Commonly used antiepileptic drug causing a rare movement disorder

Background: Phenytoin, a commonly prescribed antiepileptic drug, causes side effects like ataxia, tremor, hirsutism, gum hyperplasia, insomnia, confusion, headache and vertigo when used for longer duration. However, chorea is a rarely reported side effect of phenytoin and is completely reversible on stopping treatment.  Case presentation: A twenty-one-year-old Indian male patient, who had generalized epilepsy and had been on sodium valproate for 2 years, presented with acute onset chorea four days after starting phenytoin sodium. He had normal serum phenytoin levels. A thorough evaluation was done, which suggested phenytoin as a possible cause of chorea. Phenytoin was withdrawn, resulting in a dramatic subsiding of chorea. A rechallenge with the drug resulted in reappearance of choreiform movements. These disappeared again after drug withdrawal, implicating phenytoin as the possible etiological agent for chorea. Conclusion: Phenytoin rarely induces involuntary movements as an adverse effect. During phenytoin therapy, if a patient develops involuntary movements, phenytoin toxicity should be suspected even with normal drug levels. This is important as drug withdrawal leads to complete symptomatic improvement thereby avoiding extensive workup for other secondary causes.