Which of the following is an early sign of phenytoin toxicity?

• A. Rash
• B. Photosensitivity
• C. Horizontal nystagmus
• D. Headache

Answer: (C)

Recognizing early signs of phenytoin toxicity relies on CNS effects that appear before more global sedation or confusion. Horizontal nystagmus happens when phenytoin levels begin to rise into the toxic range and starts to disrupt ocular motor control. This involuntary side-to-side eye movement is a classic early indicator because the vestibulo-ocular pathways are sensitive to the drug’s CNS depressant effects, so you often see it before ataxia or marked drowsiness.

As levels continue to rise, you may notice worsening symptoms like ataxia, slurred speech, and greater drowsiness or confusion, with more severe toxicity potentially leading to seizures or coma. That progression helps guide timely dose adjustment to prevent further complications.

Rash and photosensitivity are adverse reactions associated with phenytoin but are not reliable early signs of toxicity; headache can occur for various reasons and isn’t specific to early phenytoin toxicity. The key early warning you’re looking for is the horizontal nystagmus.