Efficacy of Switching to Levetiracetam After S-1-Induced Phenytoin Concentration Increase: A Case Report

S-1, an oral anticancer drug, interacts with phenytoin (PHT) to increase PHT serum concentration. Although the PHT dosage is usually adjusted, few studies have examined the effects of switching from PHT to another antiepileptic drug. Here, we report the details of a case in which a patient with gastric cancer continued adjuvant chemotherapy after switching from PHT to levetiracetam (LEV) to prevent interactions with S-1. A man in his 60s with advanced gastric cancer received adjuvant chemotherapy with S-1 120 mg/day (cycles of two weeks of administration followed by one week of rest). He had experienced generalized tonic-clonic seizures 48 years prior and had been taking PHT (170 mg/day) and carbamazepine (250 mg/day). On day 22 of treatment, the PHT concentration increased from 3.72 to 11.76 µg/mL. On day 25, he developed dizziness and fell. Gradual PHT dose reduction and a switch to LEV improved his symptoms, and he remained seizure-free over nine treatment cycles. The findings of this case report suggest an approach for switching from PHT to another antiepileptic drug when PHT levels increase due to interactions with S-1. Switching to LEV may result in fewer interactions.