Not just xanax but all benzo's. I used to drink grapefruit juice by the gallon and couldn't understand why I was so groggy the next day when I was taking Vals, Xanys, Klons or even ativan. Then I read about the interaction and how grapefruit inhibits metabolism of benzos. So be careful, you could really build up a high concentration especially with longer acting ones like V's and be cloudy for days.