Advice and Information - Warfarin and Other Medication
This fact sheet is intended to help those affected by atrial fibrillation (AF) understand warfarin and the effects other medications may have on the effectiveness of this therapy.

Warfarin is a medication which inhibits the clotting process, thereby reducing the risk of AF- related stroke.

People on warfarin are required to have regular blood tests to check for INR (international normalised ration), a measure of the blood’s clotting capability. Warfarin works by interfering with how the liver uses the Vitamin K taken in our diet. Many other medications are also broken down by the liver and they may affect how effectively warfarin thins the blood.

Medications can increase the effect of warfarin and thus a lower dose may need to be taken, or they may decrease the effect of warfarin so a higher dose may need to be taken.

People taking warfarin together with long-term medications do not need to worry, but when some medications are started, or doses are adjusted, increased monitoring of the INR is required so that the right dose of warfarin is given and the blood remains within INR range.


Medications that increase the effects of warfarin include:

• Amiodarone heart rhythm medication
• Aspirin pain killer
• Allopurinol used in gout
• Ampicillin antibiotic
• Atorvastatin cholesterol medication
• Celecoxib anti-inflammatory pain killer
• Cefalexin antibiotic
• Cefradine antibiotic
• Cimetidine indigestion and stomach ulcer healing medication
• Citalopram antidepressant
• Ciprofloxacin antibiotic
• Clarithromycin antibiotic
• Clopidogrel antiplatelet medication
• Cranberry juice drink
• Dexamethadone oral steroid medication (at high dose only)
• Diclofenac anti-inflammatory pain killer
• Dipyridamole antiplatelet medication
• Disulfiram used in alcohol misuse therapy
• Entacapone used in Parkinson’s disease
• Erythromycin antibiotic
• Esomeprazole indigestion and stomach ulcer healing medication
• Etodolac anti-inflammatory pain killer
• Ezetimibe cholesterol medication
• Fluconazole as oral antifungalnmedication
• Flutamide used in prostate cancer
• Fluoxetine antidepressant
• Glucosamine health supplement for joint pain
• Ibuprofen anti-inflammatory pain killer
• Itraconazole as oral antifungal medication
• Ketoconazole as oral antifungal medication
• Levothyroxine thyroid replacement hormone
• Mefenamic acid anti-inflammatory pain killer and used for period pain
• Meloxicam anti-inflammatory pain killer
• Metronidazole antibiotic
• Miconazoleas oral or topical anti-fungal medication
• Mirtazepine antidepressant
• Neomycin antibiotic
• Norfloxacin antibiotic
• Omeprazole indigestion and stomach ulcer healing medication
• Orlistat anti-obesity medication
• Oxytetracycline antibiotic
• Piroxicam anti-inflammatory pain killer
• Pantoprazole indigestion and stomach ulcer healing medication
• Paroxetine antidepressant
• Prednisolone oral steroid medication (at high dose only)
• Proguanil malaria treatment and prophylaxis
• Rosuvastatin chlolesterol medication
• Sertraline antidepressant
• Sibutramine anti-obesity medication
• Simvastatin cholesterol medication
• Sixaxentan used to treat hypertension
• Sodium valporate anti-epileptic medication
• Sulfinpyrazone gout medication
• Tamoxifen used in breast cancer
• Tetracycline antibiotic
• Testosterone male hormone supplement
• Tramadol pain killer
• Trimethoprim antibiotic
• Venlafaxine antidepressant


Medications that decrease the effect of warfarin include:

• Barbiturates sedative medication
• Bosentan used to treat hypertension
• Carbamazepine anti-epileptic medication
• Griseofulvin as an oral antifungal medication
• Phenytoin anti-epileptic medication
• Primidone anti-epileptic medication
• Rifampicin antibiotic
• St John’s Wort antidepressant (use with warfarin is not recommended)
• Sucralfate indigestion and stomach ulcer healing medication
• Vitamin K vitamin supplement


If you are on warfarin and you are concerned about interactions with other drugs you are taking, or you are considering taking drugs, cold and ’flu medications or herbal remedies, you should seek advice from a GP, an anticoagulation clinic, a pharmacist or a qualified healthcare professional.