Warfarin FAQs

Our Thrombosis Experts Answer Your Questions

When is warfarin taken?

Warfarin should be taken daily at approximately the same time every day. It is often best to take this medication in the late afternoon or evening as this allows for dose adjustment if needed, based on testing. If patients forget to take a dose, they should contact their medical provider for specific instructions.

What medications should be avoided with the use of warfarin?

There are many medications that may increase the anticlotting effect of warfarin. Unless specifically instructed by their healthcare provider, patients should avoid medications that affect the clotting system, such as aspirin. Some antibiotics can affect the level of anticoagulation while on warfarin as well.

Some people taking warfarin who have a high risk of clotting also need to take antiplatelet agents. These drugs, like warfarin, also interfere with the clotting mechanism. It is important to discuss all medications you take before initiating anticoagulation with warfarin.

Herbal medicines, although available without a prescription, may also affect the level of anticoagulation your body experiences. For example, supplements that may affect clotting include:

• Ginseng

• Ginkgo biloba

• Bromelain

• Flaxseed

• Fish oil

• Vitamin E in large doses

• Garlic

• Ginger

• Bilberry fruit

Learn more about herbal products and warfarin

Should patients avoid vitamin K foods while they are taking warfarin?

Although warfarin works through interference with the vitamin K pathway, which is important in making some of the body’s clotting factors, anticoagulation patients on warfarin do not have to avoid all vitamin K-containing foods. These patients should, however, remember the following guidelines:

• Eat large amounts of foods with vitamin K can reduce the effectiveness of warfarin

• Consistency is key, and eating the same number of servings of foods with vitamin K will help maintain your international normalized ratio

• Making drastic changes in your diet, such as trying to lose weight by eating more green leafy vegetables or cutting out all green leafy vegetables, will affect your level of anticoagulation

Get more information about food and warfarin

What is the prothrombin time/international normalized ratio?

The prothrombin time, referred to as the PT, measures the time needed for a clot to form in a tube of the patient’s blood after chemicals have been added to activate the clotting reaction artificially.

The prothrombin time is obtained either by blood drawn from the patient’s vein or by an instrument that provides an international normalized ratio result from a blood sample taken from a finger stick.

Because different laboratories use different reagents to perform the prothrombin time, results can vary between laboratories. Therefore, the prothrombin time was standardized through the development of the international normalized ratio system so that results from different laboratories could be compared and interpreted.

The  international normalized ratio is calculated to adjust for the varying sensitivities of diverse reagents used in different laboratories. It is standardized only for the measurement of the effect of warfarin on the prothrombin time to monitor therapy.

How often should a patient’s anticoagulation level be monitored?

The frequency of monitoring needed for each patient depends on how well they respond to warfarin and any other medical issues they may have. Some patients require weekly monitoring, while others with stable responses who require long-term therapy may be monitored every other week or once a month.

What information do I need to know about my anticoagulation?

Ask your healthcare provider to give you educational materials about the use and monitoring of warfarin.

You should be aware of your target international normalized ratio range and the results of your tests. You should maintain a record of your international normalized ratio values and warfarin dose. This information will allow you to be a better partner in your healthcare.

When is a low molecular weight heparin more appropriate than warfarin?

Healthcare providers may prescribe low molecular weight heparin instead of warfarin in a variety of situations. These situations include, but are not limited to patients, who:

• Experience clotting events while on warfarin

• Are pregnant

• Have new or very large clots

• Are about to undergo surgery or dental work

The use of low molecular weight heparin requires intensive patient education before the start of therapy. This should be coordinated through your healthcare provider’s office.

Can a patient who is taking warfarin still get a clot?

It is possible for patients to develop a blood clot while taking warfarin. If the international normalized ratio is not in the target range and the patient is not being anticoagulated enough, a clot may occur. A change in the brand of warfarin may lead to fluctuations in the international normalized ratio.

Other variables, such as the presence of underlying cancer, may place patients at risk of developing a clot, even if warfarin is taken as prescribed and the international normalized ratio is within the target range.

If you have more questions about warfarin and laboratory testing, call the IHTC.