10 Warfarin Interactions to Watch Out For

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 provider 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. But this isn’t a complete list, so remember to always talk with your healthcare provider before making any changes to your medications or supplements while you’re taking warfarin.

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1. Some antibiotics, such as Cipro and Bactrim

Antibiotics are one of the most common types of medications that interact with warfarin. 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 healthcare provider 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 healthcare provider 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) and voriconazole (Vfend). Your healthcare provider will usually lower your warfarin dose if you need to take an oral or injected antifungal. 

But what about using an antifungal that isn’t swallowed or injected? Miconazole (Monistat, Lotrimin AF) is a common over-the-counter (OTC) antifungal that’s used topically for infections like athlete's foot. It’s also used to treat 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 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 provider. 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)

• Statins, such as simvastatin (Zocor), and fluvastatin (Lescol XL)

• Propranolol

If you’re taking these medications with warfarin, your healthcare provider may adjust your warfarin dose to help prevent bleeding problems. But be sure to let your provider 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 they’re taken with warfarin.

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.

It’s best to avoid NSAIDs while taking warfarin, unless your healthcare provider instructs you otherwise. If you need an OTC treatment for pain or fever, talk to your healthcare provider about the safest option for you.

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 oral anticoagulants (DOACs). Examples of these medications include: 

• Clopidogrel (Plavix)

• Apixaban (Eliquis)

• Rivaroxaban (Xarelto)

• Dabigatran (Pradaxa) 

In most cases, you should avoid taking these medications with warfarin. Doing so could put you at risk of bleeding.

7. SSRIs, such as Zoloft

Selective serotonin reuptake inhibitors (SSRIs) are medications most often taken for depression. Some common examples are 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 healthcare provider 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

• St. John’s wort 

• Ginseng 

• Ginkgo biloba 

• Garlic

• Dong quai 

• Danshen

• Goldenseal

Keep your healthcare provider 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 healthcare provider will make sure your warfarin dose accounts for the vitamin K you normally consume. But if your diet changes, let your provider 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 provider. 

When should I contact my healthcare provider about a warfarin interaction?

Staying in close communication with your healthcare provider is important when you’re taking warfarin. Check with them before starting or stopping any medication or supplement. And reach out if you’ve had a major change in your diet or overall health.

Contact your healthcare provider right away if you notice any unusual bruising, or if you experience bleeding from your nose or while brushing your teeth. It’s important to catch bleeding problems early, before they get worse.

Seek emergency care if you notice any of these more serious bleeding signs:

• Coughing or vomiting blood (may look dark-colored)

• Blood in your pee (may look pink or red)

• Blood in your poop (may look black or like coffee grounds)

You should also be aware of signs that warfarin isn’t working as well as it should. Let your provider know right away if you notice any signs of a blood clot, such as:

• Sudden pain or swelling in one leg

• A red or warm area in one leg

• Difficulty breathing

• Chest pain

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Written by:

Daphne Berryhill, RPh

Daphne Berryhill, RPh, has two decades of experience as a clinical pharmacist. She spent most of her career in the Chicago area practicing in-home infusion.

Edited by:

Stacia Woodcock, PharmD

Stacia Woodcock, PharmD, is a pharmacy editor for GoodRx. She earned her Doctor of Pharmacy degree from the University of Kentucky and is licensed in New York and Massachusetts.

Reviewed by:

Joshua Murdock, PharmD, BCBBS

Joshua Murdock, PharmD, BCBBS, is a licensed pharmacist in Arizona, Colorado, and Rhode Island. He has worked in the pharmacy industry for more than 10 years and currently serves as a pharmacy editor for GoodRx.

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