Phenytoin Toxicity Potentially Induced by Addition of Lacosamide in a Long-Term Anticonvulsant Regimen: A Case Report of Rare Drug Interaction

Abstract

Phenytoin toxicity remains a critical concern when serum levels exceed therapeutic thresholds, with the contribution of drug interactions to this toxicity being well-established. However, the interaction between phenytoin and lacosamide, another anticonvulsant, has seldom been reported. This case report describes a 61-year-old male with a longstanding history of effective seizure control using phenytoin and primidone. Following a recent seizure, lacosamide was added to his regimen. Approximately one-month post-adjustment, the patient exhibited symptoms indicative of phenytoin toxicity, including ataxia, slurred speech, and diminished coordination. This case highlights the necessity for vigilance regarding potential drug interactions in patients with long-term anticonvulsant therapies and underscores the need for cautious regimen modifications. It also prompts further investigation into the mechanisms underlying the interaction between phenytoin and lacosamide to enhance patient safety and treatment efficacy.