Which side effect is commonly associated with antihistamines?

Drowsiness is a well-known side effect associated with antihistamines, particularly the first-generation antihistamines, which can cross the blood-brain barrier more easily than second-generation antihistamines. This sedative effect occurs because these medications block histamine receptors in the central nervous system, leading to feelings of fatigue and lethargy.

As a result, individuals taking these medications often experience decreased alertness, which can affect daily activities such as driving or operating machinery. Awareness of this side effect is crucial for anyone being prescribed or taking antihistamines, as it may limit their functionality or necessitate adjusting dosages or alternatives based on individual reactions.