What Is Warfarin?
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Warfarin - Weigh the Pros and Cons

Written by 
Dr. Anju Virinder Mohan
 and medically  reviewed by Dr. Muhammad Zohaib Siddiq

Education: MBBS, FCPS Cardiology

Professional Bio:

Dr. Muhammad Zohaib Siddiq is a General Practitioner specializing in Cardiology with 13 years of clinical experience. He completed his MBBS at the Liaquat University of Medical and Health Sciences.... 

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Published on Dec 13, 2022 and last reviewed on Jul 17, 2023   -  6 min read


Warfarin makes blood flow easily through the veins or arteries. Read below to know more about its dosage, use of side effects, and precautions.

What Is Warfarin?

Warfarin is an FDA-approved (Food and Drug Administration) drug that reduces the formation of blood clots. The anticoagulant reduces the risk of stroke, heart attacks, or other severe heart conditions. Warfarin is a vitamin K antagonist that makes blood flow easily through the blood vessels. The drug is available only on prescription by a doctor.

What Is Warfarin Used to Treat?

Warfarin is an anticoagulant sold under various brand names, used to treat and prevent the risk of future blood clots, especially if one has had one previously, such as in venous thrombosis and pulmonary embolism. The drug prevents blood clots in people with irregular, mechanical heart valves or blood clotting disorders such as thrombophilia. Warfarin helps to reduce the risk of death and recurrent myocardial infarction. The drug may be used to prevent recurrent stroke and transient ischemic attacks.

How Does Warfarin Work?

Warfarin blocks the function of vitamin K and disrupts the coagulation cascade to reduce the frequency and extent of thrombus formation. It can deplete functional vitamin K reserves and thereby reduce the synthesis of active clotting factors by competitively inhibiting multi-unit vitamin K epoxide reductase complex. The onset of action of Warfarin is 36 to 48 hours, with a peak plasma time of three days. The drug is metabolized primarily in the liver by oxidation.

What Is the Dosage of Warfarin?

The usual Warfarin dose for adults is 10 mg for the first two days, followed by 3 mg or 9 mg. Warfarin tablets are available in strengths of 0.5 mg, 1 mg, 3 mg, and 5 mg. Warfarin liquid comes with a plastic syringe to measure the right amount of medicine.

Dosage of Warfarin

How to Use Warfarin?

Warfarin comes as a tablet and is usually taken once a day with or without food. The drug should be at around the same time each day and as advised by the physician. If one takes more than the prescribed dose of Warfarin, it is essential to call the doctor immediately. Warfarin is started on a low dose, and gradually the dose is increased or decreased based on the results of the blood tests. It is essential to test the prothrombin time (PT) or INR tests regularly and before starting Warfarin to avoid the risk of bleeding, especially in the first three months of the treatment. Warfarin should be stored at room temperature and away from direct light, moisture, extreme cold, or heat.

What Are the Side Effects of Warfarin?

Warfarin increases the risk of bleeding and can cause severe allergic reactions, which can be life-threatening.

The side effects may include

What Are the Interactions of Warfarin?

Several drugs, dyes, supplements, and foods may interact with Warfarin, lowering its effectiveness or increasing the bleeding risk.

These include

Drug Interaction:

  • Anticoagulants such as Apixaban, Dabigatran, and Rivaroxaban.

  • Antiplatelets such as Clopidogrel, Ticagrelor.

  • Aspirin or Aspirin-containing products.

  • Medicines for heart problems include Amiodarone, Pacerone, Quinidine, Propafenone, or Nexterone.

  • Cholesterol-lowering medicines such as Bezafibrate, Gemfibrozil, Clofibrate, or Cholestyramine.

  • Antibiotics such as Erythromycin, Co-trimoxazole, and Norfloxacin.

  • Acetaminophen such as Tylenol.

  • Antifungal medications such as Fluconazole and Miconazole gel.

  • Ibuprofen Such as Advil, Montrin or Naproxen such as Aleve.

  • Phenytoin.

  • Sucralfate.

  • Carbamazepine.

  • Rifampin.

  • Phenobarbital.

  • Sertraline.

Interaction with Supplements:

  • Dong Quai.

  • Ginseng.

  • St. John’s wort.

  • Ginkgo Biloba.

  • Vitamin E.

  • Coenzyme Q10.

Interaction with Foods:

  • Cranberries or cranberry juice.

  • Green Tea.

  • Garlic.

  • Grapefruit.

  • Black licorice.

  • Alcohol.

It is important to note that an increase in vitamin K levels in the body can promote clotting and reduce the effectiveness of Warfarin.

Interaction with Disease:

  • Diabetes - Therapy with Warfarin should be administered cautiously in patients with severe diabetes as there may be an increased risk of bleeding.

  • Liver Disease - Therapy with anticoagulants should be done cautiously in patients with severe liver disease due to decreased drug clearance.

  • Renal Dysfunction - People with renal impairment have decreased plasma protein binding and may require dose adjustments.

  • Protein C Deficiency - Initiation of Warfarin may cause thrombotic occlusion of venules in the subcutaneous tissues and should be discontinued if tissue necrosis develops.

What Precautions Should Be Taken When Taking Warfarin?

Warfarin sodium comes with a black box warning, and regular monitoring of INR should be performed on all treated patients.

  • Allergy - Warfarin is unsuitable for people who have allergic reactions to Warfarin or any other medicine as it may cause symptoms such as trouble breathing, hives, or tongue or throat swelling.

  • Surgery or Dental Treatment - Warfarin increases the risk of bleeding, so it may be required to get blood tests and stop taking Warfarin for a few days before having surgery or a dental procedure.

  • High Blood Pressure - There may be a higher risk of bleeding when Warfarin is given to people with high blood pressure. It may cause cerebral hemorrhage when given to malignant or uncontrolled hypertension patients.

  • Alcohol - Large amounts of alcohol should be avoided while on Warfarin therapy as it can cause one to bleed easily.

  • Bleeding Disorders - The use of Warfarin is contraindicated in patients with active bleeding or a hemorrhagic diathesis or coagulation defects associated with hemophilia and thrombocytopenia as there is an increased risk of uncontrollable hemorrhage or bleeding complications.

  • Head Trauma - Warfarin is contraindicated in people with a previous history of intracranial hemorrhage or recent history of extracranial bleeding.

  • Pregnancy and Breastfeeding - Warfarin is not recommended during pregnancy as it can cause congenital disabilities and bleeding problems for the baby. Warfarin passes into the breast milk in small amounts.

  • Kidney Disease - Warfarin causes renal damage in patients with chronic kidney disease.

  • Gastrointestinal Bleeding - Warfarin is contraindicated in people with large esophageal varices history of peptic ulceration as it may increase the risk of gastrointestinal bleeding.

  • Geriatrics or Debilitated - Older adults are more sensitive to Warfarin due to lower body weight, reduced liver and renal function, and low dietary vitamin K intake.

  • Brushing Teeth or Shaving - When taking Warfarin, it is advised to be careful during toothbrushing and shaving.


Warfarin treats blood clots in conditions such as deep vein thrombosis or pulmonary embolism. It can cause severe bleeding. Therefore, people taking Warfarin should be closely monitored and get their prothrombin time, and regular INR (international normalized ratio) tests should be done.

Frequently Asked Questions


Are Vitamin K and Warfarin the Same?

Vitamin K and Warfarin are antagonists. Vitamin K helps to clot the blood. In the case of Warfarin, it increases the duration of the blood clots, thereby slowing down the process. It is specifically used in individuals with heart problems to prevent clot formation, which may obstruct blood flow.


How Long Can Someone Stay On Warfarin?

Usually, the drug may be prescribed for three to six months as a general guideline. However, it may vary with each individual depending on the severity of the health condition. In some patients, it may be prescribed for a lifetime.


Is Warfarin a Blood Thinner or Anticoagulant?

Warfarin is an anticoagulant. It slows down the process of clot formation. Other examples of blood thinners include heparin. On the contrary, anti-platelet drugs help to prevent clot formation by inhibiting the enzymes that help with the clumping of platelets.


What Fruit And Drinks Should Be Avoided When Taking Warfarin?

The following drinks and fruits are to be avoided during the intake of warfarin.

 - Grapefruit.

 - Cranberry.

 - Alcohol.


Does Warfarin Make an Individual Tired?

No, there have been no associated changes in fatigue or tiredness with Warfarin. Usually, individuals under medication may be medically compromised and feel tired. Proper treatment helps to recover from the disease.


Does Warfarin Affect Blood Pressure?

Warfarin causes increased systolic blood pressure. It may be due to coronary calcifications. It possibly worsens hypertension, especially in individuals with uncontrolled diabetes.


Is Warfarin a High Risk?

Warfarin is considered to be high-risk, and it is an anticoagulant. These medications tend to increase the risk of bleeding. This effect can be reversed with vitamin K and its substitutes.


Why Is Warfarin Best Taken at Night?

Generally, most drugs are preferred at night, as the metabolic process slows down at sleep. The drug ensures proper blood flow and slows down clot formation, which reduces the risk of heart block. It is important to take the drug regularly for better effects.


Does Warfarin Affect Memory?

Yes, Warfarin is known to be associated with certain medical conditions that affect health. They are mentioned below.

 - Vascular dementia.

 - Alzheimer’s disease.


Can Warfarin Damage the Kidneys?

Patients under warfarin are susceptible to kidney damage as the drug is metabolized in the kidney. Hence it should be taken with care in patients with pre-existing kidney diseases. Careful monitoring helps to prevent complications and improves the life span of an individual.


Can Warfarin be Taken in the Evening?

The drug is usually taken once daily, specifically in the evening. It is important to take the drug at the same time every day to increase efficacy and avoid missing a dose. In case of missing a dose, it is better to inform the doctor.


Can Warfarin Cause a Stroke?

Yes, there are chances of stroke in heart patients despite anticoagulant therapy. The drug is just a precaution to prevent the risk of blood clotting. It does not treat any condition. Hence, the affected individuals should be monitored frequently for dose regulation.

Article Resources

Last reviewed at:
17 Jul 2023  -  6 min read

Dr. Muhammad Zohaib Siddiq

Dr. Muhammad Zohaib Siddiq



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