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>From : The Medical Letter, Vol. 37 (Issue 955) August 15, 1995 "CONCLUSION - Grapefruit juice inhibits presystemic metabolism of some drugs metabolized by the cytochrome P450 isozyme CYP3A4, including several calcium- channel blockers, cyclosporine and terfenadine. With drugs like these,which are largely metabolized presystemically, grapefruit juice may increase serum concentrations by allowing more unmetabolized drug to enter the systemic circulation. The clinical significance of these effects has not been established. Grapefruit juice also inhibits CYP1A2, an isozyme involved in the metabolism of drugs such as caffeine, theophylline and tacrine (Cognex); only the the metabolism of caffeine has been shown to be significantly affected. The effect of grapefruit juice on other drugs metabolized by these and other P450 isozymes remains to be determined." Mechanism of action described. Drugs listed include: calcium channel blockers, cyclosporine, terfenadine, midazolam, estrogens, and caffeine. Report also states: "Orange juice apparently does not affect drug metabolism." [log in to unmask] (Mary Manfredi)