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Warfarin (By mouth)

Prevents and treats blood clots. May lower the risk of serious complications after a heart attack. This medicine is a blood thinner.

What works?

Learn more about the effects of these drugs. The most reliable research is summed up for you in our featured article.

Warfarin is used to prevent or treat blood clots, including deep venous thrombosis or pulmonary embolism. It is also used for blood clots that may be caused by certain heart conditions, open-heart surgery, or after a heart attack. Warfarin is an anticoagulant (blood thinner) that decreases the clotting ability of the blood. This medicine is available only with your doctor's prescription… Read more
Brand names include
Coumadin, Jantoven
Drug classes About this
Anticoagulant

What works? Research summarized

Evidence reviews

The optimal warfarin dose for patients beginning therapy

Warfarin is commonly prescribed to prevent blot clots in patients with medical conditions such as atrial fibrillation, heart valve replacement or previous blood clots. Warfarin is an effective treatment which has been used for many years but needs to be closely monitored, especially at the beginning of treatment, as there is a wide variation in response to dose. Monitoring of the response to dose is done using an International Normalized Ratio (INR) and it is important that patients remain within a narrow range (typically 2 to 3 INR) due to the need to balance the goal of preventing blood clots with the risk of causing excessive bleeding.

The addition of vitamin K to improve anticoagulation stability for patients starting or already on warfarin

People with irregularity in heart activity, mechanical heart valves, and clotting disorders are at increased risk of developing blood clots, which could lead to stroke or death. Taking warfarin significantly reduces this risk. However, taking too much warfarin can lead to excessive bleeding, while taking too little reduces its benefit. To monitor this, patients taking warfarin must have regular blood tests to check if their dose of warfarin is stable enough to find the correct balance. There is some evidence that adding a small dose of vitamin K to the warfarin improves this balance. In this review, our primary outcomes were to assess if the addition of low‐dose vitamin K to warfarin had an effect on the time taken to the first INR in range; the mean within the therapeutic range; or any adverse events, such as thromboembolic events, haemorrhage, or mortality. We found two studies that met our inclusion criteria. Neither study reported the time taken to the first INR in range. One study was only available in an abbreviated format, so we were unable to interpret the results fully. Nonetheless, it was suggested that the addition of vitamin K had no benefit. A second six‐month study gave a small dose of vitamin K (150 mcg daily) or placebo to participants taking warfarin with existing poor INR control. This study reported the mean time in therapeutic range as a percentage and found that in the group of participants deemed to have poor INR control, the addition of 150 mcg oral vitamin K significantly improved their anticoagulation control. However, the study was relatively small. Neither study reported any adverse events, such as thromboembolism, haemorrhage, or death. We conclude that further larger, higher quality studies are needed to conclude whether adding vitamin K to warfarin for patients starting or already on warfarin improves their anticoagulation control.

Warfarin initiation nomograms of 5 mg and 10 mg for venous thromboembolism

Venous thromboembolism is the presence of a blood clot that blocks a blood vessel within the venous system; it includes deep vein thrombosis (DVT) and pulmonary embolism (PE), which can be fatal. Venous thromboembolism occurs in 40% to 60% of patients after major orthopedic surgery.

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Summaries for consumers

The optimal warfarin dose for patients beginning therapy

Warfarin is commonly prescribed to prevent blot clots in patients with medical conditions such as atrial fibrillation, heart valve replacement or previous blood clots. Warfarin is an effective treatment which has been used for many years but needs to be closely monitored, especially at the beginning of treatment, as there is a wide variation in response to dose. Monitoring of the response to dose is done using an International Normalized Ratio (INR) and it is important that patients remain within a narrow range (typically 2 to 3 INR) due to the need to balance the goal of preventing blood clots with the risk of causing excessive bleeding.

The addition of vitamin K to improve anticoagulation stability for patients starting or already on warfarin

People with irregularity in heart activity, mechanical heart valves, and clotting disorders are at increased risk of developing blood clots, which could lead to stroke or death. Taking warfarin significantly reduces this risk. However, taking too much warfarin can lead to excessive bleeding, while taking too little reduces its benefit. To monitor this, patients taking warfarin must have regular blood tests to check if their dose of warfarin is stable enough to find the correct balance. There is some evidence that adding a small dose of vitamin K to the warfarin improves this balance. In this review, our primary outcomes were to assess if the addition of low‐dose vitamin K to warfarin had an effect on the time taken to the first INR in range; the mean within the therapeutic range; or any adverse events, such as thromboembolic events, haemorrhage, or mortality. We found two studies that met our inclusion criteria. Neither study reported the time taken to the first INR in range. One study was only available in an abbreviated format, so we were unable to interpret the results fully. Nonetheless, it was suggested that the addition of vitamin K had no benefit. A second six‐month study gave a small dose of vitamin K (150 mcg daily) or placebo to participants taking warfarin with existing poor INR control. This study reported the mean time in therapeutic range as a percentage and found that in the group of participants deemed to have poor INR control, the addition of 150 mcg oral vitamin K significantly improved their anticoagulation control. However, the study was relatively small. Neither study reported any adverse events, such as thromboembolism, haemorrhage, or death. We conclude that further larger, higher quality studies are needed to conclude whether adding vitamin K to warfarin for patients starting or already on warfarin improves their anticoagulation control.

Warfarin initiation nomograms of 5 mg and 10 mg for venous thromboembolism

Venous thromboembolism is the presence of a blood clot that blocks a blood vessel within the venous system; it includes deep vein thrombosis (DVT) and pulmonary embolism (PE), which can be fatal. Venous thromboembolism occurs in 40% to 60% of patients after major orthopedic surgery.

See all (53)

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