Complex sentence: The second-generation antihistamine cetirizine is in fact one of the metabolites of hydroxyzine produced in the human body, therefore having a narrower spectrum of effects, making it an effective antihistamine but removing some or all of the anxiolytic and analgesic-sparing properties.
Split into simpler sentences: The second-generation antihistamine cetirizine is in fact one of the metabolites of hydroxyzine produced in the human body. Therefore it has a narrower spectrum of effects, making it an effective antihistamine but removing some or all of the anxiolytic and analgesic-sparing properties.