What is a common side effect of antihistamines?

Drowsiness is a well-known side effect of antihistamines, particularly the first-generation antihistamines. These medications work by blocking histamine receptors in the brain, which can lead to sedation. This characteristic makes them effective for treating allergic reactions and conditions such as hay fever, as they can help alleviate symptoms while also inducing sleepiness. In comparison, while other choices might be side effects of different medications or conditions, drowsiness specifically aligns with the pharmacological action of antihistamines. Dry mouth can also be a side effect, but it is less consistent than drowsiness, which is why most people associate antihistamines primarily with increased sleepiness.