Warfarin Dosing

Warfarin pills laying on a sheet of paper

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If you've been treated for deep venous thrombosis, you can expect at least three months of continued anticoagulants or blood-thinning medication after you leave the hospital.

Deep venous thrombosis is a dangerous condition in which your body forms a clot, typically in the leg, that can potentially break off and travel to the lungs, where it will clog circulation. This deadly event is known as pulmonary embolism. With continued anticoagulant therapy after you leave the hospital, you minimize the risk of re-forming such deadly blood clots. Note that anticoagulant therapy is not only given with deep venous thrombosis. For example, if you are currently being treated for stroke or a hypercoagulable condition, you can expect a lifetime of anticoagulation therapy.

Anticoagulation can be maintained by several medications, including low-molecular-weight heparin (subcutaneous injection), fondaparinux (subcutaneous injection), or oral Xa inhibitors like dabigatran. In this article, we'll focus on warfarin (Coumadin), which is commonly available as an oral medication. When choosing an anticoagulant therapy, please keep in mind that options do exist, and your physician can further discuss these options with you. Many warfarin clinics are run by other health care providers, such as pharmacists and nurses.

How Warfarin Works

Warfarin interferes with the hepatic synthesis of clotting factors, which are vitamin-K-dependent, thus preventing the process of coagulation and the formation of any new clots. Until it settles in, warfarin is paired with a parenteral or injectable anticoagulant like Lovenox (enoxaparin injection). 

Warfarin is available in both oral and intravenous preparations—most people take oral warfarin. People are started on about 5 mg of warfarin a day for the first few days. Warfarin dose is then adjusted in order to maintain therapeutic INR levels, measures of coagulation status that I'll touch on in a bit. If interested, the website www.warfarindosing.org offers a free warfarin-dose calculator.

Who Needs Higher Doses?

Higher doses of warfarin may be required for the following populations:

  • African Americans
  • Obese people
  • People with hypothyroid conditions
  • People who are alcohol-dependent

Who Needs Lower Doses?

Conversely, the following populations should receive lower doses of warfarin:

  • Elderly people
  • People of Asian heritage
  • People with overactive thyroid (hyperthyroid)
  • People who have heart failure
  • People with liver disease
  • Anyone with a history of major surgery
  • Anyone with polymorphisms in the CYP2C9 or VKORC1 genes

In addition to the above patient characteristics, warfarin dosage is also adjusted depending on the medications that you're taking. For example, phenytoin (an antiepileptic or anticonvulsant drug) increases the action of warfarin and thus lowers INR levels.

The laboratory measure INR (international normalized ratio) is used to determine your coagulation status and adjust your dosage of warfarin. In most people, normal INR levels range from 0.8 to 1.2. People on warfarin are typically maintained at an INR level between 2 and 3, meaning that these patients are significantly more anticoagulated (have thinner blood) than average people. While receiving anticoagulation, INR levels should be regularly monitored by your physician.

How Diet Affects Dosage

Because warfarin is a vitamin K antagonist, a diet rich in vitamin K can lower your INR levels. Specifically, green and leafy vegetables tend to be high in vitamin K. On the other hand, potatoes, fruits, and cereals are low in vitamin K. You can still continue eating nutritious portions of green and leafy vegetables; however, try to remain consistent in your daily consumption so that your INR levels don't fluctuate.

Risks

Adverse effects of warfarin therapy are typically limited to nausea, cramps, and so forth. However, warfarin increases your ​risk for hemorrhage or bleeding, which can be quite serious. In case of serious hemorrhage on account of treatment with warfarin, physicians can administer vitamin K to help coagulate or clot the blood.

People who have a history of hemorrhage should be careful when taking warfarin. Furthermore, when taking warfarin, it's best to use an electric razor and electric toothbrush to limit the risk of bleeds. Also, be sure to inform your dentist that you're on warfarin before any dental work is done.

Avoid Herbal and Other Supplements While Taking Warfarin

Because warfarin can interact with a wide range of drugs and throw your INR levels out of whack, it's best that you avoid over-the-counter medications or herbal supplements while on this medication. For example, neither ginkgo biloba and coenzyme Q10 are safe to take while taking warfarin. Even some herbal teas may be strong enough to interact in unsafe ways with medication. Err on the side of safety: if you're on warfarin, check with your doctor before starting to take any new herbal or other supplement, and be sure to tell them about any herbal teas or supplements you may currently use.

A Word From Verywell

If you or a loved one need to take warfarin for deep venous thrombosis, stroke, or a hypercoagulable condition, please remember that anticoagulant therapy with this drug is an involved process. To establish and maintain proper anticoagulation status, you will need to work closely with your physician as well as, preferably, a dietitian. Let your health care providers know if they are any changes to your diet. For example, if you want to start eating salads but have not in the past, your dietitian or other health care provider will work with you to ensure that your coumadin dose is adjusted as needed. Keeping your diet consistent is key. Remember to remain vigilant, get your INR levels checked regularly, and work with your health care team. 

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