Decreased hypoprothrombinemic response to warfarin secondary to the warfarin-nafcillin interaction.

Patients who are receiving multiple medications must be screened for significant drug interactions. The specific mechanism of a drug interaction determines whether the patient may experience a subtherapeutic effect or a potentially toxic reaction. Three cases of relative warfarin resistance, possibly related to high-dose nafcillin, are described in this report. Several reports have suggested that penicillinase-resistant penicillins, such as nafcillin and dicloxacillin, exert an important enzyme-inducing effect in patients receiving warfarin. This potential interaction must be appropriately recognized and managed in order to maintain adequate anticoagulation in this patient population.