here's an excerpt form wiki
Grapefruit juice has also been reported to increase both the bioavailability of some benzodiazepines, (such as diazepam) and greatly slow the rate of metabolisation.[12] An easy way to tell if a medication may be affected by grapefruit juice is by researching whether another known CYP3A4 inhibitor drug is already contraindicated with the active drug of the medication in question. Examples of such known CYP3A4 inhibitors include cisapride (Propulsid), erythromycin, itraconazole (Sporanox), ketoconazole (Nizoral), and mibefradil (Posicor)