Allergra is not metabolized by the liver whereas claritin, clarinex, and zyrtec are.
Our livers are already under pressure by having CF, and with all the meds we take, it's nice to give the liver a break.
Because allergra is not heptatically metabolized, I have known some top allergists to rx 180 mg 2-3 times a day if need (this is NOT referring to Allegra-D, which contains pseudo ephedrine..... I'm referring to the plain Allegra 180mg). You can NOT do this with zyrtec or claritin.