Key takeaways:
Warfarin (Coumadin, Jantoven) is an oral medication used to prevent and treat blood clots. A large number of medications, supplements, and foods interact with warfarin.
Some common warfarin interactions include certain antibiotics, nonsteroidal anti-inflammatory drugs, and antifungals. Supplements such as St. John’s wort, ginkgo biloba, and ginseng can also cause issues. Alcohol and foods rich in vitamin K interact with warfarin, too.
To keep you safe, your healthcare team will regularly measure your blood’s clotting ability and adjust your warfarin dose if needed. Always check with your prescriber before making any changes to your diet or any medications or supplements you take.
Warfarin (Coumadin, Jantoven) is a blood thinner medication used to prevent and treat blood clots. Less clotting is a good thing if you’ve had a blood clot or are at risk for one. But if your blood is too thin, serious bleeding problems can occur. In fact, the FDA requires warfarin to have a boxed warning (the most serious type of medication safety warning) for this bleeding risk.
To keep you safe, you’ll have regular blood tests while you’re taking warfarin. These tests will measure your international normalized ratio (INR). A high INR means your blood is too thin and isn’t forming clots properly, making bleeding more likely. A low INR means your blood is too thick, and unwanted blood clots may happen. Your healthcare team will choose a target INR range that’s best for you and adjust your warfarin dose to keep it there.
More than 200 medications, foods, and supplements can interact with warfarin. But don’t worry, we’ll help sort this all out. Here, we’ll cover 10 of the most important warfarin interactions to watch out for. This isn’t a complete list, so remember to always talk with your healthcare team before making any changes to your medications or supplements while you’re taking warfarin.
1. Some antibiotics, such as Cipro and Bactrim
Antibiotics are one of the most common types of medications that cause a warfarin interaction. They may kill off gut bacteria that help make vitamin K. Vitamin K helps your blood clot. So less of it can lead to a higher INR, which means there’s a higher risk of bleeding.
Some antibiotics also affect certain liver enzymes (proteins) that help break warfarin down. This can push your INR out of its target range.
Rifampin is one antibiotic that causes your body to break down warfarin faster. This can leave you less protected from blood clots. Your prescriber will likely increase your warfarin dose if you start taking rifampin.
But the opposite effect can happen, too. Some antibiotics block your liver from breaking down warfarin. This can raise warfarin levels. And higher warfarin levels can make your blood too thin and put you at risk of bleeding. Some examples of antibiotics that have this effect include:
Ciprofloxacin (Cipro)
Metronidazole (Flagyl)
Sulfamethoxazole / trimethoprim (Bactrim)
It’s best to avoid these antibiotics when taking warfarin. But if a different antibiotic isn’t an option, your prescriber will likely adjust your warfarin dose while you’re taking them.
2. Some antifungals, such as Diflucan
Some antifungal medications also raise warfarin levels in the body. This may increase your risk of bleeding. Some examples include:
Fluconazole (Diflucan)
Itraconazole (Sporanox)
Ketoconazole (Nizoral)
Voriconazole (Vfend)
Your prescriber will usually lower your warfarin dose if you need to take an oral or injected antifungal.
Warfarin and your diet: Learn how foods rich in vitamin K can affect warfarin and how to manage your diet while taking it.
Warfarin side effects: Here are common and rare warfarin side effects to be aware of, and what to do if they occur.
Blood thinner medications: Read about warfarin and other blood thinner medications to better understand the different options available.
But what about using an antifungal that isn’t swallowed or injected? Miconazole (Monistat, Zeasorb) is a common over-the-counter (OTC) antifungal that’s used topically for infections like athlete's foot and vaginal yeast infections. Using one of these products may also increase your risk of bleeding if you’re taking warfarin. This interaction is usually less serious with topical or vaginal antifungals compared to oral or injectable antifungals because not as much of them enters your bloodstream. But it’s still good to be aware of.
If you need a topical or vaginal antifungal while taking warfarin, talk to your healthcare team. They can help you choose the safest product for your needs and keep an eye on your INR while you’re using it.
3. Some anti-seizure medications, such as phenytoin
Carbamazepine (Tegretol, Carbatrol) and phenytoin (Dilantin) are anti-seizure medications that can cause your body to clear warfarin faster and lower your INR. If you start them while you’re taking warfarin, you might need a higher warfarin dose to keep your INR in its target range.
Phenytoin may also increase warfarin’s effect when you first start taking it. So your INR might go up early on in treatment, and then go down later. If you’re taking phenytoin and warfarin together, you’ll likely have your INR tested more often to help watch for these interactions.
4. Other medications that raise warfarin levels
There are also other medications that may cause increased warfarin levels and raise your risk of bleeding. A few to be aware of include:
Amiodarone (Pacerone)
Omeprazole (Prilosec)
Diltiazem (Cardizem)
Statin cholesterol medications, such as simvastatin (Zocor) and fluvastatin (Lescol XL)
If you’re taking these medications with warfarin, your prescriber may adjust your warfarin dose to help prevent bleeding problems. But be sure to let them know if there are any changes made to your medications.
5. NSAIDs, such as ibuprofen
Nonsteroidal anti-inflammatory drugs (NSAIDs) include medications such as ibuprofen (Advil, Motrin) and naproxen (Aleve). They aren’t blood thinners, but they can increase your risk of bleeding. And this is especially true if you take them with warfarin.
It’s best to avoid NSAIDs while taking warfarin, unless your healthcare team instructs you otherwise. If you need an OTC treatment for pain or fever, talk to your prescriber about the safest option for you.
Aspirin is a unique NSAID that’s often used as a blood thinner at low doses. In many cases, it’s not recommended to take low-dose aspirin and warfarin at the same time unless your prescriber tells you to.
6. Other blood thinners, such as Plavix
There are several other medications used to treat and prevent blood clots. The two main types are antiplatelet medications and direct-acting oral anticoagulants (DOACs). Examples of these medications include:
Clopidogrel (Plavix)
Apixaban (Eliquis)
Rivaroxaban (Xarelto)
Dabigatran (Pradaxa)
You should avoid taking these medications with warfarin unless your prescriber tells you to. Doing so could put you at risk of bleeding.
7. SSRIs, such as Zoloft
Selective serotonin reuptake inhibitors (SSRIs) are medications commonly prescribed for depression. Some examples include fluoxetine (Prozac) and sertraline (Zoloft).
SSRIs can also increase your risk of bleeding. But the risk is small, so they’re generally OK to take with warfarin. That being said, your prescriber may want to check your INR more often when you first start taking them together.
8. Herbal and dietary supplements
Interactions between supplements and warfarin are difficult to predict. There’s often limited information about supplements from clinical studies. And they aren’t very well-regulated. So they may not always contain the exact ingredients they say they do.
Listed below are some examples of supplements that may interact with warfarin:
Cranberry
Ginseng
Dong quai
Danshen
Keep your healthcare team in the loop before starting or stopping any supplement. They’ll be able to manage any interactions and adjust your warfarin dose if necessary.
9. Vitamin K–rich foods
Vitamin K in your diet can also interact with warfarin. The more vitamin K you consume, the higher the warfarin dose you’ll need.
Many foods contain vitamin K, but certain foods have more than others. Some common vitamin K–rich foods include:
Leafy green vegetables, such as spinach and kale
Cruciferous vegetables, such as brussel sprouts and broccoli
Salad dressings
Tuna in oil
You should also be aware of multivitamins, nutritional shakes, and energy drinks that contain vitamin K.
Keep in mind: If you like eating foods that contain a lot of vitamin K, that’s fine. Just be consistent about how much of them you eat and how often you eat them. Your prescriber will make sure your warfarin dose accounts for the vitamin K you normally consume. But if your diet changes, let them know. They may need to adjust your warfarin dose.
10. Alcohol
Alcohol can also affect how warfarin works in your body.
Episodes of binge drinking (five or more drinks at a time) can raise warfarin levels and make bleeding more likely. But if you drink every day, alcohol can have the opposite effect. It can lower warfarin levels and raise your risk of a blood clot.
It’s best to limit your alcohol consumption if you’re taking warfarin. And if you do consume alcohol, do so in moderation (no more than 1 to 2 drinks in a day). If you have questions or concerns about your alcohol use while taking warfarin, talk to your healthcare team.
Frequently asked questions
The most serious warfarin side effect is severe bleeding. This is why you’ll need frequent blood tests while you’re taking it. These help ensure warfarin stays at a safe level in your body. They also help catch an increased bleeding risk so your warfarin dosage can be adjusted before a serious bleeding event happens.
It’s likely OK to drink coffee while taking warfarin, as long as you’re consistent with how much and how often you drink it. Caffeine may increase warfarin levels in the body, which can raise your risk of bleeding. But if you drink caffeine regularly, your prescriber will adjust your warfarin dosage accordingly to ensure it stays at a safe level. Just be sure to avoid making any changes to your coffee (or caffeine) habits without talking to your prescriber first.
You don’t have to completely avoid any foods while you’re taking warfarin. But you do need to be consistent with how much vitamin K is in your diet. Making changes to how much vitamin K you consume can affect warfarin levels in your body. This can raise your risk of blood clots or bleeding. So it’s best to talk to your prescriber before making any major changes in your diet.
The most serious warfarin side effect is severe bleeding. This is why you’ll need frequent blood tests while you’re taking it. These help ensure warfarin stays at a safe level in your body. They also help catch an increased bleeding risk so your warfarin dosage can be adjusted before a serious bleeding event happens.
It’s likely OK to drink coffee while taking warfarin, as long as you’re consistent with how much and how often you drink it. Caffeine may increase warfarin levels in the body, which can raise your risk of bleeding. But if you drink caffeine regularly, your prescriber will adjust your warfarin dosage accordingly to ensure it stays at a safe level. Just be sure to avoid making any changes to your coffee (or caffeine) habits without talking to your prescriber first.
You don’t have to completely avoid any foods while you’re taking warfarin. But you do need to be consistent with how much vitamin K is in your diet. Making changes to how much vitamin K you consume can affect warfarin levels in your body. This can raise your risk of blood clots or bleeding. So it’s best to talk to your prescriber before making any major changes in your diet.
The bottom line
Warfarin (Coumadin, Jantoven) interacts with many medications. Some of the most common are certain antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), and antifungal medications. Herbal supplements like ginkgo biloba and St John’s wort can also cause a problem. So can alcohol and foods rich in vitamin K, such as spinach and broccoli.
It’s best to check with your healthcare team before you make any changes to your medications, supplements, or diet. They can look out for any interactions and manage them with a change to your warfarin dose, if needed.
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