Phenytoin Toxicity During Neoadjuvant Concurrent Capecitabine and Radiation Therapy for Rectal Adenocarcinoma: Case Report of a Drug Interaction.

Phenytoin toxicity occurs when serum levels exceed the therapeutic level, leading to symptoms such as nystagmus, slurred speech, and decreased coordination. This toxicity is sometimes caused by drug interactions. Interactions between phenytoin and capecitabine are not commonly documented. We report the case of a 52-year-old man taking phenytoin for atypical meningioma who developed symptoms of phenytoin toxicity while receiving capecitabine in the treatment of rectal adenocarcinoma.