BackgroundAcyclovir is one of the most common prescribed antiviral drugs. Acyclovir nephrotoxicity occurs in approximately 12–48% of cases. It can present in clinical practice as acute kidney injury (AKI), crystal-induced nephropathy, acute tubulointerstitial nephritis, and rarely, as tubular dysfunction. Electrolytes abnormalities like hypokalemia, were previously described only when given intravenously.Case presentationA 54 year-old female presented with weakness and lower extremities paresis, nausea and vomiting after receiving oral acyclovir. Physical examination disclosed a decrease in the patellar osteotendinous reflexes (++ / ++++). Laboratory data showed a serum creatinine level of 2.1 mg/dL; serum potassium 2.1 mmol/L. Kidney biopsy was obtained; histological findings were consistent with acute tubular necrosis and acute tubulointerstitial nephritis. The patient was advised to stop the medications and to start with oral and intravenous potassium supplement, symptoms improved and continued until serum potassium levels were > 3.5 meq/L.ConclusionsThe case reported in this vignette is unique since it is the first one to describe hypokalemia associated to acute tubular necrosis induced by oral acyclovir.