PT test one or more times a week to once a month if you’ve had stable results. Follow your provider’s instructions for how often you need to get blood tests and when to adjust your daily warfarin dose.
Other medications may increase or decrease warfarin’s anticoagulant effect. You might need to get blood tests more often when you stop, start or increase the dose of medications that are likely to affect the way warfarin works. You may need a different dose of warfarin, too. Talk to your healthcare provider who manages your warfarin if you have any changes in your other prescription medications. Even over-the-counter (OTC) medicines may interfere with warfarin.
Other information about your blood tests includes:
Other instructions for taking warfarin include:
You’ll need warfarin as long as you’re at risk of a blood clot that can harm you. Keep taking warfarin as long as your provider prescribes it. Never stop taking warfarin on your own without talking to your provider.
Benefits of warfarin include:
Bleeding is the most common side effect of warfarin. It can appear as any of several different symptoms. Call your provider if you notice any of the following signs of bleeding:
If you have any of these warfarin side effects, your provider may want to do a blood test, change your dose, stop the medication or give you medicine to stop the bleeding.
Warfarin starts to work in 24 to 72 hours. Your blood may clot less easily 36 to 72 hours after you start taking warfarin. While your provider is waiting for warfarin to kick in, they’ll sometimes give you a different anticoagulant, usually injectable, that works right away.
With the risk of bleeding, it’s a good idea to take care with devices or tools that can hurt you. It’s also important to take your daily dose without skipping a day.
Contact your provider if:
You should get emergency treatment if:
While taking warfarin, you should avoid:
You need vitamin K for normal blood clotting. But large changes in the amount of vitamin K in your diet can change the way warfarin works. If you eat foods high in vitamin K, it’s important to eat the same amount of these foods from week to week.
Tell your provider if you’re planning to:
Changing your eating habits may mean that you’ll be getting more or less vitamin K in the foods you eat. If you change your eating habits, your provider may want to check your blood more frequently to see how warfarin is working.
These vegetables have high amounts of vitamin K:
If you want more servings of vegetables in your daily meal plan, choose vegetables that aren’t high in vitamin K, like:
A note from Cleveland Clinic
Healthcare providers have prescribed warfarin for decades, but it’s a finicky medicine. Foods and other medicines are just two of the things that can affect how well it works from day to day. That’s why it’s so important to keep up with the blood tests that check how well it’s working. Since people bleed more easily while taking warfarin, it’s a good idea to use an electric razor and a toothbrush with soft bristles. Being careful with meal prep can keep you safer, too.
Last reviewed by a Cleveland Clinic medical professional on 06/09/2023.
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