Antibiotics taken with warfarin may decrease INR

IN PATIENTS on warfarin, dicloxacillin decreases INR, while phenoxymethylpenicillin and amoxycillin slightly raise levels.

A study of 240 Danish patients on warfarin, with a mean INR level of 2.59, found treatment with dicloxacillin decreased INR by a mean of 0.62.

More than 60% of patients had subtherapeutic INRs within 2–4 weeks of exposure to dicloxacillin.

Exposure to the other antibiotics led to small increases in INR of 0.10 and 0.14 respectively. Although infections may increase INR, dicloxacillin’s effect may