Drugs to Treat Blood Clots in the Veins - Warfarin

What is warfarin?

Warfarin (Coumadin) is a blood thinner that makes the blood less sticky, preventing new clots from forming and stopping existing clots from growing. Warfarin acts more slowly than heparin and LMWH, but unlike these other medications it can be given as a pill instead of an injection. Warfarin is the most commonly used drug for long-term blood clot prevention in women at risk.

Warfarin is also given to women who have had a DVT or pulmonary embolism (and sometimes a heart attack or stroke) to reduce the risk of blood clots in the future. It can also prevent blood clots after surgery or while you are confined to bed in the hospital.

Who might receive warfarin to treat or prevent DVT and PE?

After initial treatment with other blood-thinning drugs such as LMWH, you will usually be prescribed warfarin to take at home to prevent blood clots. Taking warfarin reduces your risk of future blood clots by more than 90%.²

How long you need to take warfarin depends on how likely you are to have another blood clot. Most women who have had a DVT or pulmonary embolism should take warfarin for at least 3 months.² If you have risk factors for blood clots that are no longer present (such as recent surgery or pregnancy), 3 months is usually long enough. If you have conditions that continue to put you at risk, you will need to take warfarin for 6 months or more.³ Women who have had more than one DVT in the past, or who have blood clotting problems or other conditions that put them at very high risk for clots, may need to take warfarin for the rest of their lives.

Warfarin may also be given to prevent DVT if you are undergoing a surgical procedure that carries a high risk of blood clots, such as hip or knee surgery.⁵ Warfarin is less convenient than LMWH because you need to have frequent blood tests to measure your blood's clotting ability.

Who should not receive warfarin?

Warfarin should not be used in pregnant women because it can cause birth defects or excess bleeding during delivery.² Other blood thinning drugs such as LMWH are less likely to cause these problems and may be the treatment of choice for pregnant women.^2,8

Warfarin may be less effective at preventing blood clots in women with cancer, or who have trouble maintaining safe blood clotting levels while taking warfarin.² These women may need to take a different type of blood-thinning drug to prevent clots.

What are the risks of warfarin?

The main risk of warfarin is excess bleeding, which occurs in about 3% of patients each year. Because women who have had a DVT or PE have a 5% to 7% risk of dying of DVT or pulmonary embolism within a year, the benefits of preventing clots usually outweigh the risk of bleeding problems.²

I am receiving warfarin. What should I know?

If you have been prescribed warfarin, you will need to have regular blood tests to prevent your blood from becoming too thin or too thick. You will be tested daily for the first few days, then two or three times a week for a few weeks, and less often after your blood clotting has stabilized. This process can be frustrating and bothersome, and it may take several weeks of adjustments before finding the best dose for you.

Warfarin can cause excessive bleeding from even minor cuts such as a razor nick. If you have to take warfarin, you will be advised to avoid activities that might cause injury. Your diet can affect how well warfarin works: getting too much vitamin K in your diet makes bleeding problems more likely while taking warfarin, so you may need to restrict foods that contain vitamin K. These include green vegetables (such as lettuce and broccoli), avocado, and egg yolks. Ask your doctor about other dietary considerations while you are taking warfarin.

Women taking warfarin for 3 to 10 weeks or longer may develop purple toe syndrome, in which the toes and feet turn a dark, purple, blue, or mottled color. This side effect may be reversible, but if left untreated, it may cause gangrene.

Learn more: Blood-Thinning Pills: How to Use Them Safely