Phenytoin, an antiepileptic drug with nonlinear pharmacokinetics, can cause toxicity, especially with drug interactions that affect its metabolism.Here, A 70-year-old man with a history of stroke and epilepsy developed worsening ataxia and falls after starting cimetidine. His serum phenytoin level was 30 mcg/ml, above the therapeutic range. MRI showed mild cerebellar degeneration. while his Phenytoin toxicity, exacerbated by cimetidine inhibiting phenytoin metabolism, leading to cerebellar dysfunction and he managed with Phenytoin was discontinued, and valproate was initiated. The patient's condition improved with supportive care. Chronic phenytoin use can cause cerebellar ataxia. Drug interactions, like with cimetidine, can increase phenytoin levels and toxicity risk. In short, Monitoring phenytoin levels and avoiding drug interactions is crucial to prevent toxicity. Switching to alternative antiepileptic drugs can improve outcomes in affected patients.