Which drug classification is likely to cause drowsiness as a side effect?

Antihistamines are a drug classification commonly associated with drowsiness as a side effect. This effect arises because many antihistamines, particularly first-generation antihistamines, can cross the blood-brain barrier. Once in the central nervous system, they block the action of histamine, a chemical that promotes wakefulness. This sedative effect can be particularly useful for patients struggling with allergies when symptoms disrupt sleep, but it also means that individuals should be cautious when taking these medications if they need to remain alert.

In contrast, expectorants primarily work by thinning and loosening mucus in the airways, allowing for easier coughing and are not known to cause drowsiness. Bronchodilators, used primarily to relax and open the airways in conditions like asthma, mainly focus on improving respiratory function and do not typically induce sedation. Decongestants aim to relieve nasal congestion by constricting blood vessels and do not usually lead to drowsiness. Thus, among the options presented, antihistamines distinctly stand out as those likely to cause this sedative side effect.