INTRODUCTION: Terbinafine (Lamisil), a common treatment for onychomycosis, is infrequently associated with hepatotoxicity. Asymptomatic transaminase elevations occur in < 1% of users with normalization typically occurring after Terbinafine discontinuation. Rare cases of severe hepatic injury requiring transplantation have been reported. While most studies recommend against routine lab monitoring with Terbinafine, the potential seriousness of hepatotoxicity may warrant specific guidelines for testing of liver enzymes during treatment. We report an unusual case of Terbinafine-induced liver injury. CASE DESCRIPTION/METHODS: A 28-year-old male without previously diagnosed medical conditions presented with jaundice, dark colored urine, and itching of his abdomen, back, palms, and soles. He had been on Terbinafine 250 mg once a day for onychomycosis for 45 days. His pre-treatment liver function tests were normal. He occasionally used Ibuprofen. He denied other medications, herbal products and health supplements. He denied alcohol, tobacco and recreational drug use. His exam was notable for jaundice with no stigmata of chronic liver disease. Labs included a normal CBC, elevated liver enzymes [AST 189 U/L, ALT 526 U/L, bilirubin 4.7 mg/dL (direct 1.8), alkaline phosphatase 238 U/L]. Viral serologies were negative for HAV IgM, HBsAg, HBV DNA, HCV Ab, HCV RNA and HEV IgM. Anti-nuclear antibody, anti-smooth muscle antibody, and anti-mitochondrial antibody were also negative. Ceruloplasmin and ferritin were normal. An abdominal ultrasound was negative for biliary obstruction. After exclusion of other causes of liver injury, a diagnosis of drug-induced liver injury due to Terbinafine was made. Terbinafine was stopped and his liver function tests have persistently improved. DISCUSSION: Hepatic injury from Terbinafine can be hepatocellular, cholestatic, or both. While abnormalities are generally reversible upon discontinuation, prolonged cholestasis and hepatic failure requiring transplantation have been reported. The mechanism causing injury, while incompletely understood, has been hypothesized to result from binding of an allylic aldehyde metabolite to hepatobiliary proteins causing direct toxicity or altering canalicular proteins leading to immune-mediated cholestasis. It is necessary that all clinicians are aware of the potential for severe hepatic injury from Terbinafine. Further studies are needed to determine when monitoring of liver enzymes should be performed during Terbinafine treatment.