Why can you not take allegra (fexofenadine) with juice?

Enzyme interaction: Certain juices specifically grapefruit juice can interfere with the liver enzyme which metabolizes Allegra (fexofenadine) causing the level in your blood stream to be higher than normal despite taking the appropriate dose. The good news is that a higher blood level than normal rarely causes any problems as the drug has a very safe side effect profile.

Decreases absorption: Fruit juices, particularly grapefruit, interfere with absorption of Allegra (fexofenadine) and reduce its blood levels and therefore it's efficacy. This is because it competes for a receptor on cells in the intestine called organic anion transporting polypeptides (oatps). This receptor is involved in absorption.