Immune checkpoint inhibitors (ICIs) have revolutionized cancer therapy by enhancing the ability of the immune system to combat malignancies. Nivolumab and cemiplimab, monoclonal antibodies targeting programmed cell death protein 1, have exhibited notable therapeutic efficacy; however, they are associated with immune‑related adverse events (irAEs). The present study describes the cases of 2 patients, a 71‑year‑old male with metastatic esophageal adenocarcinoma and a 66‑year‑old female with metastatic squamous cell carcinoma who developed acute/subacute onset rapidly progressive myositis/myasthenia gravis (MG) following treatment with nivolumab and cemiplimab. Both patients had negative MG antibody panels and the first had uninformative electrodiagnostic testing, rendering the diagnosis challenging. Additionally, 1 patient had myocarditis and the other had hepatitis. The mechanisms of ICI‑induced myositis/MG remain unclear. Early recognition and intervention are vital for the prevention of severe morbidity and mortality. Treatment strategies, including the termination of the offending ICI medication, steroids, intravenous immunoglobulin (IVIG) and plasma exchange, should be tailored based on individual patient responses, and physicians should not hesitate to commence IVIG or plasma exchange once the diagnosis is established.. These cases underscore the importance of vigilance for myositis/MG in patients treated with ICIs, even with uninformative serological and electrodiagnostic testing, and the need for collaboration between multiple disciplines in managing complex irAEs including the associated myocarditis and hepatitis.

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