Warfarin Dosing

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

Deep venous thrombosis (or deep vein 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 have a condition that increases your risk for clots (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 (brand name Coumadin), which is commonly available as an oral medication. When choosing an anticoagulant therapy, please keep in mind that options do exist, and your healthcare provider can further discuss these options with you. Many warfarin clinics are run by other healthcare providers, such as pharmacists and nurses.

How Warfarin Works

Warfarin interferes with the hepatic synthesis of clotting factors, which are vitamin K dependent. It prevents the process of coagulation and the formation of any new clots. Until it settles in and you've been on it awhile, warfarin is paired with a parenteral, or injectable, anticoagulant like Lovenox (enoxaparin injection). 

Warfarin is available in both oral and intravenous forms—most people take oral warfarin. People are started on about 5 milligrams of warfarin a day for the first few days. Your warfarin dose is then adjusted in order to maintain therapeutic INR (international normalized ratio) levels, measures of coagulation status. (The Warfarin Dosing website offers a free warfarin-dose calculator.)

Who Needs Higher Doses?

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

• African Americans
• People who are obese
• People with hypothyroid (underactive thyroid) conditions
• People who are dependent on alcohol

Who Needs Lower Doses?

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

• Older adults
• People of Asian heritage
• People with hyperthyroid (overactive thyroid) conditions
• 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, Dilantin (phenytoin, an antiepileptic or anticonvulsant drug) increases the action of warfarin and thus lowers INR levels.

The laboratory measure INR is used to determine your coagulation status, which is needed in order to 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 warfarin, INR levels should be regularly monitored by your healthcare provider.

How Diet Affects Dosage

Because warfarin is a vitamin K antagonist, a diet rich in vitamin K can lower your INR levels. Specifically, leafy green vegetables like kale and spinach 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 leafy green vegetables, but try to remain consistent in your daily consumption so that your INR levels don't fluctuate.

Risks

Adverse effects of warfarin therapy typically are limited to nausea and cramps. However, warfarin increases your ​risk for hemorrhage, or bleeding, which can be quite serious. In case of serious bleeding due to warfarin, healthcare providers can administer vitamin K to help clot your 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 bleeding. Also, be sure to inform your dentist that you're on warfarin before any dental work is done.

A Word From Verywell

If you or a loved one needs 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 healthcare provider as well as, preferably, a dietitian.

Let your healthcare team know of any changes to your diet. For example, if you want to start eating salads but have not in the past, your dietitian or other healthcare 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 healthcare team.