What is a common side effect of antihistamines?

Antihistamines are commonly used to alleviate allergy symptoms, and one of their well-known side effects is drowsiness. This occurs because many first-generation antihistamines, such as diphenhydramine and chlorpheniramine, can cross the blood-brain barrier and bind to central nervous system histamine receptors. The resulting sedation can be beneficial for patients who also suffer from insomnia or want to mitigate their allergy symptoms at night.

While antihistamines can cause other side effects, such as dizziness or dry mouth, drowsiness is particularly significant because it impacts the patient's ability to perform tasks that require mental alertness, like driving or operating machinery. Additionally, users should be aware of this effect when considering the timing of their medication or when combining it with other sedating agents.