Nafcillin and Hypokalemia: A Forgotten Adverse Effect

Hypokalemia is a common yet often overlooked electrolyte disturbance in hospitalized patients. Among the various etiologies, drug-induced hypokalemia remains underrecognized. Nafcillin, a beta-lactam antibiotic widely used for methicillin-sensitive Staphylococcus aureus (MSSA) infections, has been infrequently reported as a cause of renal potassium wasting. We present the case of a 67-year-old male hospitalized for MSSA bacteremia complicated by epidural and shoulder abscesses, ultimately requiring extensive surgical interventions and long-term intravenous nafcillin therapy. During his hospital course, the patient developed persistent hypokalemia that was resistant to supplementation. A thorough evaluation excluded gastrointestinal, renal, and endocrine causes. The calculated fractional excretion of potassium (FEK) was 17.7%, indicating renal potassium wasting. The hypokalemia was attributed to nafcillin therapy and managed successfully with aggressive potassium and magnesium repletion. This case highlights nafcillin as a potential but underreported cause of renal potassium wasting. Clinicians should consider nafcillin-induced hypokalemia in patients receiving prolonged therapy, particularly when hypokalemia persists despite adequate supplementation and resolution of other contributing factors. Routine electrolyte monitoring and early intervention can mitigate complications and ensure safe continuation of antibiotic treatment.