Allegra 120 is a popular antihistamine used to treat allergy symptoms. While it’s generally safe when used alone, certain medications or substances can interact with it, affecting its effectiveness or increasing the risk of side effects. Rifampin, an antibiotic used to treat tuberculosis and other infections, can lower the effectiveness of Allegra 120. It speeds up the elimination of fexofenadine from the body, reducing how well Allegra 120 works for your allergies. Grapefruit and apple juice can interfere with the absorption of Allegra 120. They may increase the concentration of fexofenadine in the blood, potentially leading to unwanted side effects.