Warfarin - Use | Dose | Side Effects

This article was medically reviewed by M.Pharm, Marko Tanaskovic on August 12, 2018. To read more about an author, click here.

Warfarin is an oral coumarin anticoagulant and is used for the prevention and treatment of thromboembolic disorders. It works by inhibiting the synthesis of vitamin K-dependent coagulation factors (factor II, VII, IX and X) as well as protein S and C. Thus, warfarin prevents formation of blood clots (thrombus) but it can't remove existing blood clots. The effect of this drug occurs within one day, and it can last up to five days.

Pharmacokinetic data for warfarin

Warfarin is rapidly absorbed in the small intestine and thereafter metabolized in the liver. Food can slow the absorption of this drug, but absorption is not reduced if taken with food, and because of that Warfarin can be taken regardless of meals. The maximum concentration in blood is reached after four hours of ingestion. The half-life is 40 hours and is eliminated mainly via urine. The high percentage of the drug is bound to plasma proteins (up to 99%). It passes the placental barrier and drug concentration in the fetus can be the same as in the mother, which is why the application of this drug is not recommended during pregnancy.

Safety precautions

Warfarin should be avoided in the following conditions:

  • If you have an aneurysm in the central nervous system
  • If you have a hemorrhagic disorders
  • If you have a peptic ulcer
  • If you have lesions of the urogenital tract
  • If you are planning any surgical intervention
  • If you plan to go to the dentist
  • If you have malignant hypertension (hypertension related with cancer)
  • If you have hemophilia
  • If you have a threatened abortion
  • If you have any tendency to bleeding

Warfarin should be avoided in all of the above-mentioned situation for one reason - increased risk of bleeding.

Namely, Warfarin inhibits blood coagulation (coagulation is a process that allows your body to stop bleeding because it forms blood clots that clog the opening/hole in the blood vessels). While you are on treatment with this medicine, the process of coagulation is slowed down / reduced, so stopping bleeding in these patients is almost a "mission impossible". Therefore, you must make sure that you do not participate in any activities that could cause a bleed. Be careful during everyday activities (shaving, brushing teeth, etc.) because even these activities can lead to bleeding.

Warfarin, pregnancy and lactation

Warfarin has been placed in group X, which means that its use should be avoided during pregnancy because there is a high risk of harmful effects on the embryo / fetus. In one study with 418 pregnant women who took this drug, it was recorded that 16% of pregnant women had an abortion or stillbirth, and 15% of pregnant women gave birth to a child with abnormalities.1 Therefore, its use during pregnancy should be avoided whenever possible.

Breastfeeding should be avoided while taking this drug, although there is no evidence that this drug is excreted into breast milk.


Dosage is adjusted individually, in accordance with the values of INR (international normalized ratio of prothrombin time) of the patients. Your doctor will determine the dose you need to take.

British Society for Hematology recommends the use of this medicine in the following cases (if the value of INR is 2.5):

  • Dilated cardiomyopathy
  • Rheumatic mitral valve disease
  • Atrial fibrillation
  • Cardioversion
  • Deep venous thrombosis
  • Pulmonary embolism


Warfarin interacts with many drugs, because it binds for plasma proteins in a high percentage. Its connection with plasma proteins may be terminated by many drugs, which automatically means an increase of Warfarin concentration in the blood and increased risk of side effects.

Drugs that increase the effects of warfarin are the following:

  • Antimicrobial agents (drugs that kill bacteria and fungi), such as: erythromycin, fluconazole, itraconazole, ketoconazole, azithromycin, clarithromycin, metronidazole, isoniazid, neomycin and tetracycline.
  • Drugs that lower lipid levels, such as: fenofibrate, fluvastatin, atorvastatin, pravastatin, simvastatin, gemfibrozil, and others.
  • Non-steroidal anti-inflammatory drugs (NSAIDs), such as: ibuprofen, indomethacin, ketoprofen, fllurbiprofen, sulindac, and others.
  • Acetaminophen
  • Tramadol

Drugs that reduce the effects of this drug include:

  • Carbamazepine and barbiturates
  • Oral contraceptives
  • Corticosteroids
  • Spironolactone
  • Vitamin K
  • St. John's Wort
  • Alcohol

Tell your doctor about all the medicines you are taking.

Side effects

Side effects of warfarin include:

  • Bleeding which can occur anywhere in your body
  • Fever
  • Pruritus
  • Necrosis of the skin
  • Jaundice
  • Vasculitis
  • Alopecia
  • Pancreatitis


  1. NCBI link

Information on this website are provided for educational purposes only and are not intended for medical advice, diagnosis or treatment.

If you have any concerns or questions about your health, you should always consult with a physician or other health-care professional.

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