USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

Diphenhydramine is actually an antihistamine, not a calcium channel blocker, so understanding its primary adverse effects is essential. The main adverse effect associated with diphenhydramine is peripheral edema, which can occur as a result of its anticholinergic properties. This drug often causes vasodilation and increased capillary permeability, leading to fluid accumulation in the tissues.

While cough, anxiety, and blurred vision can occur with various medications, they are not typically highlighted as primary adverse effects of diphenhydramine. Cough may arise from other respiratory conditions, anxiety could be a general reaction to medication in some patients, and blurred vision can be a result of anticholinergic effects experienced at higher doses or among sensitive individuals. However, peripheral edema stands out in this context as a more notable side effect linked with the use of diphenhydramine, especially when considering its action and side effect profile in the context of antihistamines.