Flucloxacillin-warfarin interaction: an under-appreciated phenomenon.

Current drug databases do not acknowledge an interaction between warfarin and flucloxacillin although case reports have indicated that flucloxacillin may increase warfarin requirement to maintain therapeutic international normalised ratio (INR). To assess whether flucloxacillin therapy leads to a significant increase in warfarin dose, we conducted a retrospective, observational, cohort study of hospital-in-the-home patients previously stable on warfarin; who were treated with flucloxacillin or other antibiotics for at least 2 weeks between June 2015 and December 2016. The outcome measured was change in average warfarin dose at two time periods: 1 week prior to antibiotic treatment and the final week of antibiotic treatment.