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Coumadin: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Dec 31, 2019.

1. How it works

  • Coumadin is a brand (trade) name for warfarin. Warfarin works by blocking the formation of vitamin K-dependent clotting factors and inhibiting a vitamin K dependent enzyme complex, as well as two anticoagulant proteins. This increases the time it takes for blood to clot. This may be described as "thinning the blood".
  • Coumadin belongs to the class of drugs known as coumarins. Coumadin is also an anticoagulant.

2. Upsides

  • Decreases the body's ability to form blood clots and is used in the prevention and treatment of deep vein thrombosis (DVT) and pulmonary embolism (PE).
  • Also used to prevent blood clots from developing as a result of atrial fibrillation or cardiac valve replacement.
  • Used to decrease the risk of death after a heart attack (myocardial infarction) or stroke due to a blood clot.
  • Has a long history of use.
  • Coumadin is available as a generic under the name warfarin.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Major and fatal bleeding. Bleeding is more likely to occur within the first month.
  • Other common side effects include gastrointestinal disturbances (such as nausea, vomiting, diarrhea, taste perversion, abdominal pain, flatulence and bloating), skin reactions, hair loss (alopecia), and elevated liver enzymes.
  • Rarely, tissue necrosis or gangrene of the skin and other tissues may occur. Potentially fatal calciphylaxis (the accumulation of calcium in the small blood vessels of the fat and skin tissues) may also occur rarely.
  • Emboli may be released as a result of Coumadin therapy. These may lodge in the kidneys, pancreas, spleen, liver, or feet ("purple toes syndrome"). Some cases have progressed to necrosis or death.
  • Coumadin has a narrow therapeutic range - meaning that there is a fine line between too much and too little. For this reason, regular blood monitoring of the international normalized ratio (INR) - a standardized number that determines the ability of your blood to clot - is required. The frequency of monitoring varies; daily monitoring is required initially. Ongoing monitoring frequency depends on patient response but may need increasing for numerous reasons such as during times of illness or with dietary or medication changes.
  • Coumadin cannot break apart established blood clots, nor can it reverse damage to tissue that has already been starved of oxygen. It can; however, prevent the extension of existing blood clots and reduce the risk of part of that blood clot breaking off and lodging in another artery or vein.
  • Coumadin should not be used by women who are pregnant or planning to become pregnant, except under specialist advice (for example for women with mechanical heart valves who are pregnant and who have a high risk of thromboembolism) because warfarin can harm a developing baby.
  • May not be suitable for some people, including those with a history of kidney disease. More frequent monitoring of anticoagulation may be necessary for those with kidney problems. The risks of Coumadin treatment may also be increased in certain populations such as those with moderate to severe liver disease, an indwelling catheter, an infectious disease, severe to moderate high blood pressure, diabetes, or undergoing eye surgery. Some people are resistant to the effects of Coumadin (called hereditary resistance).
  • Should not be used as initial therapy in people with certain disorders, such as heparin-induced thrombocytopenia (HIT), although treatment may be considered after the platelet count has normalized.
  • Coumadin interacts with numerous drugs including some antibiotics, heart medications, oral contraceptives, pain medications, and acid suppressants. Coumadin also interacts with several types of botanicals, including coenzyme Q10, St. John’s wort, ginseng, echinacea, ginkgo, golden-seal, as well as grapefruit and cranberry juice. See here for a full list of interactions.

Notes: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. For a complete list of all side effects, click here.

4. Bottom Line

  • Coumadin is effective at "thinning the blood" (reducing the ability of the blood to clot); however, a number of factors affect blood levels of Coumadin including diet, ethnicity, other medications, and illness. Too much Coumadin can cause major and potentially fatal bleeding.

5. Tips

  • Be aware that foods containing vitamin K can affect Coumadin therapy. Try to eat a normal, balanced diet so that you maintain a consistent intake of vitamin K. Avoid eating too much of the same thing (for example, a whole plate of broccoli or a big bowl of salad greens). Foods high in vitamin K include kale, collards, broccoli, spinach and other green leafy vegetables. Cranberry juice and alcohol may also affect Coumadin levels so limit your intake of these.
  • No one dosage fits all. The dosage schedule for Coumadin needs to be tailored for each individual depending on their own INR response to the drug and the condition being treated. Patient factors such as age, weight, race (Asian patients may need lower dosages), body weight, sex, concomitant medications, and comorbidities all affect dosage as do some genetic factors (for example, CYP2C9 and VKORC1 genotypes).
  • An initial lower starting dose of Coumadin is recommended for seniors or people who are frail or of Asian descent.
  • Loading doses (a bigger dose at the start of treatment) are no longer routinely recommended as these increase the risk of bleeding without offering any more rapid protection against clot formation.
  • If you miss a dose, and it is the same day, you may take that dose of Coumadin. However, if you do not discover the missed dose until the next day, do not double up on the dose (just forgo the missed dose).
  • You may need to temporarily stop or change your Coumadin dosing schedule if you have planned surgery (including eye surgery) or a dental procedure. Discuss this with your surgeon or dentist prior to the procedure.
  • Monitor yourself for signs of bleeding such as blood in your stools or urine, nose-bleeds, bleeding gums, excessive menstrual bleeding or excessive bruising and seek immediate medical advice. Also take care to minimize your risk of bleeding - avoid full-contact sports, be careful with knives and try to minimize your risk of falling.
  • Always adhere to your prescribed dosage schedule. Ask your doctor before you take or discontinue ANY other drug, including over-the-counter medicines and botanical (herbal) products. Ensure you get your blood levels monitored as instructed.
  • Wear or carry identification that states you are taking Coumadin tablets, in case of an emergency.
  • Contact your doctor if you develop severe diarrhea, an infection or a fever, as concurrent illness may affect your response to Coumadin.
  • Although Coumadin and Jantoven are both brands of warfarin, small changes in their formulation may mean your body reacts differently to each brand. Most experts recommend you stick with the same brand of warfarin to help keep your INR levels stable.

6. Response and Effectiveness

  • Some slowing of the blood's ability to clot may be noticed within 24 hours; however, it can take from 72 to 96 hours for the full effects to be seen. One dose of Coumadin lasts for 2 to 5 days; however, daily dosing is needed to keep blood levels consistent. Effects are likely to accumulate with repeated dosing because of the time it takes for the affected vitamin K-dependent clotting factors to replenish. Desired INR range varies depending on the condition being treated and specific guidelines; however, the majority of guidelines aim for a target INR of 2.5 (range 2-3).
  • Treatment duration also varies, from three months to life-long depending on the condition and other patient factors; generally, until the danger of thrombosis or embolism has passed.
  • INR readings greater than 4 are associated with a higher risk of bleeding with no additional therapeutic benefit in most people.

7. Interactions

Medicines that interact with Coumadin may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with Coumadin. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with Coumadin include:

  • acetaminophen
  • amiodarone
  • anticonvulsants such as carbamazepine, phenytoin or valproic acid
  • antimicorbial agents, such as ciprofloxacin, clarithromycin, erythromycin, metronidazole
  • apixaban
  • aspirin
  • cimetidine
  • corticosteroids, such as prednisone and methylprednisone
  • dabigatran
  • medications that inhibit or induce CYP3A4 enzymes, such as fluconazole, ketoconazole, or ritonavir
  • methotrexate
  • mifepristone
  • NSAIDs, such as ibuprofen, diclofenac, etodolac, and naproxen
  • omeprazole
  • rifampin
  • St John's Wort
  • tinzaparin.

In general, any medicine that can increase the risk of bleeding (such as clopidogrel, SSRI antidepressants [eg, citalopram, duloxetine, fluoxetine, venlafaxine], fish oils) may interact with Coumadin.

Large amounts of vitamin K in the diet (such as that from spinach, green tea, chard, kale) can also reduce the effectiveness of Coumadin.

Alcohol may increase the risk of stomach bleeding with Coumadin.

Note that this list is not all-inclusive and includes only common medications that may interact with Coumadin. You should refer to the prescribing information for Coumadin for a complete list of interactions.


Coumadin (warfarin) [Package Insert]. Revised 11/2018. Bristol-Myers Squibb Pharma Company

Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use Coumadin only for the indication prescribed.

Copyright 1996-2020 Revision date: December 31, 2019.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.