When you take a medication orally, it ends up in your digestive tract, where it starts to do its work. All medicines must somehow make their way into your bloodstream, which carries them where they need to go to have an effect.
Allegra was designed to target tiny receptors on cells in your small intestines. These receptors absorb the Allegra and help it work where it needs to. Unfortunately, certain fruit juices, especially orange and grapefruit juice, interfere with this absorption.
Studies have shown that absorption can be reduced by as much as 40%. And that means, you're not really getting your full dose of Allegra! So, obviously, it's not going to work as well as you'd expect either.
Antacids have a similar "blocking" effect on the receptors.