What is the difference between zyrtec and xyzal (levocetirizine)?

Very similar: Zyrtec is the brand name for the molecule cetirizine, whereas xyzal is levocetirizine, the mirror image. Xyzal came to market when zyrtec was going generic and over-the-counter. There are subtle differences between the two, but they behave very similarly.

Same active ingred: When chemicals are synthesized there are mirror image molecules made in equal quantities. Zyrtec contains both versions of the same molecule (one version thought to be responsible for benefits, other for side effects). Xyzal (levocetirizine) is just the beneficial version.

Very similar: The generic of zyrtec is Cetirizine and generic of xyzal (levocetirizine) is levo-cetirizine. Most medications have 2 sides or mirror images in the same chemical mixture. One of the sides (levo-) is believed to be the active side when it comes to making the drug effective. The other side may be inactive. For xyzal (levocetirizine), the active enantiomer of cetitizine was isolated and called xyzal (levocetirizine). It may be more effective.

Not much: Zyrtec is a mixture of two mirror image drugs, levo and dextrocetirizine. Xyzal is a pure levocetirizine. Only the levo form is an active antihistamine. The dextro form is inactive but has no known side effects. The difference is cost, insurance coverage, and patient prejudice or preference.