Drugs That Affect the INR

Drugs that increase the INR and risk of bleed

Medications in italics are liver enzyme inhibitors and increase the INR. They act very quickly (can
be within 24 hours) and if the drug is withdrawn the effect disappears quickly depending on
the drug half-life. The INR should if possible be monitored within 72 hours of starting the
interacting drug and on withdrawal.

  • Gastrointestinal: cimetidine, omeprazole. and possibly other PPIs
  • Cardiovascular: amiodarone (liver enzyme inhibition is slow and may persist long after withdrawal requiring, weekly monitoring over 4 weeks), fibrates, ezetimibe, propafenone, propranolol
  • CNS: fluvoxamine, SNRIs, SSRIs*, tramadol
  • Antiinfectives: azole antifungals (esp. miconazole including oral gel and vaginal), co-trimazole*, macrolides* (can be serious but unpredictable), metronidazole, quinolones (can be serious but unpredictable), tetracyclines
  • Endocrine: anabolic steroids (and danazol), high dose corticosteroids, glucagon (high dose 50mg+ over 2 days), flutamide, levothyroxine
  • NSAIDs: Ibuprofen at lowest effective dose (+/-PPI) is probably safest if NSAID is required
  • Miscellaneous: alcohol (acute), allopurinol, benzbromarone, colchicine, disulfiram, fluorouracil, interferon paracetamol (prolonged use at high dose), sulfinpyrazone, tamoxifen, topical salicylates, zafirlukast
  • Herbal preparations/Food supplements: carnitine, chamomile, cranberry juice, curbicin, dong quai, fenugreek, fish oils, garlic, gingo biloba, glucosamine, grapefruit juice, lycium, mango, quilinggao

Drugs that decrease the INR

Medications in italics are liver enzyme inducers and decrease the INR. They act more slowly (up to
a week) with peak effect at 2-3 weeks and can persist for up to 4 weeks after stopping
depending on drug half-life. The INR will need checking after 1 week of concurrent therapy.

  • Miscellaneous: Alcohol (chronic), azathioprine, barbiturates, bosentan, carbamazepine,
    carbimazole, griseofulvin, mercaptopurine, nevirapine, OCP/HRT, propylthiouracil, raloxifene, rifampicin (most potent inducer), trazodone
  • Herbal preparations: avocado, co-enzyme Q10, green tea, natto, soya beans, St Johns wort (avoid)
  • Binding agents: cholestyramine, sucralfate,

Drugs that increase or decrease the INR

  • Miscellaneous: Ginseng, phenytoin, quinidine