Which of the following is a symptom of phenytoin overdose?

• A. Nystagmus
• B. Ataxia
• C. Diplopia
• D. Confusion

Answer: (B)

Phenytoin toxicity produces neurologic signs from excessive CNS depressant effect, with cerebellar involvement being a key feature. At therapeutic levels the drug helps control seizures with limited impairment, but in overdose the excess block on neuronal activity disrupts cerebellar circuits that coordinate movement. That disruption leads to uncoordinated actions and a gait that’s unstable, i.e., ataxia. It’s a hallmark of overdose because it directly reflects cerebellar dysfunction from high drug levels. Other signs like nystagmus or diplopia can occur with toxicity, and confusion or sedation can appear with more severe toxicity, but the uncoordinated movement pattern of ataxia best captures the overdose effect in this context.