Which is least harmful physically Allegra or Zyrtec?

Answer:

1st A: Not much difference medically. Both have similar side effect profiles. Check with a pharmacist if you are taking any other prescription or non-prescription medications in combination with either drug, or use an online drug interaction checker such as the one available at www.drugstore.com
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2nd A: Allegra is the only NonSedating antihistamine. The FDA allows pnly Allegra to promote as NONsedating. In fact Allegra is the only antihistamine a pilot can take and fly, all others they have to stay grounded until the medicine wears off.
Zyrtec might have a slight advantage when it comes to urticaria, but Allegra has the slight advantage with sneezing, itchy nose, itchy watery red eyes. (European Studies)
Allegra has more formulations and when it comes to pediatric use or school age children use, there is no way you could convince me to use Zyrtec, for my child. They have a hard enough time staying awake and focusing in class, I will not allow my kids to be hampered by their allergy medicine.
Other than the sedation issue, they are interchangeable.
Allegra is metabolized by the Liver
Zyrtec is metabolized by the Kidneys