Warfarin interactions

Many medicines can interfere with the way warfarin works, including prescription and non-prescription medicines, and complementary medicines such as vitamins and herbs. (See What is a medicine?)

Watch our video, in which specialist Dr John Worthington explains why it is important to have more frequent INR tests if you start or stop taking any other medicine while you are taking warfarin.

Be medicinewise about warfarin interactions

  • Always tell your doctor, pharmacist and other healthcare providers that you are taking warfarin, especially before starting a new prescription, non-prescription or complementary medicine.
  • Always check with your doctor or pharmacist before making any changes to your current medicines.
  • Find out how your other medicines may interact with warfarin. Your doctor can make minor adjustments to your dose to manage many interactions.
  • For information on medicines interactions, you can ask your doctor or pharmacist, or read the consumer medicines information (CMI).
  • Keep a list of all the medicines you are taking — including any vitamins, herbal remedies and over-the-counter medicines — to show your healthcare providers, so they can check for any possible medicine interactions.
  • Have your INR blood test done whenever your doctor schedules this after you start or stop a medicine.
  • Wear a MedicAlert bracelet to show that you take warfarin.

Which medicines interact with warfarin?

Because so many medicines can affect warfarin, it is safer to assume that any medicine may affect warfarin, and get in the habit of checking with your health professional before making changes. Always check with your doctor or pharmacist before you start or stop taking any medicine.

Some medicines need to be avoided altogether, while other medicines may have a weaker interaction, or none at all. For some medicines that interact, you may still be able to take them, but you may need to have more frequent blood testing (INR monitoring) when you start (or stop) taking them. This enables you and your doctor to check if the medicine is interfering with your warfarin and to adjust the dose as needed.

Below are some examples of commonly used medicines that can interfere with warfarin, but it is not a complete list:

  • Some antibiotics
ciprofloxacin (e.g. Ciproxin), erythromycin (e.g. Eryc), metronidazole (e.g. Flagyl), sulfamethoxazole with trimethoprim (e.g. Bactrim)
  • Anti-inflammatory and pain reliever medicines, including those used to treat arthritis
aspirin (e.g. Solprin), meloxicam (e.g. Mobic), celecoxib (e.g. Celebrex), ibuprofen (e.g. Nurofen), indomethacin (e.g. Indocid), methyl salicylate (e.g. Deep Heat Cream), paracetamol (e.g. Panadol) – when used more often than usual
  • Some heart and cholesterol medicines
aspirin (e.g. Cartia), clopidogrel (e.g. Plavix), amiodarone (e.g. Cordarone), simvastatin (e.g. Zocor), fenofibrate (e.g. Lipidil)
  • Some ulcer and reflux medicines
esomeprazole (e.g. Nexium), cimetidine (e.g. Tagamet), omeprazole (e.g. Losec)
  • Some antidepressants
fluoxetine (e.g. Prozac), fluvoxamine (e.g. Luvox), paroxetine (e.g. Aropax)
  • Corticosteroids
prednisolone (e.g. Solone)
  • Thyroid hormones
Such as thyroxine (e.g. Oroxine)
  • Some anti-epileptic medicines
Carbamazepine (e.g. Tegretol)
  • Many cold and flu medicines
Some products contain medicines (e.g. paracetamol) which may interfere with warfarin
  • Some antifungals
Miconazole (e.g. Daktarin), fluconazole (e.g. Diflucan), griseofulvin (e.g. Grisovin)
  • Some vitamins, herbs and other complementary medicines
Such as vitamin K, vitamin E, fish oil, glucosamine, feverfew, garlic, ginkgo biloba, St John’s wort

These medicines may either increase the clotting risk or increase the bleeding risk of warfarin. Do not start or stop taking any medicine without speaking to your doctor or pharmacist.