Which generation of antihistamines is associated with anticholinergic ocular effects, exemplified by Benadryl?

• A. 1st gen oral antihistamines (Benadryl)
• B. 2nd gen antihistamines
• C. Topiramate
• D. Phenytoin

Answer: (A)

Anticholinergic effects in the eye come from blocking muscarinic receptors, which reduces tear production and blurs near vision due to impaired accommodation. First-generation antihistamines cross into the eye and the brain more readily and block these receptors, so they carry notable anticholinergic ocular effects. Benadryl is diphenhydramine, a classic example of a first-generation H1 blocker, so it fits this pattern and can cause dry eyes, blurred vision, and mydriasis. Second-generation antihistamines are designed to minimize these effects by being less able to block muscarinic receptors, so they have far fewer ocular anticholinergic side effects. Topiramate and phenytoin aren’t antihistamines; they have other ocular risks (topiramate can cause angle-closure–related issues, diplopia or nystagmus with different mechanisms), but they don’t exemplify anticholinergic ocular effects from antihistamines.