Warfarin: 7 Ways to Keep Blood Levels in a Safe Range

This blood thinner can be tricky to manage, but these tips can help.
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Many Americans take warfarin (brand name Coumadin) every day to make their blood thinner. Some are on the medication for three to nine months, for treatment of a blood clot in the leg veins or the lungs. Others are on the medication indefinitely because of atrial fibrillation, an artificial heart valve, or some other chronic condition linked to a high risk of stroke.

Warfarin is an effective and inexpensive drug, but as anyone who's been on it can tell you, it's tricky to use. It requires frequent blood tests to monitor its effect, it interacts with many common foods and medications, and it can cause serious or even life-threatening bleeding. In fact, warfarin regularly shows up on lists of the top-ten medications to cause hospitalization or serious side effects. Studies have also found that most people on warfarin are within the right range (as measured by the INR, or international normalized ratio) only about 60 percent of the time.

Fortunately, it's possible to greatly reduce one's chance of warfarin-related problems by following certain precautions to maximize the likelihood of being in the right range. Here's what to do:

1. Make sure to get dosing instructions in writing, and follow them carefully.

More than almost any other commonly used medication, warfarin requires individualized dosing that may frequently need adjustment. It's also common for a person to take different doses of warfarin, depending on the day of the week (such as 2 milligrams for five days of the week and 3 mg for two days of the week). Because of this, prescription bottles for warfarin may have vague instructions, such as, "Take as directed." This allows the clinician to change the directions without ordering a new prescription, but it also means that patients and families must be vigilant about understanding the instructions.

What to do:

  • Make sure any dosage instructions or changes are provided in writing.

  • Don't skip blood tests or appointments for checking warfarin's effects.

  • Make sure your doctor provides you with instructions if so-called protime or INR results (the blood tests used to monitor warfarin) are lower or higher than goal. (The goal is usually an INR between 2.0 and 3.0; a higher INR means the blood takes longer to clot.)

  • If you're a caregiver, make sure your loved one is able to keep up with a complicated dosing regimen, or plan on providing extra help with medications.

2. Pay extra-close attention during the first three months after starting warfarin.

Although everyone on warfarin has an increased chance of bleeding, studies have found that the risk is especially high in the first three months after starting warfarin. This probably occurs because this is when people tend to need more dose adjustments and the most monitoring.

What to do:

  • Make sure to attend all INR checks and warfarin appointments, even if they seem to be frequent. Frequent appointments are usually necessary to keep new patients in range.

3. Avoid big changes in intake of leafy greens and other sources of vitamin K.

Warfarin works by interfering with the liver's use of vitamin K to make certain clotting factors. Suddenly eating more leafy greens, such as kale, spinach, and collard greens, will decrease the effectiveness of warfarin. Conversely, suddenly reducing intake of leafy greens will make the warfarin have a stronger effect, increasing the chance of bleeding.

What to do:

  • You don't have to avoid greens; you just need to have a stable weekly intake.

  • Notify your doctor if you decide to go on a spinach salad diet or otherwise make a big dietary change. Extra monitoring of your warfarin will be required.

More ways to keep blood levels in a safe range with warfarin

4. Make sure all doctors know you take warfarin, and ask them to check for interactions when antibiotics or other new medications are prescribed.

Warfarin interacts with a truly mind-boggling array of commonly prescribed medications, including many antibiotics. Some strengthen warfarin's effect (thus increasing the risk of bleeding), whereas others weaken warfarin's effect (increasing the risk of blood clots and stroke).

What to do:

  • Bring an up-to-date medication list to every doctor's visit or, better yet, bring all medication bottles.

  • Whenever a new medication is prescribed, ask the doctor to check for a possible interaction with warfarin. Many doctors have access to computer programs that can check for interactions on demand.

  • If you need to take a medication that will affect warfarin, make sure there's a plan to adjust your warfarin dose, or at least check on your INR more frequently.

  • Get all prescriptions from the same pharmacy. Pharmacies are often able to check for drug interactions, which can be a helpful backup system.

5. Be extra-careful about regularly using aspirin, acetaminophen, or NSAIDs.

Although occasional use for a headache or sore joint should be fine, these common over-the-counter medications can all increase the risk of bleeding in people on warfarin. Regular use (daily for more than a week) of acetaminophen increases INR; this can be countered by lowering the weekly dose of warfarin. Aspirin and NSAIDs such as ibuprofen don't affect INR but do cause stomach irritation, which can predispose someone taking warfarin to bleeding.

What to do:

  • Don't take aspirin, acetaminophen, or NSAIDs on a daily basis without discussing the bleeding risk with the doctor.

6. Avoid herbal remedies and supplements known to affect clotting.

Many herbal medications have been shown to at least theoretically affect certain clotting factors within the blood. Anyone on warfarin should probably avoid these. Otherwise their use should be discussed with the doctor so that extra blood monitoring can be arranged.

What to do:

  • Be careful about supplements containing any of the following: alfalfa, angelica, aniseed, arnica, asafetida, celery, chamomile, clove, fenugreek, feverfew, fucus, garlic, ginger, ginkgo, ginseng, horse chestnut, horseradish, licorice, meadowsweet, poplar, prickly ash, quassia, red clover, willow.

  • Note that dietary doses -- doses commonly used in cooking -- of garlic and ginger have not been shown to lead to increased bleeding events in people on warfarin.

7. Ask about self-monitoring at home.

Several handheld finger-stick devices now are FDA approved for checking INRs at home. Some people on warfarin do their own monitoring and adjustment, with the support of instructions and backup from a clinician's office. In 2010, a large study found that rates of bleeding on warfarin were the same whether people were monitored in a special warfarin clinic or they performed self-monitoring. However, the study also found that people reported more satisfaction and slightly better quality of life, on average, with home monitoring.

What to do:

  • If you're the independent, do-it-yourself type, or if frequent visits to the doctor are a hardship, consider asking for a self-monitoring device. You'll need to be trained to use it, and your healthcare provider will need to provide clear instructions on how to adjust your warfarin.

Dr. Leslie Kernisan

Leslie Kernisan is a clinical instructor in the University of California, San Francisco, Division of Geriatrics, and maintains a popular blog and podcast at BetterHealthWhileAging. See full bio