https://www.selleckchem.com/products/ab680.html Fenbendazole is a benzimidazole anthelmintic agent, with a broad antiparasitic range in animals such as dogs and pigs. The agent is also reported to exert antitumor effects and inhibit microtubule-associated tubulin polymerization, but its safety and tolerability profile in humans remains unclear. An 80-year-old female patient with advanced nonsmall cell lung cancer (NSCLC) was started on pembrolizumab monotherapy. The patient experienced severe liver injury 9 months later. An interview with her and her family revealed that she had been taking fenbendazole for a month, solely based on social media reports suggesting its effectiveness against cancer. After discontinuation of the self-administration of fenbendazole, the patient's liver dysfunction spontaneously resolved. The antitumor inhibitory effects of fenbendazole have been reported; however, she did not experience tumor shrinkage. This is the first case report of a patient with advanced NSCLC who self-administered the anthelmintic, fenbendazole. Twitter and Facebook are online social media platforms which have been constructively used to exchange information among cancer patients. However, sources of medical information on these platforms are often unproven, and it is difficult for nonmedical professionals to accurately select and filter complex medical information. Physicians should enquire patients about self-administration of orally ingested products, including dietary supplements, herbs, or bioactive compounds, in cases of unexpected adverse reactions.Immune checkpoint inhibitors (ICI) have drastically changed the landscape of metastatic melanoma management, thus significantly improving survival. Clinically, assessing treatment response may be challenging in a portion of cases due to a massive influx of immune cells into the tumor microenvironment, causing a transient increase in the target lesion size. This phenomenon, coined pseudoprogression, can occur in 5-