The effects of the oral anticoagulants are not normally altered by the
penicillins but isolated cases of increased prothrombin times and/or bleeding have been seen in patients given
amoxicillin, ampicillin/flucloxacillin, benzylpenicillin, co-amoxiclav, or talampicillin. Carbenicillin, in the absence of an anticoagulant, can prolong prothrombin times. In contrast, a handful of cases of reduced
warfarin effects have been seen with dicloxacillin, nafcillin, and possibly
amoxicillin.
Even though the general picture is of no interaction some individual physicians say that they have seen changes with otherwise normally non-interacting
penicillins. It has been said that experience in anticoagulant clinics suggests that the INR can be altered by courses of broad-spectrum antibacterials such as
ampicillin, even though studies do not demonstrate an interaction. For this reason consider monitoring concurrent use so that the very occasional and unpredictable cases (increases or decreases in the anticoagulant effects) can be identified and handled accordingly. Any significant changes appear to occur after 4 days of concurrent use.