email E-Mail


Warfarin is an anticoagulant, also called a blood thinner, that helps prevent blood clots from forming in your body. Blood thinners help reduce your risk for heart attack, stroke and blockages in your arteries and veins by preventing blood clots from forming and growing.

How does it work?
Although they are called blood thinners, anticoagulants do not really thin your blood, but decrease its ability to clot (coagulate). Reducing the likelihood of clotting means fewer harmful blood clots will form, which could potentially block your blood vessels causing a heart attack or stroke.

How should I take it?
Before prescribing warfarin, your doctor will first do a blood test known as INR (International Normalized Ratio) to decide how much medication you need. He or she will also decide how often you will need this blood test. After the test, your dose of warfarin may change. Your doctor will adjust your dose based on your needs. Always talk to your doctor or pharmacist about how and when to take your medication.

What should I avoid while taking this medicine?
Avoid smoking and limit your use of alcohol. Also, watch your diet because certain foods may change the way warfarin works. Large doses of vitamin K (found in fish, liver, spinach, cabbage, cauliflower, broccoli and brussels sprouts) may decrease the effects of the medicine. You don't have to stop eating these foods, but you should be conscious about eating too much of them, or making sudden changes to your diet. Some natural health products may also affect warfarin levels in different ways. Also, try to avoid cutting yourself. Use an electric shaver and a soft-bristled toothbrush. You may also want to floss gently and wear gloves while gardening and washing dishes.

Visit Health Canada to learn more about The Effects of Grapefruit and its Juice on Certain Drugs. 

What if I am taking other medicines?
Many prescription and over-the-counter drug products are known to interact with warfarin. Always talk to your doctor or pharmacist before taking any other medication including prescription, non-prescription, over-the-counter or natural health products (vitamins and minerals, herbal remedies, homeopathic medicines, traditional medicines such as traditional Chinese medicines, probiotics and other products such as amino acids and essential fatty acids).

What else should I tell my doctor?
If you need to visit your dentist, be sure to tell him or her that you are taking warfarin. Ask your pharmacist for a wallet card that says you are taking warfarin. You may also want to wear a medical alert bracelet.

What are some common side effects?
Warfarin must be monitored closely with regular blood tests to make sure your blood is not too thin (which can lead to bleeding) or too thick (which may leads to clots). The most common side effect is bleeding and bruising. Bleeding may occur in the gums, urinary system or bowels. Warfarin may cause you to bleed more than usual if you cut yourself, or may cause easy bruising. Report any stroke signs or blood clot or any physical changes to your doctor.

Lifestyle changes
Eating a healthy diet that is lower in fat, especially saturated and trans fats, being smoke free, limiting alcohol use, being physically active and reducing stress are also important in lowering the risk of heart disease. Talk to your healthcare practitioner about how you can achieve these lifestyle changes.

For more information
Health Canada provides health and medical information to help Canadians maintain and improve their health. Learn more about Safe Use of MedicinesSafety and Effectiveness of Generic Drugs and Buying Drugs over the Internet.

Drug Product Database provides information about drugs approved for use in Canada.

MedEffect Canada provides safety alerts, public health advisories, warnings and recalls.

Your ministry of health also provides useful health resources in your province or territory. For example, Ontario has a MedsCheck program providing free pharmacist consultations on safety use of drugs. British Columbia has a Senior Healthcare webpage providing information about important health programs.  

Last modified: July 2011
Last reviewed: July 2011