Diplopia can be associated with which of the following drugs?

• A. Antidepressants
• B. Antianxiety agents
• C. Prednisone
• D. Phenytoin

Answer: (D)

Diplopia from drugs often reflects disruption of ocular motor control when the CNS is affected. Phenytoin is a classic antiseizure medication that can cause double vision as an early sign of toxicity, and it is commonly seen with other fine-tuning issues like nystagmus and gait disturbance. Mechanistically, phenytoin blocks voltage-gated sodium channels to calm neural activity, but at higher levels this can impair the coordinated firing of the extraocular muscles, producing misalignment of the eyes and diplopia. Other listed drugs may affect vision in various ways, but diplopia is not a typical or prominent adverse effect of antidepressants or antianxiety agents, and prednisone’s ocular effects are usually cataracts, glaucoma, or myopathy rather than diplopia. So phenytoin best explains diplopia in this context. If this occurs, check phenytoin levels and adjust the regimen as needed.