Independent side-by-side comparison.
Neither is clearly "better" — Zyrtec (cetirizine) and Claritin (loratadine) are both second-generation antihistamines that work the same way and relieve allergy symptoms about equally well, so the choice comes down to drowsiness versus how fast it kicks in.
They're clinically similar, not the same drug: both are second-generation antihistamines that block histamine, and the NHS says "there's not much evidence to suggest any particular antihistamine is better than any other at relieving allergy symptoms." The one consistent real difference is drowsiness — the NHS cetirizine page notes cetirizine "seems to be more likely to make you feel sleepy than loratadine" (and the NHS loratadine page says the reverse: loratadine is "less likely to make you feel sleepy than cetirizine"). Onset differs slightly too: the NHS says you should feel better "within an hour" of cetirizine versus "within 1 to 3 hours" of loratadine. Both are still classed as non-drowsy/less-sedating second-generation antihistamines (MedlinePlus groups Zyrtec and Claritin together as ones that "cause less sleepiness than others").
Either is fine for everyday hay fever, hives, and allergy symptoms — but lean to Zyrtec (cetirizine) if you want the faster effect (NHS: relief usually within an hour) and aren't bothered by, or actually wouldn't mind, a bit more sleepiness.
Lean to Claritin (loratadine) if staying alert matters most (driving, work, school), since the NHS says it's less likely than cetirizine to make you feel sleepy — accepting that it may take a little longer (1 to 3 hours) to kick in.
Similar, not a clear winner: same drug class, comparable allergy relief per the NHS. Pick by side-effect/speed tradeoff — Claritin (loratadine) if you want the least chance of drowsiness, Zyrtec (cetirizine) if you want faster relief and don't mind being a touch more likely to feel sleepy. If one doesn't suit you, it's reasonable to try the other. Follow the OTC Drug Facts label and ask a pharmacist or doctor if you have kidney/liver disease, are pregnant or breastfeeding, or take other medicines.
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Ratings are based on FDA regulatory (recall-safety) data. This comparison is for general reference only — not medical advice. Always consult a licensed professional before choosing or switching a medication.