Warfarin (Intravenous)

Warfarin given intravenously is usually used for short-term treatment. Many people who need ongoing treatment will be transferred on to oral warfarin after an initial period of using the intravenous version. The US brand name for warfarin (intravenous route) is Coumadin.


Warfarin acts as an anticoagulant agent, helping to prevent clots from forming in the blood in veins and arteries. It is sometimes described as a blood "thinner". Vitamin K is blocked by warfarin to preclude the production of clotting factors. This means that the blood retains its liquid form and does not thicken and coagulate into lumps, which can have potentially fatal consequences. Warfarin cannot eliminate an established clot or thrombus, but it can prevent the clot from getting larger. It also works to prevent a thromboembolism, where a clot travels in the body and blocks a blood vessel. Clots are most serious when they move to the brain (stroke), lungs (pulmonary embolism), kidney, legs or gastrointestinal tract.

The effects of warfarin administered intravenously wear off more quickly than oral warfarin. This means that intravenous warfarin treatment is ideal for "bridging", whereby it is used temporarily in place of oral warfarin in the period before patients are due to undergo surgery. Warfarin treatment will need to be ceased altogether shortly before any surgery to ensure that the blood is able to clot. Once cuts have had sufficient time to heal, the use of warfarin may be resumed.

Type of medicine

  • Anticoagulant
  • Blood thinner

Conditions treated

  • Arrhythmia (heart rhythm disorders)
  • Blood clots arising from open-heart surgery
  • Deep vein thrombosis (DVT)
  • Heart attack
  • Heart valve replacement
  • Pulmonary embolism
  • Stroke

Side effects

While warfarin has the potential to save your life, like many medicines, it has a range of possible side effects, ranging in severity from inconvenient to potentially dangerous.

In rare cases, it is possible to experience an allergic reaction to warfarin. If you have any of the following symptoms, you should get emergency medical help immediately:

  • Difficulty breathing
  • Facial swelling - lips, tongue or throat
  • Hives (red bumps on the skin, usually itchy)

Be sure to make your doctor aware of any allergies you have, including allergies to foods, preservatives, dyes etc. If you have ever experienced an allergic reaction to any type of warfarin, you must alert your doctor to this before any further warfarin treatment.

If you cut yourself accidentally while taking warfarin you may find that the bleeding is heavy, as the blood is unable to clot as easily. In very rare cases, warfarin can cause profuse bleeding if there is a major wound. If unchecked the bleeding can in very rare cases be fatal.

Some research indicates that cases of heavy bleeding are more probable during the first four weeks of using warfarin rather than later in the course of treatment.

You are at greater risk of heavy bleeding if you have:

Other medical problems which you should mention to your doctor include spinal anesthesia, and any surgery, either recent or planned.

Other possible side effects

Gangrene and necrosis

Gangrene and skin necrosis (skin tissue dying) are possible side effects, so if you are having intravenous warfarin treatment, you should be watchful for symptoms such as a purple hue to the skin. This side effect is most likely to occur within eight days of your warfarin treatment commencing.


This rare condition is caused by the build-up of deposits of calcium in the blood vessels. Calciphylaxis is uncommon but the risk is increased if you already suffer from kidney disease. Conditions such as diabetes and atherosclerosis can exacerbate calciphylaxis.

It is very important that you tell caregivers and medical professionals that you have been receiving warfarin treatment. Even if you are having minor procedures such as a visit to the dental hygienist or a vaccination, you should always inform your dentist, nurse or doctor. You may choose to carry a card or bracelet which, in the event of a medical emergency, alerts people to the fact that you are on warfarin treatment.

You can minimise the risk of bleeding by making a few changes to your hygiene and grooming routine. For example, using waxed dental floss, a soft-bristle toothbrush and choosing an electric razor can all help to avoid unnecessary bleeding.

You may be advised to limit or avoid contact sports and other activities which could significantly increase the likelihood of a head injury.

Some symptoms require immediate attention, and you should seek professional medical advice immediately. See Warnings below for further details.


There is a risk of harm to the unborn child if warfarin is used during pregnancy. The risk is particularly significant during the first trimester. If you are undergoing intravenous warfarin treatment, speak to your medical practitioner before planning a pregnancy or if there is a possibility that you may be pregnant. Some women are transferred onto alternative treatments during pregnancy and delivery.

The exception to this contraindication is for women who have a mechanical heart valve. In this case, they may need to continue with warfarin treatment through pregnancy.


Studies have shown that there is minimal risk of any side effects on the infant if warfarin is taken by a breastfeeding mother.


There is no conclusive evidence to demonstrate whether age affects the safety or efficacy of warfarin injections for children.


Studies have not identified additional risks for older people taking warfarin intravenously. However, the dosage may need to be adjusted for elderly people who are more at risk of bleeding, for example if they are prone to falls.


Intravenous warfarin is injected through a needle into the vein and is less commonly used than oral warfarin. Unlike the oral version of warfarin, the intravenous form of warfarin must be administered by your doctor or nurse, or must be taken under medical supervision. The injection will take no more than two minutes.

Your blood clotting level will need to be monitored regularly by your doctor while you are having warfarin. The international normalized ratio (INR) test measures how quickly your blood is able to clot. Your INR value must be maintained at a level appropriate to your condition, so your doctor will need to carry out regular tests and adjust the dosage administered accordingly. The INR is usually recommended to be below 4.0, as anything higher increases the risk of bleeding, without offering any additional medical benefit. Generally, a range between 2.5 - 3.0 is set for the target INR. Testing for INR will vary according to your condition, but a frequency of between one and four weeks is usually recommended.

Warfarin dosage has to be adjusted and temporarily stopped for patients who are about to have surgery. This allows the clotting factors in the blood to become effective again so that surgical wounds can heal. Warfarin treatment can be resumed shortly afterward.

The duration of your course of treatment will vary according to the nature of your condition. Warfarin injections are usually given for a matter of weeks or months, and oral warfarin is more likely to be used for conditions requiring longer term anticoagulant treatment.

Warfarin can take around three days for the full anticoagulant effect to take place. If you are transferring to warfarin from another anticoagulant, such as heparin, you may need a short transition phase where you are treated with both medicines for a few days, until the warfarin becomes fully effective.



Some foods and drinks can alter the effects of warfarin. Your doctor will give you full details of any dietary restrictions, and it is important that you follow the instructions of your doctor.

Vitamin K

As far as possible, you need to maintain a steady amount of Vitamin K in your body while you are on warfarin. This is because warfarin works by blocking the action of Vitamin K in the body. Blood clots are formed when clotting factors in the blood are activated by platelets grouping together in clusters. Blood-clotting factors or fibrinogens are proteins which are produced in the liver, and the effective production of them depends on the presence of Vitamin K. Insufficient Vitamin K in the diet results in a risk of heavy bleeding, while excess Vitamin K leads to warfarin being unable to prevent clotting. Therefore, changes in the amount of Vitamin K consumed and absorbed quickly have an impact on the dosage of warfarin required.

Foods which can significantly increase the effect of warfarin, and therefore should be avoided, include:

  • Cranberry
  • Grapefruit

Other food, drink, and herbal products which may affect your warfarin treatment include:

  • Asparagus
  • Avocado
  • Bee pollen
  • Black tea
  • Broccoli
  • Brussels sprouts
  • Canola oil
  • Chamomile
  • Coenzyme Q10
  • Curcumin
  • Dong quai
  • Fish oil
  • Garlic
  • Ginkgo biloba
  • Ginger
  • Green tea
  • Green leafy vegetables
  • High protein food
  • Liver
  • Papaya
  • Plums
  • Pomegranate
  • Rhubarb
  • Spinach
  • Soybean
  • St John's Wort
  • Vitamin K-rich food

Alcohol and tobacco can cause interactions with warfarin. Your doctor may instruct you to limit your consumption of these products if appropriate to minimise the risk of problems arising during your warfarin treatment.

Medication, remedies and supplements

Over the counter medicines and prescription medications may affect how your warfarin treatment works. For example, you should check before taking aspirin, ibuprofen or other painkillers. Many antibiotics impact the efficacy of warfarin, and medication for allergies can have an impact on warfarin treatment too. Even some cold and flu medicines may be unsuitable, so be sure to seek medical advice before using.

Herbal remedies, food supplements and vitamins can also affect your warfarin treatment. Always consult your medical practitioner prior to taking any new medicine, supplement or remedy to ensure that it is safe to take in conjunction with warfarin. If you go to hospital for any form of procedure, operation or treatment, always tell the nurses and surgeons about your warfarin treatment.

Medicines which should not be used in conjunction with warfarin:

  • Amifampridine
  • Defibrotide
  • Tamoxifen

There are many medicines which are not recommended for use with warfarin,

  • Abciximab
  • Aceclofenac
  • Acemetacin
  • Acenoccoumarol
  • Alefacept
  • Alipogene Tiparvovec
  • Alteplase, Recombinant
  • Amiodarone
  • Amoxicillin
  • Ampicillin
  • Amtolmetin Guacil
  • Anagrelide
  • Anistreplase
  • Apixaban
  • Aprepitant
  • Aspirin
  • Azithromycin
  • Bemiparin
  • Bivalirudin
  • Bilnatumomab
  • Bromfenac
  • Bufexamac
  • Cabozantinib
  • Capecitabine
  • Carbenicillin
  • Carboplatin
  • Cefadroxil
  • Cefdinir
  • Cefepime
  • Cefixime
  • Cefotaxime
  • Cefpodoxime
  • Ceftaroline Fosamil
  • Ceftazidime
  • Ceftibuten
  • Ceftizoxime
  • Celecoxib
  • Cephalexin
  • Cephalothin
  • Cephapirin
  • Cephradine
  • Ceritinib
  • Choline Salicylate
  • Ciprofibrate
  • Ciprofloxacin
  • Citalopram
  • Clarithromycin
  • Clonixin
  • Clopidogrel
  • Cloxacillin
  • Cobicistat
  • Collagenase, Clostridium histolyticum
  • Cyclophosphamide
  • Dabrafenib
  • Dalteparin
  • Danaparoid
  • Dapsone
  • Deferasirox
  • Dexibuprofen
  • Dexketoprofen
  • Diclofenac
  • Dicloxacillin
  • Diflunisal
  • Dihydroartemisinin
  • Dipyridamole
  • Dipyrone
  • Doxifluridine
  • Doxorubicin
  • Doxorubicin Hydrochloride Liposome
  • Dronedarone
  • Drotecogin Alfa
  • Droxicam
  • Econazole
  • Edoxaban
  • Efavirenz
  • Elvitegravir
  • Enoxacin
  • Enoxaparin
  • Entacapone
  • Enzalutamide
  • Eptifibatide
  • Erlotinib
  • Erythromycin
  • Escitalopram
  • Etodolac
  • Etofenamate
  • Etoposide
  • Etoricoxib
  • Etravirine
  • Felbinac
  • Fenofibrate
  • Fenofibric Acid
  • Fenoprofen
  • Fepradinol
  • Feprazone
  • Fluconazole
  • Floctafenine
  • Fluconazole
  • Flufenamic Acid
  • Fluorouracil
  • Fluoxetine
  • Flurbiprofen
  • Fluvoxamine
  • Gatifloxacin
  • Gemifloxacin
  • Golimumab
  • Ibuprofen
  • Iloprost
  • Imatinib
  • Indomethacin
  • Infliximab
  • Influenza Virus Vaccine
  • Itraconazole
  • Ketoconazole
  • Ketoprofen
  • Ketorolac
  • Leflunomide
  • Lepirudin
  • Levocarnitine
  • Levofloxacin
  • Levomilnacipran
  • Linezolid
  • Lomitapide
  • Lornoxicam
  • Loxoprofen
  • Lumiracoxib
  • Lycium
  • Mechlorethamine
  • Mefenamic Acid
  • Meloxicam
  • Mercaptopurine
  • Methicillin
  • Methotrexate
  • Methyl Salicylate
  • Metronidazole
  • Miconazole
  • Milnacipran
  • Mirtazapine
  • Morniflumate
  • Moxalactam
  • Moxifloxacin
  • Nabumetone
  • Nadroparin
  • Nafcillin
  • Nalidixic Acid
  • Naproxen
  • Nepafenac
  • Niflumic Acid
  • Nilotinib
  • Nimesulide
  • Nimesulide Beta Cyclodextrin
  • Nintedanib
  • Norfloxacin
  • Noscapine
  • Obeticholic Acid
  • Ofloxacin
  • Oritavancin
  • Oseltamivir
  • Oxacillin
  • Oxandrolone
  • Oxaprozin
  • Oxyphenbutazone
  • Parecoxib
  • Paroxetine
  • Peginterferon Alfa-2b
  • Penicillin (G / V)
  • Pentosan Polysulfate Sodium
  • Phenindione
  • Phenprocoumon
  • Phenylbutazone
  • Piketoprofen
  • Piperacillin
  • Piracetam
  • Piroxicam
  • Pixantrone
  • Posaconazole
  • Pranoprofen
  • Prasugrel
  • Procarbazine
  • Proglumetacin
  • Proguanil
  • Propyphenazone
  • Proquazone
  • Regorafenib
  • Reteplase, Recombinant
  • Rivaroxaban
  • Rofecoxib
  • Ropinirole
  • Roxithromycin
  • Secnidazole
  • Secukinumab
  • Sertraline
  • Siltuximab
  • Simvastatin
  • Sitaxsentan
  • Streptokinase
  • Sulfamethoxazole
  • Sulfisoxazole
  • Sulindac
  • Sultamicillin
  • Tan-Shen
  • Tegafur
  • Telithromycin
  • Tenecteplase
  • Tenoxicam
  • Teriflunomide
  • Testosterone
  • Tiaprofenic Acid
  • Ticarcillin
  • Ticlopidine
  • Tinzaparin
  • Tirofiban
  • Tolfenamic Acid
  • Tolmetin
  • Torsemide
  • Treprostinil
  • Urokinase
  • Valdecoxib
  • Valproic Acid
  • Venlafaxine
  • Vilazodone
  • Vincristine
  • Vincristine Sulfate Liposome
  • Vindesine
  • Vorapaxar
  • Vortioxetine

Any of the following medicines combined with warfarin could increase the probability of some side effects occurring. However, your doctor may decide that both medicines are required for your treatment, in which case he or she will monitor your reaction to the medicines and adjust dosages as necessary.

  • Acarbose
  • Acetaminophen
  • Aminoglutethimide
  • Amitriptyline
  • Amprenavir
  • Apazone
  • Argatroban
  • Atovaquone
  • Azathioprine
  • Balsalazide
  • Benzbromarone
  • Bosentan
  • Butabarbital
  • Butalbital
  • Carbamazepine
  • Cefamandole
  • Cefazolin
  • Ceftriaxone
  • Chitosan
  • Chloral Hydrate
  • Chlorthalidone
  • Cholestyramine
  • Chondroitin
  • Cimetidine
  • Cisapride
  • Cisplatin
  • Coenzyme Q10
  • Colesevelam
  • Cyclosporine
  • Danazol
  • Desogestrel
  • Dexamethasone
  • Dexlansoprazole
  • Dienogest
  • Disopyramide
  • Disulfram
  • Doxepin
  • Drospirenone
  • Duloxetine
  • Eslicarbazepine Acetate
  • Esomeprazole
  • Estradiol Cypionate
  • Estradiol Cypionate
  • Estradiol Velerate
  • Eterobarb
  • Ethinyl Estradiol
  • Ethynodiol Diacetate
  • Etonogestrel
  • Exenatide
  • Felbamate
  • Fluoxymesterone
  • Fluvastatin
  • Gefitinib
  • Gemcitabine
  • Gemfibrozil
  • Ginseng
  • Glucosamine
  • Glyburide
  • Griseofulvin
  • Heparin
  • Ifosfamide
  • Indoprofen
  • Isoniazid
  • Isoxicam
  • Ivermectin
  • Lactulose
  • Lansoprazole
  • Levamisole
  • Levonorgestrel
  • Lopinavir
  • Medroxyprogesterone Acetate
  • Melatonin
  • Menthol
  • Mephobarbital
  • Mesalamine
  • Mesna
  • Mestranol
  • Methyltestosterone
  • Mitotane
  • Moricizine
  • Nelfinavir
  • Nevirapine
  • Niacin
  • Nilutamide
  • Norelgestromin
  • Norethindrone
  • Norgestimate
  • Norgestrel
  • Omeprazole
  • Oxypurinol
  • Pantoprazole
  • Pentoxifylline
  • Phenobarbital
  • Phytonadione
  • Prednisone
  • Primidone
  • Propafenone
  • Norelgestromin
  • Norethindrone
  • Norgestimate
  • Norgestrel
  • Omeprazole
  • Oxypurinol
  • Pantoprazole
  • Pentoxifylline
  • Phenobarbital
  • Phytonadione
  • Prednisone
  • Primidone
  • Propafenone
  • Propoxyphene
  • Quetiapine
  • Ranitidine
  • Rifabutin
  • Rifampin
  • Ritonavir
  • Rosuvastatin
  • Saquinavir
  • Secobarbital
  • Sorafenib
  • Stanozolol
  • Sucralfate
  • Sulfasalazine
  • Sulfinpyrazone
  • Tenidap
  • Terbinafine
  • Tigecycline
  • Tolterodine
  • Tramadol
  • Trastuzumab
  • Vancomycin
  • Venetoclax
  • Vitamin A
  • Vitamin E
  • Vorinostat
  • Zafirlukast
  • Zileuton

There may be other medications which could be unsuitable in conjunction with warfarin. You should always discuss any medication you are currently using with your doctor prior to taking any form of warfarin. Your doctor may be able to prescribe a suitable dosage to allow you to use such medicines with an appropriate level of warfarin, but there may be a greater risk of some side effects occurring.

Just as prescription and over the counter medicines can disrupt warfarin, so illegal substances such as marijuana can cause complications if taken while you are having warfarin treatment.


If you experience any of the following symptoms while on warfarin you should seek professional medical advice immediately:

  • Unexplained bleeding
  • Unexplained nosebleeds
  • Bleeding gums
  • Heavy bleeding from a cut or wound
  • Blood coughed up in mucus
  • Vomiting blood or a substance with the appearance of coffee grounds
  • Blood in the stools or from the rectum
  • Heavier menstrual periods than usual or bleeding from the vagina
  • Nausea, fits or loss of consciousness
  • Blurred vision, numbness, dizziness, slurred speech,
  • Severe unexplained headache
  • Toes turn a bluish purple colour and feel painful
  • Yellow tone to skin and whites of eyes
  • Severe abdominal pain
  • Joint pain or inflammation
  • Unexplained bruising


Warfarin for intravenous use is supplied as a white crystalline powder. It should be protected from moisture and light. However, since intravenous warfarin is administered by a medical professional, you will not usually need to store the medicine yourself. Warfarin powder is very soluble in water, and for intravenous use, it is combined with sterile water to make a solution. Once the solution is made up it should be used within four hours. The injection will usually be carried out by a doctor or nurse.


Warfarin offers significant, often life-saving, benefits for the patient. It prevents the formation of clots in the blood vessels and can help to stop existing clots from moving within the body to a site where they can cause a blockage, possibly impacting a vital organ, with a potentially fatal outcome.

However, there are potential risks, and these are greater for certain categories of patient, i.e. those with a range of underlying conditions such as kidney disease. Warfarin can trigger an allergic reaction in some people. Emergency medical care is needed if symptoms such as hives or difficulty breathing present themselves. There is a range of side effects which people taking warfarin may experience. These may be simply unpleasant or in rare situations possibly fatal, for example, if kidney damage occurs. Therefore, it is essential that warfarin patients have regular check-ups. Throughout your course of warfarin treatment, you will need to have regular blood tests to check that your INR is at an acceptable level. The INR test ensures that blood clotting factors are not at dangerously low levels, whilst balancing the need to minimise the risk of harmful blood clots forming.

There are many medications with which warfarin does not combine well, and which may need to be avoided, or used in carefully managed dosage. You also need to pay attention to your diet to ensure that you do not consume food or drinks which can affect the efficacy of the warfarin.

Last Reviewed:
December 25, 2017
Last Updated:
April 27, 2018