Grapefruit juice is the juice from grapefruits. It is rich in Vitamin C and ranges from sweet-tart to very sour. Variations include white grapefruit, pink grapefruit and ruby red grapefruit juice.
Grapefruit juice, and grapefruit in general, is a potent inhibitor of the cytochrome P450 CYP3A4 enzyme, which can affect the metabolism of a variety of drugs, increasing their bioavailability. In some cases, this can lead to a fatal interaction with drugs like astemizole or terfenadine. The effect of grapefruit juice with regard to drug absorption was originally discovered in 1989. The first published report on grapefruit drug interactions was in 1991 in the Lancet entitled "Interactions of Citrus Juices with Felodipine and Nifedipine," and was the first reported food-drug interaction clinically. However, the effect only became well-publicized after being responsible for a number of deaths due to overdosing on medication.[NOTE: the article referenced in  says nothing about deaths, only aching muscles]
Grapefruit juice has also been reported to increase both the bioavailability of some benzodiazepines, (such as diazepam) and greatly slow the rate of metabolization. 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).
The flavonoid existing in highest concentration in grapefruit juice is naringin, which in humans is metabolised to naringenin. Other flavonoids exist in grapefruit juice in lower concentrations as well. Orange juice does not contain naringin in as high a concentration, instead containing hesperetin. It is sometimes recommended as a substitute. Juice of limes and Seville oranges can also inhibit drug metabolism, however, as can apple juice with some drugs.
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- http://www.hon.ch/News/HSN/532584.html hon.ch
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