Differentiating fenbendazole-induced liver injury from immunotherapy hepatitis - the importance of structured causality assessment: A case report

BACKGROUND: Drug-induced liver injury poses a diagnostic challenge in oncology patients, especially those on immune checkpoint inhibitors, because symptoms often overlap with immune-mediated hepatitis. The increasing use of social media has led people to self-administer veterinary anthelmintics like fenbendazole for cancer, often at high doses and without medical guidance, despite limited data on safety or effectiveness. CASE SUMMARY: We report a case of a 47-year-old woman with metastatic colon cancer on nivolumab/relatlimab who developed severe hepatocellular liver injury after increasing her self-administered dose of fenbendazole. A detailed medical and temporal history, supported by a Roussel Uclaf Causality Assessment Method score of 8 (probable), identified fenbendazole as the most likely cause. Rapid biochemical improvement occurred after stopping the drug, with safe reintroduction of immunotherapy and no recurrence of liver injury. CONCLUSION: This case emphasizes the importance of thorough medication history-taking and structured causality assessment to distinguish between unregulated drug-induced liver injury and immune-related adverse events in cancer care. The use of social media to promote alternative therapies necessitates proactive patient counseling due to significant liver risks, and careful diagnostic evaluation can prevent unnecessary immunosuppression or treatment delays.