What is Warfarin?

Warfarin (also known under the brand names Coumadin, Jantoven, Marevan, Lawarin, and Waran) is an anticoagulant. It was initially marketed as a pesticide against rats and mice and is still popular for this purpose, although more potent poisons such as brodifacoum have since been developed. A few years after its introduction, warfarin was found to be effective and relatively safe for preventing thrombosis and embolism (abnormal formation and migration of blood clots) in many disorders. It was approved for use as a medication in the early 1950s and has remained popular ever since; warfarin is the most widely prescribed anticoagulant drug in North America.

Warfarin is a synthetic derivative of coumarin, a chemical found naturally in many plants, notably woodruff (''Galium odoratum'', Rubiaceae), and at lower levels in licorice, lavender, and various other species. Warfarin and related coumarins decrease blood coagulation by inhibiting vitamin K epoxide reductase, an enzyme that recycles oxidized vitamin K to its reduced form after it has participated in the carboxylation of several blood coagulation proteins, mainly prothrombin and factor VII. For this reason, drugs in this class are also referred to as vitamin K antagonists. In 1921, Frank Schofield, a Canadian veterinary pathologist, determined that the cattle were ingesting mouldy silage made from sweet clover that functioned as a potent anticoagulant. He separated good clover stalks and damaged clover stalks from the same hay mow, and fed each to a different rabbit. The rabbit that had ingested the good stalks remained well, but the rabbit that had ingested the damaged stalks died from a haemorrhagic illness. A duplicate experiment with a different sample of clover hay produced the same result. This report led to the subsequent research that led to dicoumarol's discovery. Coumarins are now known to be present in many plants. The smell of freshly cut grass or hay is due to a coumarin.

The identity of the anticoagulant substance in moldy sweet clover remained a mystery until 1940 when Karl Paul Link and his lab of chemists working at the University of Wisconsin set out to isolate and characterize the hemorrhagic agent from the spoiled hay. It took five years for Link's student Harold A. Campbell to recover 6 mg of crystalline anticoagulant. Next, Link's student Mark A. Stahmann took over the project and initiated a large scale extraction, isolating 1.8 g of recrystallized anticoagulant in about 4 months. This was enough material for Stahmann and Charles F. Huebner to check their results against Campbell's and to thoroughly characterize the compound. Through degradation experiments they established that the anticoagulant was 3,3'-methylenebis-(4-hydroxycoumarin), which they later named dicoumarol. They confirmed their results by synthesizing dicumarol and proving that it was identical to the naturally occurring agent. Over the next few years, numerous similar chemicals were found to have the same anticoagulant properties. The first of these to be widely commercialized was dicoumarol, patented in 1941. Link continued working on developing more potent coumarin-based anticoagulants for use as rodent poisons, resulting in warfarin in 1948. (The name warfarin stems from the acronym ''WARF'', for ''Wisconsin Alumni Research Foundation'' + the ending ''-arin'' indicating its link with coumarin.) Warfarin was first registered for use as a rodenticide in the US in 1948, and was immediately popular; although it was developed by Link, the WARF financially supported the research and was assigned the patent.

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Warfarin Contraindications


Warfarin is contraindicated in pregnancy, as it passes through the placental barrier and may cause bleeding in the fetus; warfarin use during pregnancy is commonly associated with spontaneous abortion, stillbirth, neonatal death, and preterm birth. Coumarins (such as warfarin) are also teratogens, that is, they cause birth defects; the incidence of birth defects in infants exposed to warfarin ''in utero'' appears to be around 5%, although higher figures (up to 30%) have been reported in some studies. Depending on when exposure occurs during pregnancy, two distinct combinations of congenital abnormalities can arise.) and any benefit needs to outweigh this risk when warfarin is considered as a therapeutic measure. Risk of bleeding is augmented if the INR is out of range (due to accidental or deliberate overdose or due to interactions), and may cause hemoptysis (coughing up blood), excessive bruising, bleeding from nose or gums, or blood in urine or stool.

The risks of bleeding is increased when warfarin is combined with antiplatelet drugs such as clopidogrel, aspirin, or other nonsteroidal anti-inflammatory drugs. The risk may also be increased in elderly patients and in patients on hemodialysis.

Warfarin necrosis

A rare but serious complication resulting from treatment with warfarin is warfarin necrosis, which occurs more frequently shortly after commencing treatment in patients with a deficiency of protein C. Protein C is an innate anticoagulant that, like the procoagulant factors that warfarin inhibits, requires vitamin K-dependent carboxylation for its activity. Since warfarin initially decreases protein C levels faster than the coagulation factors, it can paradoxically increase the blood's tendency to coagulate when treatment is first begun (many patients when starting on warfarin are given heparin in parallel to combat this), leading to massive thrombosis with skin necrosis and gangrene of limbs. Its natural counterpart, purpura fulminans, occurs in children who are homozygous for certain protein C mutations.


After initial reports that warfarin could reduce bone mineral density, several studies have demonstrated a link between warfarin use and osteoporosis-related fracture. A 1999 study in 572 women taking warfarin for deep venous thrombosis, risk of vertebral fracture and rib fracture was increased; other fracture types did not occur more commonly. A 2002 study looking at a randomly selected selection of 1523 patients with osteoporotic fracture found no increased exposure to anticoagulants compared to controls, and neither did stratification of the duration of anticoagulation reveal a trend towards fracture.

A 2006 retrospective study of 14,564 Medicare recipients showed that warfarin use for more than one year was linked with a 60% increased risk of osteoporosis-related fracture in men; there was no association in women. The mechanism was thought to be either reduced intake of vitamin K, which is necessary for bone health, or interaction by warfarin with carboxylation of certain bone proteins.

Purple toe syndrome

Another rare complication that may occur early during warfarin treatment (usually within 3 to 8 weeks) is ''purple toe syndrome''. This condition is thought to result from small deposits of cholesterol breaking loose and flowing into the blood vessels in the skin of the feet, which causes a blueish purple color and may be painful. It is typically thought to affect the big toe, but it affects other parts of the feet as well, including the bottom of the foot (plantar surface). The occurrence of purple toe syndrome may require discontinuation of warfarin.

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Warfarin Pharmacology


Warfarin consists of a racemic mixture of two active enantiomers—''R''- and ''S''- forms—each of which is cleared by different pathways. S-warfarin has five times the potency of the R-isomer with respect to vitamin K antagonism. (specifically the VKORC1 subunit), thereby diminishing available vitamin K and vitamin K hydroquinone in the tissues, which inhibits the carboxylation activity of the glutamyl carboxylase. When this occurs, the coagulation factors are no longer carboxylated at certain glutamic acid residues, and are incapable of binding to the endothelial surface of blood vessels, and are thus biologically inactive. As the body's stores of previously-produced active factors degrade (over several days) and are replaced by inactive factors, the anticoagulation effect becomes apparent. The coagulation factors are produced, but have decreased functionality due to undercarboxylation; they are collectively referred to as PIVKAs (proteins induced vitamin K absence/antagonism), and individual coagulation factors as PIVKA-''number'' (e.g. PIVKA-II). The end result of warfarin use, therefore, is to diminish blood clotting in the patient.

The initial effect of warfarin administration is to briefly ''promote'' clot formation. This is because the level of protein S is also dependent on vitamin K activity. Reduced levels of protein S lead to a reduction in activity of protein C (for which it is the co-factor) and therefore reduced degradation of factor Va and factor VIIIa. This then causes the hemostasis system to be temporarily biased towards thrombus formation, leading to a prothrombotic state. This is one of the benefits of co-administering heparin, an anticoagulant that acts upon antithrombin and helps reduce the risk of thrombosis, which is common practice in settings where warfarin is loaded rapidly.


The effects of warfarin can be reversed with vitamin K, or, when rapid reversal is needed (such as in case of severe bleeding), with prothrombin complex concentrate—which contains only the factors inhibited by warfarin—or fresh frozen plasma (depending upon the clinical indication) in addition to intravenous vitamin K.

Details on reversing warfarin are provided in clinical practice guidelines from the American College of Chest Physicians. For patients with an international normalized ratio (INR) between 4.5 and 10.0, a small dose of oral vitamin K is sufficient.


Warfarin activity is determined partially by genetic factors. The American Food and Drug Administration "highlights the opportunity for healthcare providers to use genetic tests to improve their initial estimate of what is a reasonable warfarin dose for individual patients". Polymorphisms in two genes are particularly important.


VKORC1 polymorphisms explain 30% of the dose variation between patients: particular mutations make VKORC1 less susceptible to suppression by warfarin. ''VKORC1'' polymorphisms explain why African Americans are on average relatively resistant to warfarin (higher proportion of group B haplotypes), while Asian Americans are generally more sensitive (higher proportion of group A haplotypes).


''CYP2C9'' polymorphisms explain 10% of the dose variation between patients, These ''CYP2C9'' polymorphisms do not influence time to effective INR as opposed to ''VKORC1'', but does shorten the time to INR >4.

  • The Fennerty 10 mg regimen is for urgent anticoagulation
  • The Tait 5 mg regimen is for "routine" (low-risk) anticoagulation
  • From a cohort of orthopedic patients, Millican ''et al.'' derived an 8-value model, including ''CYP29C'' and ''VKORC1'' genotype results, that could predict 79% of the variation in warfarin doses. It is awaiting validation in larger populations and has not been reproduced in those who require warfarin for other indications.
  • Lenzini ''et al.'' derived and prospectively validated a model including ''CYP29C'' and ''VKORC1'' genotypes. This model could predict 70% of the variation in warfarin doses in a validation cohort (versus 48% without genotype). The pharmacogenetic protocol lead to a reduction in out of range INR values as compared to a historic control.
  • www.WarfarinDosing.org, is a non-profit website programmed with dosing calculators and other decision support tools for clinicians' use when initiating warfarin therapy.

Adjusting the maintenance dose

Recommendations by many national bodies including the American College of Chest Physicians

Self-testing and home monitoring

Patients are making increasing use of self-testing and home monitoring of oral anticoagulation. International guidelines were published in 2005 to govern home testing, by the International Self-Monitoring Association for Oral Anticoagulation.

The international guidelines study stated: "The consensus agrees that patient self-testing and patient self-management are effective methods of monitoring oral anticoagulation therapy, providing outcomes at least as good as, and possibly better than, those achieved with an anticoagulation clinic. All patients must be appropriately selected and trained. Currently-available self-testing/self-management devices give INR results that are comparable with those obtained in laboratory testing." Apart from the metabolic interactions, highly protein bound drugs can displace warfarin from serum albumin and cause an increase in the INR. This makes finding the correct dosage difficult, and accentuates the need of monitoring; when initiating a medication that is known to interact with warfarin (e.g. simvastatin), INR checks are increased or dosages adjusted until a new ideal dosage is found.

Many commonly-used antibiotics, such as metronidazole or the macrolides, will greatly increase the effect of warfarin by reducing the metabolism of warfarin in the body. Other broad-spectrum antibiotics can reduce the amount of the normal bacterial flora in the bowel, which make significant quantities of vitamin K, thus potentiating the effect of warfarin. In addition, food that contains large quantities of vitamin K will reduce the warfarin effect. hypothyroidism (decreased thyroid function) makes people less responsive to warfarin treatment, while hyperthyroidism (overactive thyroid) boosts the anticoagulant effect. Several mechanisms have been proposed for this effect, including changes in the rate of breakdown of clotting factors and changes in the metabolism of warfarin.

Excessive use of alcohol is also known to affect the metabolism of warfarin and can elevate the INR. Patients are often cautioned against the excessive use of alcohol while taking warfarin.

Warfarin also interacts with many herbs and spices, some used in food (such as ginger and garlic) and others used purely for medicinal purposes (such as ginseng and ''Ginkgo biloba''). All may increase bleeding and brusing in people taking warfarin; similar effects have been reported with borage (starflower) oil or fish oils. St. John's Wort, sometimes recommended to help with mild to moderate depression, interacts with warfarin; it induces the enzymes that break down warfarin in the body, causing a reduced anticoagulant effect.

Between 2003 and 2004, the UK Committee on Safety of Medicines received several reports of increased INR and risk of hemorrhage in people taking warfarin and cranberry juice. Data establishing a causal relationship is still lacking, and a 2006 review found no cases of this interaction reported to the FDA; The mechanism behind the interaction is still unclear.

The use of warfarin as a rat poison is now declining because many rat populations have developed resistance to it, and poisons of considerably greater potency are now available. Other coumarins used as rodenticides include coumatetralyl and brodifacoum, which is sometimes referred to as "super-warfarin", because it is more potent, longer-acting, and effective even in rat and mouse populations that are resistant to warfarin. Unlike warfarin, which is readily excreted, newer anticoagulant poisons also accumulate in the liver and kidneys after ingestion.

This article is licensed under the Creative Commons Attribution-ShareAlike License. It uses material from the Wikipedia article on "Warfarin" All material adapted used from Wikipedia is available under the terms of the Creative Commons Attribution-ShareAlike License. Wikipedia® itself is a registered trademark of the Wikimedia Foundation, Inc.