I have looked at the package insert for XyzalⓇ (levocetirizine) and see no mention of drowsiness. Isn’t it that second and third generation antihistamines do not cause drowsiness or only cause drowsiness on overdose?
Levocetirizine is the levorotatory (“left-handed”) enantiomer of the second generation antihistamine cetirizine, which is a racemic mixture. Levocetirizine is sometimes referred to as a third generation antihistamine because it is a derivative of a second generation agent.
First generation antihistamines enter the brain and cause drowsiness by antihistamine actions at H1 receptors. Second generation antihistamines enter the brain less than first generation antihistamines and so cause less drowsiness. However, among the second generation antihistamines, not all are equal. Some, including cetirizine, still cause some degree of drowsiness.
Even for levocetirizine, the drowsiness is significant enough at normal clinical doses that a special precaution when driving, having high-risk work, or operating machinery is included in the product insert (see image below). The term used for drowsiness in the package insert is somnolence.
