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.

Related Questions

Why can't you take allegra (fexofenadine) with juice?

Interference. Grapefruit juice interferes with the body's metabolism of allegra, (fexofenadine) preventing the natural process. Thus the drug can have unintended effects. Read more...
Decreased absorption. If you take Allegra (fexofenadine) with juice, you can decrease the amount of the medication absorbed and therefore not receive the full effect you were hoping to achieve. Read more...

I am suffering from dermatographism since last 1.5 yrs and taking allegra (fexofenadine) 180mg daily once a day. Can I take noni juice for it cure?

Ain't none. You can take anything you want, but nothing, other than in some people time, 'cures' dermatographism. Antihistamines can suppress the symptoms, but some people just seem to have touchy mast cells and even mild skin trama can result in them releasing histamine causing the localized itching and swelling. Read more...