What is a common side effect of antihistamines?

Drowsiness is widely recognized as a common side effect of antihistamines due to their action on the central nervous system. First-generation antihistamines, such as diphenhydramine and chlorpheniramine, can cross the blood-brain barrier and block histamine receptors in the brain that promote wakefulness. This sedative effect makes these medications effective for managing allergic reactions, but it can also lead to increased drowsiness, impairing a person's ability to perform tasks that require full alertness, such as driving.

In contrast, other options like increased appetite, nausea, and diarrhea are not typically associated with antihistamine use. While some individuals may experience a slight increase in appetite, this is not a well-documented or common effect of these medications. Similarly, nausea and diarrhea are not characteristic side effects of antihistamines and do not arise primarily from their pharmacological action. This makes drowsiness the most relevant and frequently encountered side effect when using antihistamines.