Grapefruit juice contains flavones which inhibit the liver enzyme 3A4. Xanax (alprazolam) is metabolized by that enzyme. Grapefruit juice will increase the peak plasma concentration of Xanax (as if you took more of the drug than you really did), and extend the half-life (causing the length of the effect itself to increase). Grapefruit juice will make it like you took an overdose of Xanax.

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