Independent side-by-side comparison.
Neither is clearly "better" — Zyrtec (cetirizine) and Allegra (fexofenadine) are both once-daily, second-generation antihistamines that relieve the same allergy symptoms and hives, and both are classed as non-drowsy; the main practical difference is that cetirizine is a bit more likely to cause sleepiness.
Clinically very similar: both are second-generation OTC antihistamines that work the same way (blocking histamine), are taken once a day, and treat the same things (sneezing, runny nose, itchy/watery eyes, and hives). The one consistent, source-grounded difference is drowsiness — the NHS states "cetirizine seems to be more likely to make you feel sleepy than loratadine, desloratadine or fexofenadine." A practical quirk: fexofenadine (Allegra) should not be taken with orange, grapefruit, or apple juice, which can reduce how well it works, and aluminum/magnesium antacids should be spaced a few hours apart.
Either is fine for routine hay fever, allergy symptoms, or hives. Lean toward Zyrtec (cetirizine) if you want a once-daily option and don't mind a slightly higher chance of feeling sleepy; some people find cetirizine works well for them, and it has no fruit-juice restriction.
Lean toward Allegra (fexofenadine) if staying alert matters most — the NHS notes it is among the non-drowsy antihistamines least likely to cause sleepiness (e.g., for driving or work), or if cetirizine has made you drowsy in the past. Just remember to take it with water, not fruit juice, and to separate it from aluminum/magnesium antacids by a few hours.
There's no clear winner — they're clinically similar and both effective once-daily allergy options. Pick by drowsiness sensitivity: if you've felt sleepy on cetirizine or need to stay sharp, choose Allegra (fexofenadine); otherwise either works, so cost and what works for you can decide. Trying one and switching if it doesn't suit you is reasonable; ask a pharmacist or doctor if symptoms persist or you 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.