A 71-year-old woman diagnosed with unresectable locally advanced pancreatic cancer was initially treated with gemcitabine and nab-paclitaxel as first-line therapy. The tumor exhibited no significant progression
however, after 12 cycles, the patient developed drug-induced interstitial pneumonia, leading to the discontinuation of gemcitabine and nab-paclitaxel therapy. Following recovery from pneumonia, S-1 therapy was initiated as second-line treatment. During S-1 therapy, she was hospitalized because of impaired consciousness and was subsequently diagnosed with hyperammonemia induced by S-1. Although rarely reported, S-1-induced hyperammonemia is potentially a significant adverse effect. Here, we herein report the case of a patient with pancreatic cancer.