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Warfarin is an organic chemical compound that reduces blood clotting, which in oral form is quite often prescribed by doctors. It belongs to the group of the so-called vitamin K antagonists. What are the indications and contraindications for the administration of warfarin? What possible interactions should you be aware of? When can warfarin be dangerous?

Warfarincan be in the form of two isomers: R-warfarin and S-warfarin (this is 5 times more potent). Warfarin works by inhibiting the liver's synthesis of the active forms of clotting factors. These factors depend on vitamin K (II, VII, IX, X).

Warfarin is rapidly and completely absorbed and is 98-99% bound to plasma proteins. About 90% of an orally administered dose is excreted in the urine, mainly as metabolites.

After stopping treatment with warfarin, the level of prothrombin (a clotting factor in the blood plasma) does not return to the previous level after about 4-5 days.

Warfarin was not used in medicine right away, initially it was used as … rat poison.

It was not until 1954 that warfarin was approved for the treatment of people after tests were carried out.

One of the first patients saved by this drug was the president of the United States - Dwight D. Eisenhower, who, thanks to the treatment with this substance, survived quite a severe and extensive heart attack. But on the other hand, in 2003 it was announced that the probable cause of Stalin's death was the administration of warfarin by Beria and Khrushchev. So on the one hand it is a life-saving drug, but on the other hand, it can also be very dangerous.

Warfarin: indications

Warfarin in modern medicine is used both in the prevention and treatment of deep vein thrombosis and pulmonary embolism. Secondary also prevents a heart attack.

Warfarin is also prescribed for myocardial infarction, with atrial fibrillation, as well as pathology of the heart valves. The drug is given to help prevent thromboembolic complications.

Warfarin: contraindications

The use of warfarin, however, is not always possible. Contraindications are e.g. hypersensitivity to warfarin or any of the excipients, as well as a tendency to bleeding caused by conditions such as haemophilia, thrombocytopeniaor platelet dysfunction.

It cannot be used in severe liver failure and cirrhosis, or in untreated or uncontrolled hypertension.

Contraindication to warfarin administration are also: frequent falls resulting from a neurological condition or other he alth change, conditions predisposing to intracranial bleeding, as well as recent intracranial bleeding.

Other contraindications are:

  • surgical procedures within the central nervous system or the eye
  • any predisposition to bleeding from the gastrointestinal or urinary tract
  • dementia, psychosis, alcoholism and other conditions where there is no guarantee that the patient will follow the doctor's instructions

Warfarin and pregnancy and lactation

Warfarin, unfortunately, crosses the placenta, therefore it is contraindicated during the first trimester of pregnancy. There is a risk of teratogenicity and malformations of the central nervous system of the fetus have been observed.

It is also contraindicated during the last 4 weeks of pregnancy (it increases the risk of bleeding in the mother and fetus, which may lead to serious complications and even death).

It's best not to give warfarin throughout the pregnancy; if it is impossible, the treatment of a pregnant patient must be performed under the constant supervision of a doctor.

It is possible, however, to administer warfarin while breastfeeding.

Warfarin: side effects

With warfarin therapy, as with any other drug, there may be some side effects. They depend on the amount of the taken dose, as well as on the individual predispositions of the patient and other comorbidities.

The most common side effects are:

  • vomiting
  • nausea
  • diarrhea
  • bleeding

Less common:

  • purple toe syndrome
  • coumarin necrolysis of the epidermis

Very rare:

  • cholesterol embolism
  • transient increase in liver enzymes
  • vasculitis
  • tracheal calcifications
  • transient alopecia
  • cholestatic hepatitis
  • rash
  • allergic reactions

While taking this drug, the patient should be informed about possible side effects and instructed about disturbing symptoms.

Warfarin: drug interactions

Unfortunately, warfarin interacts with many drugs, so before starting treatment, tell your doctor aboutall preparations taken, including those available without a prescription.

  • Drugs that enhance the effects of warfarin

Ciprofloxacin, amoxicillin, valproic acid, azithromycin, cefuroxime, codeine, propranolol, acetylsalicylic acid, doxycycline, erythromycin, fluconazole, influenza vaccine, methotrexate, metronidazole, non-steroidal, non-steroidal anti-phenics (e.g. ), omeprazole, paracetamol (but only after 1-2 weeks of use), quinine (even in tonic drinks), tetracycline, miconazole (also in the form of a mouth gel), tramadol, vitamin A, simvastatin and vitamin E.

  • Drugs that weaken the effects of warfarin

Barbiturates, mesalazine, cyclosporine, phenobarbital, spironolactone, rifampicin, vitamin C, some preparations of natural origin (with extracts of ginseng, St. John's wort), vitamin K in high doses.

Drugs that affect platelet function (clopidogrel, acetylsalicylic acid, ticlopidine, dipyridamole, most NSAIDs, and long-term high doses of penicillins may increase the risk of severe bleeding complications, even life-threatening.

If you need painkillers while using warfarin, doctors recommend paracetamol or opioids.

An overactive thyroid gland, elevated body temperature, and decompensated circulatory failure may increase the effects of warfarin. In hypothyroidism, the effect of warfarin may be less.

In all the above-mentioned cases, you should carefully and systematically monitor the patient's condition and react to any disturbing changes.

Warfarin: food interactions

Avoid taking too much vitamin K while taking warfarin - avoid foods, supplements, and products that contain it.

Warfarin: Overdose

The potentially toxic dose in children is 0.5 mg / kg. The lowest known and reported lethal dose in adults is 6-15 mg / kg.

All symptoms of overdose are caused by a bleeding disorder.

Bleeding of varying intensity from almost any organ is possible, as well as hematuria and minor mucosal bleeding.

Poisoning caused by high doses of warfarin taken may lead to hemoptysis, bloody vomiting, tarry stools, ecchymosis, bruising, intracranial haemorrhage, and even hemorrhagic shock.

  • First aid for warfarin overdose

In some cases, gastric lavage and activated charcoal are enough. In more difficult cases, based on coagulation tests and clinical symptoms, vitamin K may be administered intravenously.

In the case of severe poisoning, much higher doses of vitamin K are administered, and if severe bleeding occurs and the patient's life is at risk, plasma coagulation factor supplementation is used.

Warfarin and herbs

St. John's wort, ginkgo biloba or cranberry are herbs that enhance the effects of warfarin, while green tea and ginseng weaken their effects.

Unaware of this, patients who want to take care of their he alth, drink a large amount of herbal infusions, take numerous supplements without consulting a doctor or pharmacist, and do not tell the doctor about it, considering it to be of little importance, and it should be remembered that optimizing treatment , no exposure to side effects, increases the effectiveness and safety of therapy.


Warfarin - as a drug known for a long time and still successfully used in medicine - has its advantages and disadvantages.

"Advantages" of warfarin

  • doctors' decades of experience in conducting therapy with this substance
  • effectiveness both in the prevention and treatment of thromboembolic complications
  • easy availability and relatively low price, which is also important for patients

"Disadvantages" of warfarin

  • unpredictable pharmacological effect
  • delayed start of action
  • slowly receding anticoagulant effect after the end of therapy
  • risk of major bleeding
  • the possibility of numerous interactions of significant importance, both with food and with a large group of other drugs, which unfortunately makes it difficult to predict possible side effects

Therefore, patients using warfarin should be under constant medical supervision and should also report any disturbing symptoms. They cannot under any circumstances modify the dose themselves, or take other medications on their own, even over-the-counter medications.

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