All You Need to Know about Warfarin Therapy: Monitoring, Side effects, Interactions & More

Warfarin therapy is for people with blood clotting disorders. People on warfarin therapy are familiar with the PT / INR blood test. The results of this periodic test help your doctor determine the effective and safe dose of warfarin. Fortunately, the Health & Care Professional Network Center offers PT / INR monitoring at your home.

If you have to take warfarin, you need to know some tips. In this article, we are going to explain why warfarin is prescribed, how it is monitored, what side effects it has, and when you should talk to your doctor about your symptoms.

Why do I need Warfarin?

Your doctor may prescribe warfarin if you are at risk for blood clots. People who are at risk for blood clots include:

  • People who have a mechanical heart valve
  • People with atrial fibrillation (a type of heart rhythm disorder)
  • People with clotting disorders
  • People with heart attack or stroke
  • People with pulmonary embolism (a clot that has traveled to the lung) or deep vein thrombosis (blood clot in the leg)

Warfarin does not increase the size of the clot and also prevents the clot from moving to other parts of the body.

Warfarin therapy: Monitoring

When your physician prescribes warfarin for you, the goal is to reduce the tendency of the blood to clot but not to stop clotting completely. Therefore, blood’s ability to clot should be monitored while you are taking warfarin. For this purpose, you should have a periodic blood test. Your doctor will determine the appropriate dosage of warfarin based on your blood test results. These tests are done in the lab or using a portable device at home to help your doctor determine the effective and safe dose of warfarin:

  • Prothrombin time (PT): This laboratory test measures the time it takes for a clot to form.
  • International Normalized Ratio (INR): The INR is calculated based on the number obtained for PT.

The higher the PT and INR, the longer it takes for the blood to clot.

If the INR value is less than the target range, the risk of clotting increases. The risk of bleeding also increases when the INR is above the target range.

Side effects of Warfarin

One of the most important side effects of warfarin is bleeding. If bleeding occurs in the brain, it can be life-threatening. Internal bleeding is also very dangerous. Fortunately, the risk of serious bleeding due to warfarin is low. But you may notice bruises on your skin or simply gum bleeding, or nosebleeds. These side effects are not so serious. It can be said that you may experience these side effects by taking any of the anticoagulants.

The bleeding risk varies from person to person. Bleeding often occurs when you have started taking warfarin for a few weeks or during the period of illness. Patients who have been taking warfarin for at least six months or those who have reached a stable dose have a much lower risk of internal bleeding. You should report any falls or accidents to your doctor immediately during warfarin therapy. You should also know the symptoms of bleeding:

Symptoms of internal bleeding:

  • Blood in the urine
  • Severe headache
  • Dark stools
  • Vomiting blood

If these symptoms occur, you must contact your healthcare provider immediately.

Symptoms of mild bleeding:

  • Easy bruising
  • Nosebleeds
  • Bleeding from the gums

You should also report changes in the severity and frequency of these symptoms to your doctor.

When to seek help

Contact your healthcare provider right away if you experience any of the following symptoms:

  • Nosebleed
  • Headache, weakness, and dizziness
  • Dark urine
  • Frequent vomiting, abdominal pain, and vomiting blood
  •  dark-colored stool
  •  Head injury
  •  Serious fall
  •  Other serious injuries that could cause bleeding

Consult your doctor if any of the following occur:

  • Darkness and bruising of the skin
  • Gum bleeding after brushing
  • Excessive menstrual bleeding
  • Bleeding between menstrual periods
  • Fever
  • Diarrhea, vomiting, and inability to eat for more than 24 hours
  • Prescribing a new drug (some drugs change the INR)
  • A planned surgery (In these cases, you may need to stop taking warfarin a few days before your surgery to decrease the risk of bleeding during and after surgery.)

Warfarin and food

Some foods and supplements may affect the effectiveness of warfarin. Therefore, you should consult your doctor before starting any change in your diet.

  • Vitamin K: When taking warfarin, You need to balance your vitamin K intake. Severe changes in vitamin K intake alter INR. Decreasing the amount of vitamin K increases the INR and the possibility of bleeding. Conversely, increasing the amount of vitamin K reduces INR and increases the risk of blood clotting. Foods that have high level of vitamin K include:
  • Kale
  • Broccoli
  • Spinach
  • Collard or turnip greens
  •  Lettuce
  • Brussels sprouts
  • Cabbage
  • Cranberry juice and grapefruit juice: These juices prevent blood from clotting. Therefore, if you are taking warfarin, we recommend that you do not drink more than one glass of either juice a day.
  • Alcohol: People on warfarin therapy should not consume large amounts of alcohol in a short time (for example, with a single meal) because alcohol changes the INR and increases the risk of bleeding.

Warfarin and medications

Some medications, herbs, and vitamins affect the function of warfarin. A list of these drugs is given in the table below:

Medications that increase the risk of bleedingExamples
AntifungalsFluconazole,  posaconazol, voriconazole
AntibioticsAmoxicillin, amoxicillin-clavulanate , ciprofloxacin, levofloxacin , metronidazole, trimethoprim-sulfamethoxazole
AntidepressantAmitriptyline, duloxetine , fluoxetine , sertraline , venlafaxine
HormonesMethyltestosterone, oxandrolone , testosterone
CancersCapecitabine, fluorouracil, imatinib ; tamoxifen
CholesterolsFenofibrate, gemfibrozil , most statin drugs
Pain relieversAspirin, celecoxib, ibuprofen, naproxen, tramadol
SteroidsMethylprednisolone, prednisone, prednisolone
Heartburn reliefsCimetidine, omeprazole
Medications that increase the risk of blood clotsExamples
Antibiotics and anti-tuberculosisDicloxacillin, nafcillin, rifampin, rifapentine
HerbsSt. Johns wort
Immunosuppressive medicationAzathioprine
Stomach and nauseaAprepitant, fosaprepitant, sucralfate
VitaminsVitamin K
Seizures or nerve painCarbamazepine, fosphenytoin, phenobarbital, phenytoin

Therefore, be sure to consult your doctor before taking any new medication.

Let’s sum up

We reiterate that if you are under warfarin therapy and you have abnormal bruising, nosebleeds, or gum bleeding, consult your doctor immediately. You may need PT / INR monitoring. If so, you can contact our consultants and get information on how to do this test at your home.

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