How often do I need INR blood tests while on warfarin?

When starting warfarin, INR is typically checked every 1-3 days until stable, then weekly for 1-2 weeks, and eventually every 4-12 weeks once your dose is stable. The target INR is usually 2.0-3.0 for most conditions, or 2.5-3.5 for mechanical heart valves. Any change in medication, diet, or health status may require more frequent testing.

What are the signs of warfarin-related bleeding?

Warning signs of bleeding include: unusual bruising, blood in urine (pink or red), blood in stool (red or black/tarry), bleeding gums, nosebleeds that won't stop, coughing up blood, vomiting blood, severe headache, dizziness, and unusual weakness. Seek immediate medical attention if you experience any of these symptoms, especially severe headache (may indicate brain bleeding) or black tarry stool.

What is the difference between warfarin and DOACs (new blood thinners)?

DOACs (direct oral anticoagulants) such as apixaban, rivarelbaan, edoxaban, and dabigatran differ from warfarin in several ways: DOACs have fixed dosing without routine INR monitoring, fewer food and drug interactions, and a faster onset/offset of action. However, warfarin remains the only oral anticoagulant recommended for mechanical heart valves and is preferred in certain situations such as severe kidney impairment or antiphospholipid syndrome.

Warfarin: Uses, Dosage & Side Effects

Name: Warfarin: Uses, Dosage & Side Effects
Creator: iMedic Medical Editorial Team
Published: 2025-04-17T08:00:00+00:00

Oral anticoagulant (vitamin K antagonist) for prevention and treatment of blood clots

Rx – Prescription Only ATC: B01AA03 Anticoagulant

Active Ingredient: Warfarin sodium
Available Forms: Tablets
Common Strengths: 1 mg, 2 mg, 2.5 mg, 3 mg, 5 mg
Brand Names: Waran, Coumadin, Jantoven, Marevan

✓ Medically reviewed | Last reviewed: January 27, 2026 | Evidence level: 1A

Warfarin is one of the most widely prescribed anticoagulant medications in the world. It prevents blood clots by inhibiting vitamin K-dependent clotting factors. Warfarin requires regular blood monitoring (INR tests) and has significant interactions with many foods, drugs and herbal supplements. Despite the availability of newer anticoagulants (DOACs), warfarin remains essential for patients with mechanical heart valves and certain other conditions.

📅 Published: April 17, 2025

🔄 Updated: January 27, 2026

⏱ Reading time: 15 minutes

✓ Written and reviewed by iMedic Medical Editorial Team | Specialists in clinical pharmacology

📊 Quick facts about Warfarin

Active Ingredient

Warfarin

Vitamin K antagonist

Drug Class

Anticoagulant

Blood thinner

ATC Code

B01AA03

Antithrombotic agent

Common Uses

AF, DVT, PE

Clot prevention

Available Forms

Tablets

1 mg – 10 mg

Prescription Status

Rx Only

Prescription required

💡 Key takeaways about Warfarin

Regular INR monitoring is essential: Warfarin has a narrow therapeutic index; INR must be checked regularly to ensure safe and effective dosing (target usually 2.0–3.0)
Many drug and food interactions: Vitamin K-rich foods, alcohol, and numerous medications can alter warfarin's effect – keep vitamin K intake consistent and inform your doctor of all medications
Bleeding is the most common side effect: Report any unusual bruising, blood in urine or stool, or prolonged bleeding immediately to your healthcare provider
Not safe in pregnancy: Warfarin can cause birth defects, especially in the first trimester. Women of childbearing age must use effective contraception
Still essential for mechanical heart valves: Despite newer alternatives (DOACs), warfarin remains the only recommended oral anticoagulant for patients with mechanical heart valves

What Is Warfarin and What Is It Used For?

Warfarin is an oral anticoagulant (blood thinner) that prevents and treats blood clots by inhibiting vitamin K-dependent clotting factors in the liver. It is one of the most extensively studied medications in history and is listed on the WHO Model List of Essential Medicines.

Warfarin belongs to a class of medications known as vitamin K antagonists (VKAs). It works by blocking the enzyme vitamin K epoxide reductase (VKORC1), which is necessary for recycling vitamin K. Without active vitamin K, the liver cannot produce functional clotting factors II (prothrombin), VII, IX, and X, as well as the anticoagulant proteins C and S. This results in a reduced ability of the blood to form clots, providing protection against thromboembolic events.

First approved for clinical use in 1954, warfarin has been a cornerstone of anticoagulation therapy for over seven decades. Its name is derived from the Wisconsin Alumni Research Foundation (WARF), where it was originally developed. Today, warfarin is prescribed to millions of patients worldwide and is available under several brand names including Coumadin, Waran, Jantoven, and Marevan.

Warfarin is primarily used to prevent and treat conditions associated with abnormal blood clot formation. The main clinical indications include:

Atrial fibrillation (AF): To prevent stroke in patients with irregular heart rhythm. Patients with atrial fibrillation have a 5-fold increased risk of stroke, and warfarin reduces this risk by approximately 64%
Deep vein thrombosis (DVT): Treatment and prevention of blood clots in the deep veins, typically in the legs
Pulmonary embolism (PE): Treatment and prevention of blood clots that travel to the lungs
Mechanical heart valves: Warfarin is the only oral anticoagulant recommended for patients with mechanical prosthetic heart valves, where DOACs have been shown to be inferior or dangerous
Antiphospholipid syndrome: An autoimmune condition associated with recurrent blood clots
Post-myocardial infarction: In selected patients after a heart attack, particularly those with left ventricular thrombus

The anticoagulant effect of warfarin does not begin immediately. While warfarin starts to inhibit clotting factor synthesis within hours, the full anticoagulant effect takes approximately 3–5 days to develop because existing clotting factors must be depleted. For this reason, patients requiring immediate anticoagulation are typically given concurrent heparin (a faster-acting anticoagulant) when starting warfarin therapy.

Important to know:

Warfarin does not dissolve existing blood clots. It prevents new clots from forming and stops existing clots from growing larger. The body's own fibrinolytic system gradually breaks down existing clots over time.

What Should You Know Before Taking Warfarin?

Warfarin has important contraindications, warnings, and precautions. It is essential to inform your doctor about all medical conditions, medications, and supplements before starting treatment. Warfarin is contraindicated in pregnancy and in patients with active bleeding.

Contraindications

Do not take warfarin if you have any of the following conditions:

Allergy to warfarin or any of the inactive ingredients in the tablet
Active bleeding or significantly increased bleeding tendency (e.g., haemophilia, thrombocytopenia)
Severe liver disease with impaired synthetic function
Uncontrolled severe hypertension (very high blood pressure)
Recent or planned surgery of the central nervous system or eye
Recent haemorrhagic stroke (bleeding in the brain)
Use of St John’s Wort (Hypericum perforatum), which significantly reduces warfarin levels
Pregnancy (first trimester and last 4 weeks) – see Pregnancy section below

Warnings and Precautions

Tell your doctor before starting warfarin if you have any of the following conditions, as they may require dose adjustments, additional monitoring, or alternative treatment:

Cerebrovascular disease: History of stroke or TIA may increase the risk of intracranial haemorrhage
Thyroid disorders: Hyperthyroidism increases sensitivity to warfarin; hypothyroidism decreases it
Heart failure with liver congestion: Impaired liver function can potentiate warfarin’s effect
Kidney impairment: Reduced renal function may alter drug metabolism and increase bleeding risk
Liver impairment: Even mild liver disease can significantly increase the anticoagulant effect
History of gastrointestinal bleeding: Including peptic ulcer disease, gastritis, or oesophageal varices
Protein C or Protein S deficiency: These inherited conditions may cause warfarin-induced skin necrosis at treatment initiation – overlap with heparin is essential
Chronic kidney disease: Increases the risk of calciphylaxis, a rare but serious complication involving painful skin lesions

Important warning – Surgical procedures and dental work:

Always inform your surgeon, dentist, or any healthcare professional that you are taking warfarin before any procedure, including dental extractions. Warfarin may need to be stopped or bridged with heparin before surgery. Never stop warfarin on your own without medical advice, as this can lead to dangerous clot formation.

Significant changes in diet should be avoided, as the amount of vitamin K in food can alter warfarin’s effectiveness. Examples include switching to an exclusively vegetarian diet, extreme dieting, or significantly changing your intake of green leafy vegetables. Conditions that affect nutrient absorption, such as diarrhoea and vomiting, can also change warfarin’s effect and may require more frequent INR monitoring.

Pregnancy and Breastfeeding

Warfarin crosses the placenta and poses significant risks during pregnancy:

First trimester (weeks 6–12): Warfarin can cause warfarin embryopathy, a syndrome including nasal hypoplasia (underdeveloped nose bridge), skeletal abnormalities, and stippled epiphyses. The risk is estimated at 5–10% of exposed pregnancies
Second and third trimesters: Risk of fetal central nervous system abnormalities, including optic atrophy and mental impairment
Last 4 weeks of pregnancy: Increased risk of fetal and maternal haemorrhage, including intracranial bleeding during delivery
Women of childbearing age must use effective contraception during warfarin treatment

Breastfeeding: Warfarin is considered safe during breastfeeding. Studies have shown that warfarin does not pass into breast milk in clinically significant amounts, and no adverse effects have been observed in breastfed infants of mothers taking warfarin.

Driving and machinery: Warfarin does not affect your ability to drive or operate machinery.

How Does Warfarin Interact with Other Drugs?

Warfarin has one of the most extensive drug interaction profiles of any medication. Hundreds of drugs, herbal products, and dietary supplements can increase or decrease warfarin’s effect, potentially leading to dangerous bleeding or ineffective anticoagulation.

Warfarin is metabolised primarily by liver enzymes, particularly CYP2C9 (for the more potent S-enantiomer) and CYP1A2, CYP2C19, and CYP3A4 (for the R-enantiomer). Any drug that affects these enzymes can alter warfarin levels. Additionally, drugs that affect platelet function, vitamin K metabolism, or protein binding can potentiate or diminish warfarin’s clinical effect.

The following table summarises the most clinically important drug interactions. This is not exhaustive – always consult your doctor or pharmacist before starting, stopping, or changing any medication while on warfarin.

Major Interactions (Avoid or Use with Extreme Caution)

Major Drug Interactions with Warfarin
Drug / Class	Effect on Warfarin	Clinical Significance
NSAIDs (ibuprofen, naproxen, diclofenac)	Increased bleeding risk	Avoid combination. NSAIDs inhibit platelet function and can cause GI ulceration. Use paracetamol instead
Aspirin (acetylsalicylic acid)	Markedly increased bleeding risk	Avoid unless specifically prescribed by your doctor for a specific indication (e.g., dual therapy post-stent)
Amiodarone	Increases warfarin effect (CYP2C9 inhibition)	Reduce warfarin dose by 30–50% when starting amiodarone. Effect persists for weeks after stopping
Fluconazole / Miconazole	Strongly increases warfarin effect	Potent CYP2C9 inhibitors. INR can rise dramatically. Close monitoring essential
Metronidazole	Increases warfarin effect	Inhibits CYP2C9. Reduce warfarin dose and increase INR monitoring during use
Rifampicin	Strongly decreases warfarin effect	Potent CYP enzyme inducer. Can make warfarin ineffective. Dose increase of 2–5 fold may be needed
Carbamazepine / Phenytoin	Decreases warfarin effect	CYP enzyme inducers. Higher warfarin doses typically required. Monitor INR closely
St John’s Wort	Strongly decreases warfarin effect	Contraindicated. Induces CYP enzymes and P-glycoprotein. Can render warfarin ineffective

Moderate Interactions (Use with Caution)

Moderate Drug Interactions with Warfarin
Drug / Substance	Effect on Warfarin	Recommendation
Paracetamol (acetaminophen)	Mild increase in INR at doses >2 g/day	Preferred pain reliever, but use the lowest dose for the shortest time. Avoid regular high-dose use
Glucosamine	May increase warfarin effect	Monitor INR more frequently if starting or stopping glucosamine supplements
Omeprazole / Esomeprazole	May slightly increase warfarin effect	Minor CYP2C19 interaction. Monitor INR; dose adjustment rarely needed
Antibiotics (broad-spectrum)	May increase warfarin effect	Antibiotics can alter gut flora that produces vitamin K. Monitor INR during any antibiotic course
Cranberry juice / extract	May increase warfarin effect	Avoid regular consumption of cranberry products. Case reports of significant INR elevation
Alcohol	Variable – acute use increases, chronic use decreases effect	Moderate, consistent alcohol intake is acceptable. Avoid binge drinking. Chronic heavy use induces CYP enzymes

Food and Dietary Interactions

Vitamin K is essential for the production of clotting factors and directly counteracts warfarin’s effect. The key principle is consistency – you do not need to avoid vitamin K-rich foods, but you should eat a consistent amount from day to day. Sudden increases in vitamin K intake can reduce warfarin’s effect, while sudden decreases can increase it.

Foods particularly high in vitamin K (more than 100 mcg per serving) include:

Green leafy vegetables: Kale (highest source), spinach, collard greens, Swiss chard, turnip greens, broccoli, Brussels sprouts
Herbs: Fresh parsley, coriander (cilantro), basil
Oils: Soybean oil, canola oil
Other: Green tea (in large amounts), avocado, edamame

What Is the Correct Dosage of Warfarin?

Warfarin dosing is highly individualised based on INR response. There is no fixed dose – your doctor will adjust your dose to maintain your INR within the target range, typically 2.0–3.0. The usual starting dose for adults is 2–5 mg once daily.

Warfarin has a very narrow therapeutic index, meaning the difference between an effective dose and a harmful dose is small. The dose is guided entirely by INR (International Normalised Ratio) blood test results. Factors that influence the optimal dose include age, body weight, genetics (CYP2C9 and VKORC1 polymorphisms), diet, concurrent medications, and liver function.

Adults

Standard Adult Dosing

Starting dose: Typically 2–5 mg once daily. Some protocols use a loading dose of 5–10 mg for the first 2 days, but lower starting doses (2–3 mg) are recommended for elderly patients, those with liver disease, heart failure, or those at increased bleeding risk.

Maintenance dose: Usually 2–10 mg daily, adjusted to maintain the target INR. The average maintenance dose is approximately 5 mg/day, but individual doses can range from less than 1 mg to more than 20 mg per day.

Administration: Take at the same time each day, with or without food. Many patients take warfarin in the evening, which allows for dose adjustment on the same day as an INR result.

Elderly Patients

Elderly Dosing (Age ≥65)

Elderly patients are more sensitive to warfarin and generally require lower doses. Start with 1–2 mg daily and titrate slowly. Elderly patients have a higher risk of bleeding, particularly intracranial haemorrhage, and require more frequent INR monitoring. Age-related decline in liver function, polypharmacy, and the risk of falls are important considerations.

Children

Paediatric Dosing

Warfarin use in children is less common and requires specialist supervision. The initial dose is typically 0.1–0.2 mg/kg/day, adjusted to maintain the target INR. Children, especially infants, may require higher weight-adjusted doses than adults due to faster drug metabolism. INR monitoring is more frequent in paediatric patients.

INR Monitoring and Target Ranges

INR Target Ranges by Indication
Indication	Target INR	Notes
Atrial fibrillation	2.0 – 3.0	Standard range for stroke prevention
DVT / Pulmonary embolism	2.0 – 3.0	Treatment duration typically 3–6 months, or longer if recurrent
Mechanical mitral valve	2.5 – 3.5	Higher range due to increased thrombotic risk. Lifelong treatment
Mechanical aortic valve	2.0 – 3.0	Some guidelines recommend 2.5–3.5. Lifelong treatment
Antiphospholipid syndrome	2.0 – 3.0	Higher target (3.0–4.0) may be used in recurrent thrombosis

Monitoring frequency: When starting warfarin, INR is typically checked every 1–3 days until stable, then weekly for 1–2 weeks, and eventually every 4–12 weeks once the dose is stable. Any change in medication, diet, or health status requires more frequent INR testing.

Missed Dose

If you miss a dose of warfarin, take it as soon as you remember on the same day. If you do not remember until the following day, skip the missed dose and take your normal dose at the usual time. Never take a double dose to make up for a forgotten dose, as this can lead to dangerously elevated INR and serious bleeding. Inform your healthcare provider if you miss a dose, especially if you miss two or more consecutive doses.

Overdose

An overdose of warfarin can lead to severe or life-threatening bleeding. Symptoms of overdose may include:

Blood in urine (pink, red, or brown urine)
Blood in stool (red blood or black, tarry stools)
Excessive bruising or bruising without apparent cause
Bleeding gums or prolonged bleeding from cuts
Severe headache (may indicate intracranial bleeding)
Coughing up blood or vomiting blood

Overdose – seek immediate medical attention:

If you suspect a warfarin overdose, contact your doctor, go to the nearest emergency department, or call your local poison control centre immediately. The antidote for warfarin is vitamin K (phytomenadione), which can be given intravenously for rapid reversal. In life-threatening bleeding, prothrombin complex concentrate (PCC) or fresh frozen plasma may be used for immediate reversal.

What Are the Side Effects of Warfarin?

The most important side effect of warfarin is bleeding, which can range from minor bruising to life-threatening haemorrhage. Other side effects are less common but can include skin necrosis, hair loss, and “purple toe syndrome.” Always contact your doctor immediately if you experience signs of bleeding.

Like all medicines, warfarin can cause side effects, although not everyone experiences them. The risk of side effects, particularly bleeding, depends on how well the INR is controlled. Patients with an INR consistently above the target range have a significantly higher risk of bleeding events. Understanding these side effects and knowing when to seek medical attention is crucial for safe warfarin therapy.

Very Common (affects more than 1 in 10 patients)

Frequency: >10%

Bleeding from various organs: This is the most significant side effect and the primary risk of warfarin therapy. Minor bleeding (nosebleeds, bleeding gums, bruising) is very common. Always contact your doctor if you experience any bleeding

Common (affects up to 1 in 10 patients)

Frequency: 1–10%

Increased sensitivity to warfarin during long-term treatment, potentially requiring dose reduction over time

Uncommon (affects up to 1 in 100 patients)

Frequency: 0.1–1%

Anaemia (due to chronic blood loss)
Nausea and vomiting
Diarrhoea

Rare (affects up to 1 in 1,000 patients)

Frequency: 0.01–0.1%

Skin rash, urticaria (hives), and pruritus (itching)
Hair loss (alopecia)
Liver impairment and jaundice
Localised vasculitis (inflammation of skin blood vessels)
Skin necrosis and tissue damage: A serious condition that typically occurs within the first few days of treatment, especially in patients with protein C or S deficiency. Presents as painful, darkened skin areas that can progress to tissue death

Very Rare (affects up to 1 in 10,000 patients)

Frequency: <0.01%

Hypersensitivity reactions
Blue/purple toe syndrome: Painful, bluish discolouration of the toes caused by cholesterol crystal embolisation. Typically occurs 3–8 weeks after starting warfarin
Dark or black stool (may indicate gastrointestinal bleeding)
Calciphylaxis: Calcium deposition in blood vessel walls leading to progressive, painful skin ulcers. More common in patients with chronic kidney disease. This is a serious condition requiring immediate medical attention
Anticoagulation-related nephropathy: Reduced kidney function due to excessive anticoagulation causing microscopic bleeding into the kidney tubules

When to seek immediate medical attention:

Contact your doctor or seek emergency care immediately if you experience: unusual or heavy bleeding, blood in urine or stool, unexplained severe bruising, severe headache or dizziness, difficulty breathing, coughing up blood, painful skin lesions, or purple/blue discolouration of the toes. These may indicate serious complications requiring urgent treatment.

How Should You Store Warfarin?

Store warfarin tablets at room temperature, away from light and moisture. Keep out of reach of children. Do not use after the expiry date.

Proper storage of warfarin is important to ensure the medication remains effective throughout its shelf life:

Temperature: Store at room temperature, ideally below 25°C (77°F). Avoid exposure to extreme heat or cold
Light: Keep in the original packaging to protect from light
Moisture: Store in a dry place. Do not keep in the bathroom medicine cabinet where humidity is high
Children: Keep out of sight and reach of children. Warfarin ingestion by children can be extremely dangerous
Expiry: Do not use after the expiry date printed on the packaging. The expiry date refers to the last day of that month
Disposal: Do not dispose of warfarin in household waste or flush down the drain. Return unused or expired tablets to your pharmacy for safe disposal

What Does Warfarin Contain?

Each warfarin tablet contains warfarin sodium as the active ingredient. Tablets come in various strengths (1 mg to 10 mg) and are colour-coded for easy identification. Inactive ingredients vary by manufacturer.

The active substance in warfarin tablets is warfarin sodium. The commercially available form is a racemic mixture of R- and S-warfarin. The S-enantiomer is 2–5 times more pharmacologically active than the R-enantiomer and is primarily responsible for the clinical anticoagulant effect.

Common inactive (excipient) ingredients found in warfarin tablets include:

Lactose monohydrate – a sugar-based filler. Patients with lactose intolerance should consult their doctor
Maize starch (corn starch) – used as a binder and disintegrant
Calcium hydrogen phosphate dihydrate – tablet filler
Povidone – binding agent
Magnesium stearate – lubricant used in tablet manufacturing
Dyes – vary by strength for identification (e.g., indigotine E132 for blue tablets)

Warfarin tablets are typically colour-coded by strength for safety. While colours vary by manufacturer and country, a common system includes: 1 mg (pink), 2 mg (lavender), 2.5 mg (green or blue), 3 mg (tan), 5 mg (peach), 7.5 mg (yellow), and 10 mg (white). Tablets usually have a score line allowing them to be split in half for dose adjustment.

Sodium content: Warfarin tablets contain less than 23 mg of sodium per dose, making them essentially sodium-free. This is relevant for patients on sodium-restricted diets.

Frequently Asked Questions About Warfarin

What is warfarin used for?

Warfarin is an anticoagulant (“blood thinner”) used to prevent and treat blood clots. It is most commonly prescribed for atrial fibrillation (to prevent stroke), deep vein thrombosis (DVT), pulmonary embolism (PE), and for patients with mechanical heart valves. Warfarin works by blocking vitamin K-dependent clotting factors in the liver, reducing the blood’s ability to form dangerous clots.

What foods should I avoid while taking warfarin?

You do not need to completely avoid any foods, but you should keep your vitamin K intake consistent from day to day. Foods high in vitamin K include dark green leafy vegetables (kale, spinach, broccoli, Brussels sprouts), avocado, and soybean oil. Cranberry juice should be avoided as it may increase bleeding risk. The goal is not to eliminate these foods but to eat roughly the same amount each week so your warfarin dose remains stable.

How often do I need blood tests (INR) while taking warfarin?

When you first start warfarin, INR is checked every 1–3 days until your levels stabilise. The frequency then decreases to weekly, then every 2–4 weeks. Once your dose is stable, testing may occur every 4–12 weeks. Any changes to your medications, diet, or health (such as illness, vomiting, or diarrhoea) may require more frequent testing. Self-monitoring devices are available in some countries, allowing patients to test INR at home.

What is the difference between warfarin and newer blood thinners (DOACs)?

DOACs (direct oral anticoagulants) such as apixaban (Eliquis), rivaroxaban (Xarelto), edoxaban (Lixiana), and dabigatran (Pradaxa) are newer alternatives to warfarin. Key differences: DOACs have fixed dosing, do not require regular INR monitoring, have fewer food and drug interactions, and have a faster onset and offset of action. However, warfarin remains the only recommended oral anticoagulant for mechanical heart valves and is preferred in certain conditions like severe kidney impairment and antiphospholipid syndrome. Warfarin is also significantly less expensive than DOACs in many countries.

Can I take paracetamol (acetaminophen) with warfarin?

Yes, paracetamol is the recommended pain reliever for patients taking warfarin, as it is much safer than NSAIDs (ibuprofen, naproxen) or aspirin. However, regular or high-dose paracetamol (more than 2 grams per day for several consecutive days) can increase your INR. Use the lowest effective dose for the shortest time needed. If you need to take paracetamol regularly, inform your doctor so INR can be monitored more frequently.

Can I drink alcohol while taking warfarin?

Moderate, consistent alcohol consumption (1–2 drinks per day) is generally considered acceptable for most warfarin patients. However, binge drinking or excessive alcohol intake can significantly increase INR and bleeding risk. Chronic heavy alcohol use can also induce liver enzymes that metabolise warfarin, potentially reducing its effectiveness. The key is moderation and consistency – avoid sudden changes in your drinking pattern. Always discuss alcohol use with your prescribing doctor.

Is warfarin safe during pregnancy and breastfeeding?

Warfarin is not safe during pregnancy, particularly during weeks 6–12 (first trimester) when it can cause birth defects (warfarin embryopathy), and during the last 4 weeks when it increases the risk of bleeding in both mother and baby. Women who need anticoagulation during pregnancy are usually switched to heparin. Women of childbearing age taking warfarin must use effective contraception. Warfarin is considered safe during breastfeeding – it does not pass into breast milk in significant amounts.

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Ageno W, Gallus AS, Wittkowsky A, et al. Oral anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: ACCP Evidence-Based Clinical Practice Guidelines. Chest. 2012;141(2 Suppl):e44S-e88S. doi: 10.1378/chest.11-2292

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