Which side effect is most closely associated with antihistamine antiemetics?

Antihistamine antiemetics, such as diphenhydramine and meclizine, are commonly used to prevent and treat nausea and vomiting. These medications work by blocking histamine receptors in the vestibular system of the brain, which is involved in motion sickness. However, due to their antihistaminergic properties, they also inhibit acetylcholine, leading to significant anticholinergic side effects.

The most notable side effects associated with these drugs include dry mouth and sedation. Dry mouth occurs because antihistamines can reduce saliva production, leading to xerostomia. Sedation is a common side effect because antihistamines can cross the blood-brain barrier, affecting the central nervous system and causing drowsiness. These properties make antihistamine antiemetics effective for treating nausea but may also limit their use in situations where alertness is necessary.

Other options such as increased anxiety, hyperactivity, or severe gastrointestinal issues are not commonly associated with antihistamine antiemetics. While some medications can cause anxiety or gastrointestinal disturbances, these effects are more characteristic of different classes of drugs or conditions rather than a direct result of antihistamine antiemetics.