Warfarin belongs to the class of medications called anticoagulants. It is sometimes referred to as a "blood thinner," although it does not actually thin the blood.

Warfarin helps to prevent blood clots from forming or from getting bigger, but it does not dissolve blood clots. Warfarin is used for the treatment of blood clots in the veins, arteries, lungs, and heart. It is also used to prevent clots for people with conditions that put them at an increased risk of developing blood clots (e.g., some abnormal heart rhythms [atrial fibrillation], leg circulation problems, heart attack or congestive heart failure). It is also used to reduce the risk of blood clots due to surgical procedures or trauma.

Blood clots in the circulation are dangerous because they can cause medical problems such as heart attacks, stroke, and pulmonary embolism. Warfarin helps to reduce blood clotting within 24 hours of taking the medication. The full effect may take 72 to 96 hours to occur.

Warfarin works by partially blocking the reuse of vitamin K in your liver. Vitamin K is needed to make clotting factors that help the blood to clot and prevent bleeding. Vitamin K is found naturally in foods such as leafy, green vegetables, and certain vegetable oils. If you are taking warfarin, you may continue to eat these foods, but do not make any drastic changes to your diet.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

The dose of warfarin is individualized by your doctor according to blood clotting time. Blood clotting time is determined by a laboratory test, called an INR, which should be performed at regular intervals. It is very important to keep your lab appointments, as there is a narrow range between too much and too little of the medication. Too much medication may cause you to bleed more. Too little medication may lead to harmful clots forming.

Different circumstances in your life (e.g., eating certain foods or using certain medications) can cause the medication to work more or less effectively. Ask your doctor or pharmacist for a list of these foods and medications.

It is important to manage your lifestyle and habits appropriately when taking warfarin:

• do not make drastic changes in your diet, such as eating large amounts of green, leafy vegetables
• do not attempt to change your weight by dieting without first checking with your doctor or pharmacist
• do not participate in any activity or sport that may result in serious injury
• avoid drinking alcohol
• avoid drinking cranberry juice or consuming other products that contain cranberries
• avoid cutting yourself

It is very important that you take warfarin exactly as prescribed by your doctor. Take your dose of warfarin at the same time each day. If you miss a dose of warfarin, notify your health care provider right away. Take the missed dose as soon as possible and continue with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Do not take warfarin if you:

• are allergic to warfarin or any ingredients of the medication
• are pregnant
• are undergoing certain types of surgery
• are going to be receiving local anaesthetic given by injection into the spinal column
• do not have access to an adequate lab facility to get regular blood tests done
• have a high risk of abortion, eclampsia, and preeclampsia (complications of pregnancy often associated with sudden onset of very high blood pressure)
• have bleeding tendencies associated with active ulcers or overt bleeding of the stomach, genitourinary, or respiratory tracts; heart infection; brain aneurysm; or bleeding associated with many other medical conditions
• have certain blood disorders
• have malignant hypertension
• have recently had or are planning to have surgery of the central nervous system or the eye or surgery associated with trauma resulting in large open surfaces
• have senility, alcoholism, or psychosis, or other conditions where you may not be able to cooperate with taking the medication and having the necessary lab tests on a regular basis

Anticoagulation (blood thinning) should not take place in any situation where the risk of bleeding might be greater than the potential benefits of anticoagulation.

Many medications can cause side-effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• changes in skin colour
• dizziness
• fatigue
• general feeling of being unwell
• headache
• weakness

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• heavier-than-normal menstrual bleeding
• rash or itching
• signs of anemia (low red blood cells; e.g., dizziness, pale skin, unusual tiredness or weakness, shortness of breath)
• signs of clotting problems (e.g., unusual nosebleeds, bruising, blood in urine, coughing blood, bleeding gums, cuts that don't stop bleeding)
• signs of "purple toe syndrome" (e.g., changing, mottled colour of toes, pain and tenderness of toes, sores that don't heal)
• signs of serious skin damage (e.g., sores, changed skin colour or temperature)
• sudden shortness of breath
• unusual pain or swelling

Stop taking the medication and seek immediate medical attention if any of the following occur:

• signs of a serious allergic reaction (i.e., swelling of face or throat, hives, or difficulty breathing)
• signs of bleeding (dark, tarry stools, bleeding in eye, blood in stools, blood in vomit or vomit that looks like coffee grounds, blood in urine, coughing up blood)
• signs of bleeding in the brain (e.g., sudden or severe headache; sudden loss of coordination; vision changes; sudden slurring of speech; or unexplained weakness, or numbness)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin using a medication, be sure to inform your doctor of any medication conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Bleeding: The most serious risk associated with warfarin is bleeding in any tissue or organ. The risk of bleeding is related to how much is taken and for how long.

It is extremely important to have regular blood tests (as recommended by your doctor) to ensure that the correct level of blood thinning is occurring. These blood tests measure your INR level to determine the dose of warfarin. Your health care provider will adjust the dose of warfarin depending on the INR level to ensure you are not receiving too little medication (which may result in blood clots forming) or too much medication (which may result in bleeding).

Congestive heart failure: People with congestive heart failure may be more sensitive to the effects of warfarin. If you have any heart problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Dental or surgical treatment: It may be necessary to stop taking warfarin briefly before scheduled dental or surgical procedures. Make sure that everyone involved in your health care is aware that you are taking this medication.

Foods and supplements: Many foods and food ingredients can cause changes to the way warfarin acts in the body. Avocado, cocoa, cranberry, and green tea can influence the effects of warfarin. Ginger and garlic can affect warfarin use by the body, as can other herbal products such as gingko biloba, saw palmetto, and milk thistle. Talk to your doctor or pharmacist about using these products.

Kidney function: People with decreased kidney function are more likely to experience injury to the kidneys when taking warfarin. Individuals with long-term kidney disease may experience calcium accumulation causing blood vessels to harden.

If you have kidney problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Liver function: Liver disease or reduced liver function may cause this medication to build up in the body, causing side effects, including severe bleeding. If you have liver problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Medical conditions and other medications: If you have other medical conditions and are taking medications, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Some conditions and medications affect the way warfarin works and may affect the dosing of warfarin. Your doctor will recommend you get regular lab tests done.

Non-steroidal anti-inflammatory medications (NSAIDs): It is recommended that anyone taking NSAIDs such as acetylsalicylic acid (ASA) or ibuprofen should be closely monitored to ensure that no change in anticoagulation dosage is required. NSAIDs can cause stomach ulcers or bleeding. If you notice any signs of bleeding, such as frequent nosebleeds, unexplained bruising, or black and tarry stools, notify your doctor as soon as possible.

Purple toes syndrome: Purple toes syndrome is a complication of warfarin treatment and consists of a dark, purplish or mottled colour of the toes, usually occurring 3 to 10 weeks (or later) after starting treatment with warfarin. Major features of this syndrome include:

• increasing and decreasing of the colour over time
• pain and tenderness of the toes
• purple colour of bottom surfaces and sides of the toes that turns white under moderate pressure and fades with elevation of the legs

Inform your doctor at once if you notice these symptoms.

Pregnancy: Warfarin should not be used during pregnancy. It may pass to the developing fetus and cause birth defects or death as a result of bleeding. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: It is not known if warfarin passes into breast milk. If you are breast-feeding and are taking this medication, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Children: The safety and effectiveness of this medication have not been established for children less than 18 years of age. However, the use of warfarin by children is necessary in certain situations.

There may be an interaction between warfarin and any of the following:

• acetaminophen
• acetylsalicylic acid (ASA)
• adalimumab
• alcohol
• alemtuzumab
• allopurinol
• alteplase
• aminosalicylate drugs (e.g., mesalamine, olsalazine, sulfasalazine)
• amitriptyline
• amoxicillin
• anagrelide
• antiarrhythmics (e.g., amiodarone, dronedarone, propafenone)
• anticancer medications (e.g., capecitabine, fluorouracil, ifosfamide, mercaptopurine, methotrexate)
• apalutamide
• apixaban
• aprepitant
• atovaquone
• azathioprine
• "azole" antifungals (e.g., fluconazole, ketoconazole, voriconazole)
• barbiturates (e.g., butalbital, phenobarbital)
• bicalutamide
• birth control pills
• bismuth subsalicylate
• bosentan
• caffeine
• cannabis
• cephalosporins (e.g., cefazolin, cefepime, cefotetan, cefoxitin, cefuroxime, cephalexin)
• CF transmembrane regulators (e.g., elexacaftor, ivacaftor, lumacaftor, tezacaftor)
• chloral hydrate
• cholestyramine
• cimetidine
• clopidogrel
• cloxacillin
• cobicistat
• colesevelam
• coenzyme Q10
• colestipol
• corticosteroids (e.g., dexamethasone, hydrocortisone, prednisone)
• cranberry
• dabigatran
• danazol
• dapsone
• deferasirox
• dipyridamole
• dong quai
• edoxaban
• entacapone
• enzalutamide
• estrogens (e.g., conjugated estrogen, estradiol, ethinyl estradiol)
• ethacrynic acid
• fibrates (e.g., bezafibrate, fenofibrate, gemfibrozil)
• garlic
• ginger
• ginkgo biloba
• ginseng
• glucagon
• glucosamine
• goji
• green tea
• heparin
• hepatitis C antivirals (elbasvir and grazoprevir, glecaprevir and pibrentasvir, ledipasvir, voxilaprevir)
• HIV non-nucleoside reverse transcriptase inhibitors (NNRTIs; e.g., efavirenz, nevirapine)
• HIV protease inhibitors (e.g., atazanavir, darunavir, lopinavir, ritonavir, tipranavir)
• icosapent ethyl
• interferon
• isoniazid
• ivermectin
• lactulose
• leflunomide
• letermovir
• levocarnitine
• linezolid
• lomitapide
• low molecular weight heparins (e.g., dalteparin, enoxaparin, tinzaparin)
• macrolide antibiotics (e.g., azithromycin, clarithromycin, erythromycin)
• mavacamten
• mefloquine
• metformin
• methimazole
• metronidazole
• mifepristone
• milk thistle
• mirtazapine
• multivitamins with vitamin A, D, E or K
• nabilone
• nirmatrelvir
• nonsteroidal anti-inflammatory drugs (NSAIDs; e.g., celecoxib, diclofenac, ibuprofen, naproxen)
• obeticholic acid
• obinutuzumab
• omega-3 fatty acids
• orlistat
• peginterferon
• penicillins (e.g., ampicillin, penicillin, piperacillin)
• pentoxifylline
• phytonadione (Vitamin K)
• prasugrel
• progestins (e.g., dienogest, drospirenone, levonorgestrel, medroxyprogesterone, norethindrone)
• proguanil
• propylthiouracil
• protein kinase inhibitors (e.g., ceritinib, dasatinib, imatinib, lorlatinib)
• proton pump inhibitors (PPIs; e.g., esomeprazole, lansoprazole, omeprazole)
• quetiapine
• quinolone antibiotics (e.g., ciprofloxacin, levofloxacin, moxifloxacin, norfloxacin)
• rifabutin
• rifampin
• rifaximin
• rivaroxaban
• ropinirole
• sarilumab
• satralizumab
• St. John's wort
• saw palmetto
• seizure medications (e.g., carbamazepine, divalproex, phenytoin, primidone, valproic acid)
• selective serotonin reuptake inhibitors (SSRIs; e.g., citalopram, fluoxetine, paroxetine, sertraline, vortioxetine)
• serotonin/norepinephrine reuptake inhibitors (SNRIs; e.g., desvenlafaxine, duloxetine, venlafaxine)
• siltuximab
• "statins" (e.g., fluvastatin, rosuvastatin, simvastatin)
• sucralfate
• sulfonamide antibiotics (e.g., sulfadiazine, sulfamethoxazole)
• sulfonylurea diabetes medications (e.g., gliclazide, glyburide)
• tamoxifen
• teriflunomide
• testosterone
• tetracyclines (e.g., doxycycline, minocycline, tetracycline)
• thyroid hormones (e.g., levothyroxine, liothyronine, thyroid)
• ticagrelor
• tobacco (smoked)
• tocilizumab
• tolterodine
• tramadol
• trazodone
• vilazodone
• vitamin E

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription) and herbal medications that you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or illegal drugs can affect the action of many medications, you should let your prescriber know if you use them.