Which side effects are typically associated with antihistamines?

The side effects commonly associated with antihistamines include drowsiness and diminished alertness. This is primarily due to their mechanism of action; antihistamines inhibit the activity of histamine, a neurotransmitter involved in wakefulness. By crossing the blood-brain barrier, first-generation antihistamines, in particular, are notable for their sedative effects, which can significantly impair cognitive function and alertness. This sedative effect is beneficial for conditions like allergies or insomnia but poses risks, especially in activities requiring high levels of attention, such as flying or operating machinery.

In contrast to the other choices, which may describe effects more often linked to other types of medications or physical responses, the sedating effects of antihistamines are well-documented and widely recognized in both clinical settings and consumer warnings.