Meds Not to Mix: Your Guide to Taking OTC Drugs Safely

Grapefruit Juice and OTC Antihistamine, Fexofenadine (Allegra)

A study conducted by the University of Western Ontario in Canada revealed that grapefruit products and some citrus fruits, like oranges, contain chemicals that can affect various medications in your body, often times causing the medicine to stay in your body longer. "In grapefruit juice, it is thought that naringin, a flavonoid compound naturally found in the fruit, interferes with the absorption of some drugs," says Joseph S. Bertino Jr., Pharm.D and associate professor of pharmacology at Columbia University. "Grapefruit juice turns on a pump in the small intestine that blocks some drugs from moving through and into the blood."

If you were to take Allegra with grapefruit juice, the liquid can actually decrease the amount that reaches the bloodstream, making it less -- if not completely -- ineffective. The way grapefruit juice affects drug absorption is dependent on the source of the juice (fresh, frozen, or bottled), but Bertino says that to be safe, unless told otherwise, take medications with only water.