S5806 Drug-Induced Liver Injury From Off-Label Fenbendazole Use in Metastatic Breast Cancer: A Case Report

Introduction: Drug-induced liver injury (DILI) is a diagnostic challenge, especially when linked to unregulated or repurposed agents. We present a case of DILI in a patient with metastatic breast cancer, likely due to self-administration of fenbendazole—a benzimidazole anthelmintic approved only for veterinary use. Although preclinical data suggests antiproliferative effects, rigorous human data are lacking. Despite no FDA approval, its use among cancer patients is rising, fueled by anecdotal reports on social media. To date, only 2 published case reports describe severe fenbendazole-associated hepatocellular DILI, both resolving after drug cessation. Case Description/Methods: A 49-year-old woman with newly diagnosed ER-/PR-/HER2+ grade 3 invasive ductal carcinoma of the left breast with axillary nodal involvement presented for routine pre-chemotherapy workup. She was found to have markedly elevated liver enzymes: aspartate aminotransferase (AST) 679, alanine aminotransferase (ALT) 1,119, Alk Phos 122, TBili 0.5 (up from 1 week prior: AST 258, ALT 402, Alk Phos 106, TBili 0.4). She was asymptomatic and denied recent illnesses, travel, and alcohol, illicit drug, and supplement use, reporting only taking acetaminophen 500 mg daily for pain. Her son later disclosed she had been taking veterinary-grade fenbendazole orally 4 days per week for 4-6 weeks, influenced by social media posts promoting its anticancer and analgesic benefits. Workup showed negative hepatitis A, B, and C serologies, acetaminophen level <5 µg/mL, ANA 1:320, PT and international normalized ratio at 14.0 and 1.3, and no signs of hepatic encephalopathy. Abdominal ultrasound showed mild hepatic steatosis without biliary dilatation; contrast CT abdomen/pelvis revealed no hepatic metastases or obstruction. Hepatology was consulted for presumed DILI and N-acetylcysteine was empirically initiated to aid with liver function recovery. By day 4, liver enzymes improved (AST 545, ALT 874, Alk Phos 93, TBili 0.6), and the patient remained clinically stable. She was discharged with counseling to discontinue fenbendazole and to present for outpatient LFT monitoring and oncology follow up. Discussion: Fenbendazole, though intended for veterinary use, poses a real risk for hepatocellular DILI when misused in humans. This case, alongside previous reports, highlights the need for routine screening of non-prescribed agents, especially those promoted online. Early recognition, drug withdrawal, and supportive care can lead to prompt recovery. However, systematic safety evaluation and patient education are urgently needed.