Joan J. Fuller
What is Warfarin?
Warfarin is a medication which prevents blood clots from forming in your body. It helps prevent heart attacks and strokes. It is used to treat high blood pressure, pulmonary embolisms (blood clots in the lungs), thrombosis (clotting of blood vessels) and deep vein thrombosis (a clot that forms in the leg).
The drug is manufactured by Pfizer Inc., a pharmaceutical company based in New York City. Warfarin was originally made from the leaves of the Chinese sweet wormwood plant (artemisia vulgaris).
The drug was first discovered in a German lab in 1937.
The medication was initially used to kill rats, but now is used by humans to prevent deadly conditions. It is the most commonly used anticoagulant, or “blood thinner,” in the world. Anticoagulation is used to treat and prevent blood clots, which can lead to heart attacks, strokes and pulmonary embolisms.
The drug works by blocking Vitamin K from participating in the process that causes your blood to clot. For this reason, patients need to maintain a steady intake of Vitamin K in their diet. Common foods that contain high levels of the vitamin include cabbage, broccoli, brussels sprouts, green beans and peas.
People who take the medication should avoid these foods, as eating too much will actually counteract the drug’s intended effect.
Warfarin comes in tablet and liquid form. It is typically taken once a day, with the dose adjusted based on your body weight. It can take anywhere from four to seven days for the drug to build up to an effective level.
Doctors will often recommend patients get blood work done before starting on the drug and then once a month while taking it to ensure the proper dosage is being administered.
The drug comes with a number of potential side effects, including:
Skin pain
Itching
Reduced feeling of sensitivity to heat and cold
Joint pain and swelling
Nosebleeds
Bloated sensation in the abdomen
Excessive bruising
Nosebleeds
Vision problems, including double vision and loss of vision in one eye
Coughing up blood
Slowed heart rate
Stomach pain
Breathing difficulties
Headaches
Difficulty speaking or swallowing
Awareness of your heartbeat (palpitations)
Fainting
Anxiety
Depression
Insomnia
Weird dreams
If any of these symptoms persist or worsen, or if you notice any other unusual symptoms, it is important to see your doctor immediately.
Warfarin is an incredibly effective medication, but can cause life-threatening side effects. It is essential that you are aware of the risks and take the necessary precautions to avoid accidental overdose. It is also important that you strictly adhere to your doctor’s recommended dosage.
If you are concerned about your dosage or experience any side effects, do not hesitate to contact your physician.
There are a number of foods that can interact with Warfarin, and can increase the effects and danger to the patient. While you should always be mindful of the foods you eat while on Warfarin, it is especially important to avoid the following:
Alcohol
In general, the interaction between alcohol and Warfarin can pose a serious health threat to patients. Alcohol can increase the effects and side effects of Warfarin, as well as decrease how well the drug works. Drinking alcohol while on Warfarin is typically not recommended.
Cranberries and cranberry juice
These foods can increase how much Warfarin in your body, increasing the effects and danger to the patient. If you are taking Warfarin, be sure to avoid cranberries and cranberry juice.
Garlic and onions
These foods can also increase how much Warfarin in your body.
If you are taking Warfarin, be sure to avoid garlic and onions.
Vitamin K
High levels of Vitamin K can decrease how well Warfarin works in your body. Foods high in Vitamin K include broccoli, Brussels sprouts, cabbage, kale, and spinach.
Be sure to limit your consumption of these foods while on Warfarin. Your physician may also recommend you take a Vitamin K supplement while on the medication.
NSAID Pain Relievers
Some over-the-counter pain relievers, like ibuprofen (Motrin, Advil) and naproxen (Aleve), can increase the negative side effects of Warfarin. It is recommended that you avoid these pain relievers while on Warfarin and discuss other pain management options with your physician.
FDA Warnings
The FDA has required a number of warnings on the label of Warfarin to alert patients to serious risks and side effects. These include warnings that Warfarin:
Should not be used by patients with deep vein thrombosis, pulmonary embolism, recent stroke or heart attack, or unexplained hemorrhage.
Should not be used by women who are pregnant or may become pregnant, unless other drugs are not effective in treating the condition
If you experience any of these serious side effects, stop taking Warfarin and contact your doctor immediately or seek immediate medical attention:
Blood in vomit
Blood in urine
Black, tarry stools
Convulsions
Headaches that are severe
Shortness of breath
Slurring of speech
Sudden loss of vision or double vision
Swelling in your arms, face or legs
Unusual bruising or bleeding from minor cuts
Yellowing of the skin or eyes
Other Side Effects and Warnings
The following are other side effects that have been associated with Warfarin. These are less serious, but should be reported to your physician immediately:
Allergic reactions including hives, itching, facial swelling, wheezing, and shock
Bleeding that will not stop
Tremor or uncontrollable shaking
Difficulty speaking
Extreme fatigue (weakness, tiredness)
Inability to move your legs
Swelling in the arms or legs
If you experience any of these less serious side effects, also report them to your physician:
Depression
Diarrhea
Dizziness or light-headedness
Headache
Insomnia
Irritability
Nausea or vomiting
Skin rash, itching, or hives
Stomach pain or cramps
Trouble concentrating
Unusual excitement, nervousness or anxiety
Yellowing of the skin or eyes
It is also important to note that Warfarin should not be taken with other blood thinners, such as heparin, clopidogrel (Plavix), dipyridamole (Persantine), and many other prescription drugs. These combinations greatly increased bleeding risk.
It is also recommended that you avoid alcohol while on Warfarin. Alcohol can increase the negative effects of the drug and may also lead to increased bleeding.
High doses of Vitamin E and other supplements such as ginkgo biloba may also increase bleeding risk when taken with Warfarin.
If you are going to have surgery or dental work, talk to your physician about stopping Warfarin a few days before the procedure. Stopping the drug suddenly is not recommended and can increase the risk of bleeding.
If you believe that you may be pregnant, do not take Warfarin.
Do not change the dose of Warfarin on your own. Always consult with a medical professional to prevent complications.
Do not stop taking Warfarin, or change your dosage without first talking to your physician. Even stopping the drug for a short period of time can cause serious side effects.
Do not take any over-the-counter medications (including flea or tick medication for your pets) without first talking to your physician. Many of these contain ingredients that may increase the risk of bleeding while taking Warfarin.
If you experience any other side effects that concern you, make sure to discuss them with your physician.
NOTE: You must complete the full course of medication to make sure that your condition is no longer being affected by clots. Stopping treatment early can be dangerous.
What Conditions is Warfarin Used For?
Warfarin is used to treat and prevent blood clots. The medication prevents clots from forming or growing larger in your blood. This is important because when clots form in blood vessels they can cause death of the tissue in that area. For example, a clot in the brain can cause a stroke. A clot that forms in the lungs can cause death if not treated immediately.
There are many factors that affect your risk of forming a clot. Age is a major factor. Women generally have a greater risk of forming blood clots than men because of biological and lifestyle factors.
Certain conditions can also increase your risk of having a blood clot. These include:
Recent surgery or injury
Having had a previous blood clot
Heart disease
Pulmonary Embolism (PE) – this is when an artery in your lungs is blocked by a clot
Stroke
Certain cancers
Use of an implanted medical device, such as a heart pump or dialysis machine
Using an electric blanket
Traveling for long periods of time in a vehicle, airplane or boat
Obesity
Using birth control pills or device, or hormone replacement therapy
Pregnancy
Use of an IUD (Intra-Uterine Device) for birth control
Menopause (if you are over 55)
Smoking
Elevated cholesterol levels
Warfarin is also used to prevent the clotting of blood in donor organs.
How Long Does it Take for Warfarin to Work?
Most people will begin to see an effect in one to two days after starting the medication. The medication will reach it’s full effect in three to seven days.
NOTE: It can take up to two weeks for your blood levels to become the correct amount of Warfarin in your body. During this time you may need to have your dosage adjusted. Make sure to tell your physician if you experience any bruising or bleeding during this time.
What are the Possible Side Effects of Warfarin?
All medications have side effects. The key is weighing the benefits of the drug versus the side effects. The following are some of the more common side effects of Warfarin (note: this isn’t a complete list).
Skin rash
Headaches
Difficulty Breathing
Bruising or bleeding more easily than normal
Unusual vaginal bleeding (in females)
Mood changes
Difficulty talking
Blindness
Numbness
Tingling
Slurred speech
Muscle weakness
Severe abdominal pain
Difficulty swallowing
Painful or difficult urination
If you experience any of these or notice anything else that concerns you, make sure to discuss it with your physician.
NOTE: Side effects vary for each individual. The above side effects are based on several patients that have taken Warfarin. Your experience with the medication may be different.
What are the Possible Interactions with Other Medications?
There are several medications that can interact with Warfarin. Make sure to tell all of your healthcare providers about every prescription and over the counter medicine you are taking. This includes herbs and supplements. Be aware that herbal medicines and supplements can sometimes have an effect on the way Western medications work, same with Western medications having an effect on herbal remedies.
Some of the medications that may interact with Warfarin include:
Anticoagulants such as acenocouberol and phenindione
Birth control pills, patches or rings containing estrogen and/or progestin
Diabetes medications including Glipizide, Glimepiride, Tolazamide and Tolbutamide
Antibiotics such as Cephalexin, Ciprofloxacin and Norfloxacin
Antiviral medication such as Delavirdine, Efavirenz, Lopinavir and Ritonavir
Antifungals such as Ketoconazole, Itraconazole and Fluconazole
Stinging Nettles (Herbal Remedy)
Alcohol (especially when taken with methopheline)
Heparin
If you experience any of the following symptoms while taking Warfarin, contact your physician immediately:
Swelling
Itching
Skin Rashes
Hives
Difficulty Breathing
Wheezing
Severe Itching of the Skin
Swelling of the Face, Lips or Tongue
Difficulty Swallowing
Chest Pain
Shortness of Breath
Confusion
Fatigue
Loss of Appetite
Nausea or Vomiting
Decreased Urination
Weight Gain
Bloating
Stomach or Chest Pain
Dark Urine
Yellowing of the Skin or Eyes
How Do I Store Warfarin?
Keep out of reach of children.
Do not allow anyone else to take your medication.
Store in a cool dry place, away from direct sunlight.
What Should I Do If I Forgot to Take a Dose?
Take the missed dose as soon as you remember. If you are closer to the time of your next dose, then skip the missed dose and continue your regular dosing schedule. Do not take a double dose.
What Should I Do If I Overdose?
If you or anyone else accidentally takes too much Warfarin, immediately contact the Poison Control Center at 1-800-222-1222.
What Should I Do If I Miss a Dose?
If you miss a dose, take it as soon as you remember. If you are close to your next dose, then skip the one you missed and resume your normal dosing schedule. Do not take a double dose unless otherwise directed by your physician.
What Should I Do If I Lose My Prescription Bottle?
We are required to have a record of your prescription on file. Call our office at 555-555-1212 to provide an alternative contact method or refill information in case your bottle is lost.
What Should I Do If I Have Further Questions About My Medication?
If you have questions, call your physician. If you experience any problems with your medication, contact your physician immediately.
Sources & references used in this article:
- Human P450 metabolism of warfarin (LS Kaminsky, ZY Zhang – Pharmacology & therapeutics, 1997 – Elsevier)
https://www.sciencedirect.com/science/article/pii/S0163725896001404 - Warfarin reversal (JP Hanley – Journal of Clinical Pathology, 2004 – jcp.bmj.com)
https://jcp.bmj.com/content/57/11/1132.short - Apixaban versus warfarin in patients with atrial fibrillation (CB Granger, JH Alexander, JJV McMurray… – … England Journal of …, 2011 – Mass Medical Soc)
https://www.nejm.org/doi/full/10.1056/nejmoa1107039 - Warfarin, aspirin, or both after myocardial infarction (M Hurlen, M Abdelnoor, P Smith… – … England Journal of …, 2002 – Mass Medical Soc)
https://www.nejm.org/doi/full/10.1056/nejmoa020496 - Drug interactions with warfarin: what clinicians need to know (DN Juurlink – Cmaj, 2007 – Can Med Assoc)
https://www.cmaj.ca/content/177/4/369.short - The warfarin-aspirin symptomatic intracranial disease study (…, Warfarin-Aspirin Symptomatic Intracranial Disease … – Neurology, 1995 – AAN Enterprises)
https://n.neurology.org/content/45/8/1488.short
