Warfarin Interactions: Drugs, Diseases, Foods

Your liver produces clotting factors that help your blood to clot and slow blood flow if you are cut or experience trauma. 

Without blood clotting factors, you’d experience major bleeding any time you had a deep cut, which could be life-threatening.

However, having abnormal blood clotting can also cause serious issues if the blood clot blocks blood flow in your arteries or veins. 

A heart attack can occur from a blood clot in a vessel that supplies blood flow to your heart, and a stroke can happen when a blood clot blocks the blood flow to your brain.

If you’re at increased risk of developing potentially life-threatening blood clots, your healthcare provider might prescribe warfarin, an anticoagulant that can reduce the risk of forming dangerous blood clots.

But what are the safety concerns and interactions with warfarin? We’ll cover that in this article!

What is warfarin?

Warfarin is the drug name of an anticoagulant sold under the brand names Coumadin and Jantoven. 

The FDA approved warfarin in 1954, and until recent years warfarin was the primary anticoagulant medication used to reduce the risk of stroke and heart attack in patients with atrial fibrillation.

Warfarin acts by blocking vitamin K (a nutrient found in foods and some vitamins and supplements) from making clotting factors that help form clots. 

While blood clotting is important to prevent excessive bleeding, having your blood clot easily could form a potentially dangerous clot, especially if you’re already at greater risk of developing blood clots from conditions like atrial fibrillation.

The dosage for warfarin usually falls between two and five milligrams once daily, but your healthcare provider might change the dose as needed to achieve a therapeutic dose, usually without exceeding 10 milligrams daily. 

The therapeutic dose is the amount that keeps your INR in the target range. When you’re taking warfarin, you’ll need your INR tested regularly, as well as your prothrombin time. 

The INR (international normalized ratio) is calculated using the prothrombin time and measures how long it takes your blood to clot.

What are the side effects of warfarin?

Some of the potential side effects of taking warfarin may include (italics = most common):

  • Increased bleeding
  • Increased bruising
  • Gas
  • Abdominal pain
  • Bloating
  • Change in the way things taste
  • Loss of hair
  • Feeling cold or having chills

While it’s rarer, warfarin comes with a black box warning from the FDA due to its ability to cause severe bleeding. 

If you experience any of these signs of abnormal bleeding or wounds while taking warfarin, reach out to your healthcare provider.

  • Bloody, red, or tarry bowel movements (a sign of a bowel bleed)
  • Spitting or coughing up blood
  • Heavy bleeding with your menstrual period
  • Pink, red, or dark brown urine
  • Coughing up or vomiting material that looks like coffee grounds
  • Small, flat, round red spots under the skin
  • Unusual bruising or bleeding
  • Continued oozing or bleeding from minor cuts

Which drugs can warfarin interact with? 

Aspirin (acetylsalicylic acid)

Taking warfarin with aspirin can increase your bleeding risk since aspirin is also meant to prevent blood clots

Some healthcare providers recommend taking low-dose aspirin to reduce the risk of heart attack and stroke in lower-risk patients. 

If you were already taking aspirin and are prescribed warfarin, you may need to discontinue aspirin based on the advice of your healthcare provider. 

Taking aspirin and warfarin together may also irritate the lining of your stomach and lead to stomach ulcers.

Other medications

Some over-the-counter and prescription drugs might interact with warfarin, which can increase your risk of bleeding or reduce warfarin’s effectiveness and increase your risk of forming a blood clot.

Some of the medications that might interact with warfarin include:

  • Acetaminophen (Tylenol, others) or acetaminophen-containing products
  • Antacids or laxatives
  • Certain antibiotics
  • Antifungal medications, such as fluconazole (Diflucan)
  • Cold or allergy medicines
  • Ibuprofen (Advil, Motrin IB, etc.) or naproxen sodium (e.g. Aleve)
  • Medications that treat irregular heart rhythms, such as amiodarone

Other blood-thinning medications

You shouldn’t take two anticoagulant medications at the same time because it can increase your risk of severe bleeding. 

Some prescription anticoagulants besides warfarin include:

Certain supplements

Some vitamins and supplements might interact with warfarin. For example, taking a “super greens” powder containing concentrated amounts of green leafy vegetables can alter the way warfarin works, potentially making it less effective.

Some herbs and supplements that might interact with warfarin can include:

  • Alfalfa
  • Angelica
  • Aniseed
  • Arnica
  • Asafoetida
  • Chamomile
  • Echinacea
  • Fenugreek
  • Feverfew
  • Garlic
  • Ginger
  • Ginkgo biloba
  • Ginseng
  • Horse chestnut
  • Melilot
  • Prickly ash
  • Quassia
  • Red clover
  • Saint John’s wort
  • Saw palmetto
  • Spirulina
  • Sweet woodruff
  • Tonka beans

How much you take of these herbs and supplements can impact how much they might interact with warfarin. 

For instance, the risk of bleeding from vitamin E toxicity is more likely with doses above 1,000 milligrams per day, much higher than the typical amount in vitamin E supplements.

vitamins for energy

Warfarin disease interactions

Issues with thyroid function

Having hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid) might impact the effectiveness of warfarin. 

Some theories are that thyroid medication might increase the binding of warfarin to your liver, as well as the potential destruction of clotting factors.

Liver disease

While warfarin can safely be used in patients with liver disease who require anticoagulation, it can be more difficult to maintain a therapeutic INR while taking warfarin. 

One of the reasons is that your liver is the primary site of clotting factors, so liver disease directly impacts blood clotting. 

If your natural blood clotting process is impaired, it’s known as coagulopathy.

Patients with liver disease are more prone to hemorrhaging (the release of blood from a broken blood vessel) and can have poor anticoagulation control, but it doesn’t necessarily mean that warfarin is completely contraindicated for patients with liver disease.

Large esophageal varices

Esophageal varices are abnormal, enlarged veins in your esophagus (the tube that connects your throat to your stomach). 

Esophageal varices are common in people with serious liver disease, and their presence can indicate an increased risk of bleeding.

Warfarin should be avoided in patients with large esophageal varices to reduce the risk of severe bleeding.


Blood thinners like warfarin shouldn’t be taken during pregnancy. Taking these types of blood thinners can increase the risk of hemorrhaging (losing a large amount of blood due to a broken blood vessel) and problems for the mother and baby, including spontaneous abortion.

Blood thinners are only prescribed during pregnancy if the potential benefits outweigh the risks. 

If you’re taking a blood thinner and are planning to become pregnant, you should notify your healthcare provider immediately.


If you’re going to have surgery or had surgery within the past 72 hours, you should avoid taking warfarin due to the risk of bleeding. 

Your healthcare provider can offer guidance in terms of when to discontinue and resume warfarin if you have a scheduled surgery.


Since alcohol acts as a natural blood thinner, warfarin should be avoided in those with alcoholism due to the risk of severe bleeding.

History of severe bleeding, peptic ulcers, etc

If you have a known history of severe or serious bleeding (such as intracranial bleeding), warfarin might not be a safe option for you. 

However, your healthcare provider can offer personalized guidance based on your health history.

Low platelet count (thrombocytopenia)

Taking warfarin if you have a low platelet count can increase your risk of bleeding. In these cases, your healthcare provider might suggest a more conservative goal for your INR to reduce this risk until your thrombocytopenia resolves.

Cognitive functions impairing your ability to take warfarin safely

For patients with severe dementia or cognitive decline, warfarin might not be recommended unless there is a caregiver who can assist with administering medications. 

If you’re unable to take the correct dose due to your cognitive function (e.g., you miss doses or take too much warfarin), you may be at increased risk of forming a dangerous blood clot or experiencing severe bleeding.

Does alcohol interact with warfarin?

Alcohol acts as a natural blood thinner by preventing blood cells from sticking together. If you drink large amounts of alcohol while taking warfarin, it could increase your risk of severe bleeding. 

If you choose to drink alcohol while taking warfarin, aim to keep your alcohol intake low to moderate.

beta glucan

Food interactions with warfarin

Vitamin K

Vitamin K is a nutrient found in some foods, especially green vegetables. It’s a common misconception that you should avoid vitamin K altogether while taking coumadin.

The most important thing to consider in your warfarin diet is to keep your intake of vitamin K-rich foods consistent. 

If you make drastic changes to your vitamin K intake while taking warfarin, it can alter its effectiveness. 

For instance, if you go from eating spinach and kale salads with your meals to not having any salads, your warfarin dose might result in a slower clotting time, which can put you at risk of severe bleeding.

Some vitamin K-rich foods to keep consistent while taking warfarin include:

High vitamin K foods

  • Kale
  • Spinach
  • Collards
  • Turnip greens
  • Beet greens
  • Dandelion greens
  • Mustard greens

Other moderate vitamin K foods

  • Spinach
  • Brussel sprouts     
  • Broccoli   
  • Onions 
  • Lettuce 
  • Cabbage     
  • Asparagus    
  • Endive    
  • Parsley   
  • Okra    
  • Kiwi fruit (moderate source of vitamin K)

Certain fruit juices

Grapefruit juice interacts with many medications and can impact the way they work by increasing their concentration in your blood.  

If the level of warfarin is increased in your blood, you could experience increased bleeding. You should avoid drinking grapefruit juice while taking warfarin.

Contrary to some popular belief, there aren’t enough well-designed studies on large populations proving a negative interaction between cranberry juice and warfarin. 

Therefore, drinking cranberry juice in moderation is likely safe while taking warfarin.


Several diseases and health conditions can interact with warfarin, so you should speak with your healthcare provider to determine if the benefits of taking warfarin outweigh the risks.

You don’t need to avoid any particular food, but instead, aim to keep your intake of vitamin-K-rich foods and drinks consistently while taking warfarin.

Certain supplements can interact with warfarin, so be sure to tell your healthcare provider about any natural herbs or supplements you’re taking.

Explore More

warfarin diet

Warfarin Diet: Foods To Eat And Avoid.


  1. Walling, Anne D. Concurrent Disease Alters Warfarin Effectiveness. 2000;62(10):2318-2321
  2. Efird LM, Mishkin DS, Berlowitz DR, Ash AS, Hylek EM, Ozonoff A, Reisman JI, Zhao S, Jasuja GK, Rose AJ. Stratifying the risks of oral anticoagulation in patients with liver disease. Circ Cardiovasc Qual Outcomes. 2014.
  3. Zikria J, Goldman R, Ansell J. Cranberry juice and warfarin: when bad publicity trumps science. Am J Med. 2010 May;123(5):384-92.

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