Warfarin Interactions

Ciprofloxacin prolongs half life of warfarin (it’s a p450 inhibitor) so raises INR (although studies fail to establish a predictable reaction).

All quinolone antibiotics (ciprofloxacin, levofloxacin, moxifloxacin etc) have reports of raised INR.

St Johns Wort is an enzyme inducer so can make warfarin less effective.

Mnemonics: Erections go up, Cobras will being you down!

Drugs that interact with warfarin to raise INR: (INR goes up – erections go up)

  • …floxacins
  • Allopurinol
  • Clarithrimycin
  • Amiodarone
  • Cimetidine
  • Miconazole (and the other azoles)
  • SSRIs (sertaline)
  • Ezetimide

For all aged men sidenafil can cause erections

…floxacins, amiodarone, allopurion, miconaxole, ssris, cimetidine, clarithrimycin, ezetimide

Drugs that reduce INR – Enzyme inducers:


  • Carbamazepine
  • Carbomazole
  • Ocp/ hrt
  • Barbiturates
  • Azithiaprine
  • Rifampicin
  • St. John’s wart


Carbimazole/carbamazepine, OCP/HRT, Barbituates, Rifampicin, Azothiaprine, St Johns Wort