Everything you ever wanted to know about Coumadin (warfarin)


This information is intended as general educational material and is not intended to be professional medical advice. You should consult your primary care provider for specific information.

The purpose of this post is to help answer your questions about Coumadin aka warfarin.

What does warfarin do? Warfarin is one of several types of medications which “thin” the blood. That is, they oppose the tendency of the blood to form clots.

There are two opposing “systems” in the blood — one tends to make clots and is activated when there is a cut or other trauma, and the other slows down the tendency to clot, so that clots don’t just keep on growing. Clotting occurs when certain substances (which are generally specific proteins made by the liver) interact in specific ways with other elements of the blood called platelets. Warfarin tips the balance a little bit, but not totally, in favor of decreasing clotting.

How does it work? The liver is the body’s main factory. Here, sugars and fats and proteins are broken down and others are made. It is the site where the body makes the proteins it uses to cause the blood to clot. The body’s action in blood clotting is quite complex, and uses a series of proteins secreted into the blood by the liver to interact with other substances found in the blood.

Warfarin slows the production of some of these clotting proteins that are made in the liver. It does so by inhibiting the action of Vitamin K which promotes the formation of some of the clotting proteins.

Who can be helped with warfarin? There are several conditions in which warfarin is used. Let me hasten to add that not everyone with these conditions will benefit from warfarin. The list below does not list all possible conditions for which warfarin is used.

  • Clots in the legs or other parts of the body (“Deep Venous Thrombosis”) which have the potential to break loose and go to the lungs (“Pulmonary Emboli”). Pulmonary emboli can be very dangerous.
  • People with many types of artificial heart valves.
  • Some people who have a specific type of irregular heart beat known as “Atrial Fibrillation”. In this condition, clots can form in the left atrium, and then become dislodged and travel to other parts of the body. If they become lodged in an artery to the brain, they can cause a stroke.
  • Some people who have had strokes or “Transient Ischemic Attacks” (“TIA’s”).
  • Some people who have had heart attacks.
  • Some people with blocked arteries.
  • After knee or hip surgery, generally for just a few weeks or months.
  • Some people with certain types of abnormal clotting disorders.
What’s a “Pro time”, and what’s the “INR”? The “Pro Time” (or “prothrombin time” or “PT”) is the name of the test that is used to monitor the effects of warfarin. It measures the tendency of your blood to clot as compared with a normal (“control”) sample. It is expressed as the number of seconds it takes for the blood to clot, and may be expressed as a ratio of the blood sample to the control.

The “INR” (the “International Normalized Ratio”) expresses the speed of blood clotting and corrects for any inconsistencies in the control samples. It is a more accurate expression of the action of warfarin and it is currently recommended that the “INR” be used as the number to adjust the dose of warfarin.

What should my INR be? It depends on the reason you’re taking Coumadin (warfarin). For most conditions, the INR is optimally kept in the range of 2.0-3.0.
What are the side effects? The major side effect of Coumadin (warfarin) is simply bleeding. This is generally minimal as long at the PT/INR is kept in the therapeutic range. People on warfarin, even at desirable levels, may notice that they bruise more easily than usual.

Otherwise, warfarin is well tolerated. Side effects which sometimes occur include:

  • Hair loss
  • Nausea, vomiting, cramping or diarrhea (these are quite uncommon in my experience).
  • Low white blood cell counts.

These effects however are quite uncommon. As with other medications, other “rare” side effects have been reported. You should ask your health care provider if you have any concerns about whether these or different side effects may be occurring.

What are signs of bleeding? The general rule is that bleeding which is occurring for “no good reason” needs to be checked out.

This may include:

  • Bleeding from cuts or from your nose that does not stop.
  • Throwing up blood, or material that looks like “old coffee grounds”.
  • Finding blood in your bowel movements, or seeing very black and “tarry” stools. Tarry stools may indicate bleeding in the intestinal tract, which can turn this color and consistency as it passes through the GI tract.
  • A large bruise which keeps on growing
  • More bleeding than usual when you brush your teeth. People on Coumadin (warfarin) often have a little bleeding from the gums after brushing, but an increase in the amount may indicate that the dose is too high.
  • Blood in the urine.
  • Unexpected or more bleeding from the vagina. Menstrual flow may be considerably heavier.
  • Persistent headaches or dizziness
  • Weakness, being a lot more tired than usual. These may be signs of anemia from possible blood loss.
What if I get in an accident or fall? You should get checked out. This goes for falling and hitting your head, or other traumas experienced in everyday life.
Are there any other medications or supplements I need to be careful about? Coumadin (warfarin) has a large number of interactions with other drugs. They occur for a variety of reasons:

  • Some medications thin the blood through other mechanisms, such as blocking the action of platelets, and this may lead to an increase in bleeding.
  • There can be changes in how drugs are transported in the blood.
  • Medications such as antibiotics interfere with the action of bacteria, which has a lot to do with the availability of vitamin K.

The list of possible interactions is long, and there are medications which increase and some which decrease the effects of Coumadin (warfarin). The most important medications are:

  • Aspirin, which is also a blood thinner. Aspirin works by blocking the stick factor of platelets, the formed elements of the blood involved with clotting. Aspirin is sometimes given on purpose with Coumadin (warfarin) in certain conditions. However, unless it has been specifically prescribed for you, aspirin should be avoided while you are on Coumadin (warfarin). Persantine, Ticlid, and Plavix are similar drugs.
  • Acetaminophen (Tylenol and other “non-aspirin containing analgesics”) may interfere with the metabolism of warfarin. Recommendations vary, but I ask people to give me a call if they are taking more than about 6 tablets of these agents a week.
  • Antibiotics (some, not all) may interfere by reacting with the way Coumadin (warfarin) is carried in the blood, by interfering with the metabolism in the liver, or by changing the metabolism of Vitamin K.
  • Non-steroidal anti-inflammatory agents, which are widely prescribed, and available over-the-counter, for aches and pains of arthritis.
What about my diet? The clotting factors in the liver depends on the availability of Vitamin K. Therefore, the amount of Vitamin K in the diet can change the effects of Coumadin (warfarin).

The most important point to remember is to keep the amount of Vitamin K fairly constant. Wide swings in intake may cause the PT/INR to change a significant amount.

What foods are high in Vitamin K?
  • Kale, spinach, escarole, beets, and many green leafy vegetables.
  • Broccoli, cauliflower, cabbage and sprouts.
  • Green tea
What foods are low in Vitamin K?
  • Meats, poultry, and shellfish
  • Many vegetables such as squash, radishes, turnips (without the greens!), potatoes, eggplant, mushrooms, onions (not spring onions).
  • Breads, crackers, bagels, pasta
  • Rice and pastas
  • Most fruits (the skins may contain a fair amount of Vitamin K) and their juices
  • Dairy products
  • Coffee, tea (but not green tea), and colas
Is there a particular time of day I should take Coumadin (warfarin)? Take Coumadin (warfarin) at bedtime, because if you need to have a change in dose it is best to take warfarin as soon as possible which will usually be after the test result is made known during the day.
What if I want to become pregnant? You must stop Coumadin (warfarin) while you are pregnant. It is associated with significant birth defects. Let your physician know immediately. You may be a candidate for Heparin therapy (see below).
What else do I have to be careful about? You should take care not to overindulge in alcohol. This can affect the way the liver metabolizes, and we’ve already talked about the importance of this. Different physicians handle this matter differently as well. My advice to my patients is to drink no more than two drinks per evening. “Binge” drinking is particularly to be avoided — it can “shock” the liver, and interfere significantly with the protime.
Coumadin (warfarin) sounds like a major pain. . .  isn’t there anything else I can take? Although aspirin is a blood thinner, it may not be potent enough for many circumstances. Other agents like (dipyridamole) Persantine, (ticlodipine) Ticlid, and others do not do enough in many cases. “Heparin” is an agent that has been around for years. It is frequently given continuously intravenously for some conditions, but this is not practical for long term therapy. However, it has been given subcutaneously once or twice daily for long term therapy in some situations. This can be associated with osteoporosis, low platelet counts, and a few other side effects. Newer formulations minimize these side effects. It is administered with a needle under the skin. It is not used widely at the current time, but may be a good alternative in some people in some circumstances.
What if I miss a dose? If you remember within 12 hours of the missed dose, go ahead and take it then. Otherwise, skip that dose. Make a note of it, and tell the nurse practitioner that prescribes you Coumadin (warfarin) instructions next time you talk to them.
How long do I need to take warfarin? That depends on the reason you are taking the medication, and risk factors that may arise.
How often do I need to get my blood test? When just getting started, the protime/ INR (international normalizing ration) is measured every 3-7 days or even checked daily in some circumstances until the dose stabilizes, then the interval gradually increases to as long as once per month. This is a gradual process (approximately 2-3 months). When you get sick or have a diet or medication change (like an antibiotic) this needs to be done more frequently while the change is in place.
Isn’t there an easier way to get my blood tested? If you live in the NYC area you can have a the Nurse Practitioner come to your home, office or hotel to draw your blood, and have it sent to a lab covered by your insurance, or you can take a prescription into a draw station close to your location, your results will be forwarded to Metro Medical Direct, and you will get your results in the secure patient portal and your prescription can be electronically sent to your pharmacy. So even if you are traveling you can still have your blood level managed.
What if I need to go to the dentist, or have a medical procedure, or surgery? You will need to let the professional who is doing the procedure know that you are on Coumadin (warfarin) at the time you schedule the appointment or procedure. Coumadin (warfarin) generally should be stopped for 3-4 days prior to a dental or surgical procedure.
What to include on your Coumadin (warfarin) calendar? Get a pocket sized calendar or use your google calendar

Write down the PT and INR results on the day it was performed

  • Write “draw date” for the day you are to get your next test.
  • Fill in the dose you are prescribed daily until the next blood draw date.
  • Each time you take the daily dose mark it off.
  • Keep track of your last INR result, your current dose, and your next scheduled blood draw.
  • Keep a Coumadin (warfarin) CALENDAR to help you
  • Make sure anyone who prescribes or recommends any medication or supplement to you that you are on Coumadin (warfarin)
  • Make known to your health care provider any unusual bleeding that seems to occur for no good reason
  • Any changes to your diet can cause changes in the Coumadin (warfarin’s) potency.  This may require dose changes.
  • Generally, aspirin should not be used unless specifically prescribed, and acetaminophen should be used in only modest quantities: speak to your nurse practitioner for more specific information.
  • Consider obtaining a “Medic Alert” or similar bracelet stating that you are on Coumadin (warfarin).

Raymond Zakhari, NP


To request a medical house call for a Coumadin (warfarin) management clinic in your home, office, or hotel while in New York City.


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