Cetirizine and Levocetirizine are non-sedating second-generation antihistamine drugs. These drugs are used to combat various allergic problems. Cetirizine is a mixture of levocetirizine and dextrocetirizine. But levocetirizine is many times more selective for H1-Histamine receptor compared to dextrocetirizine. Thus, it produces fewer side effects (like sedation) compared to cetirizine which is a racemic mixture of both.
Cetirizine = (R)-Levocetirizine + (S)-Dextrocetirizine