If you are finding Allegra to be more effective sometimes than others, or if you've seen it on Zyrtec ads, it's true that fruit juices affect absorption of Allegra (fexofenadine).  This can be up to 50%!!

  You don't have to go off of it, though.  Just don't drink fruit juices (and all of them are thought to have the same effect) within 4 hours of taking Allegra.  Don't take more Allegra to counteract this effect, because it will be impossible to know what dosage you're actually getting.

  I'm guessing that whole fruits will do the same thing, but I haven't looked to see that.

This is what it said about why the interaction might inhibit absorption, but I haven't looked further:  

"It is most likely caused by fruit juices inhibiting an organic anion transporting peptide (OATP1A2).  By inhibiting OATP1A2, the absorption of Allegra is reduced."

Another article said:

"It is especially important to check with your doctor before combining Allegra with the following: aluminum- and magnesium-containing antacids (such as Maalox), erythromycin, or ketoconazole."

It also said to check with your doctor if you have phenylketonuria, are pregnant, or nursing.