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The grapefruit is a sub-tropical citrus tree grown for its fruit which was originally named the "forbidden fruit" of Barbados. wikipedia

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Drug interactions

Grapefruit can have a number of interactions with drugs, often increasing the effective potency of compounds. Grapefruit contains naringin, bergamottin and dihydroxybergamottin, which inhibit the cytochrome P450 isoform CYP3A4 in the intestine.
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The effect of grapefruit juice with regard to drug absorption was originally discovered in 1989. However, the effect became well-publicized after being responsible for a number of deaths due to overdosing on medication.wikipedia

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commercial products of grapefruit juice sold in California. These included Minute Maid, Dole, Tropicana, Ocean Spray, Ralps, Albertson, Stater Bros, Vons, Langers, etc. The concentrations of naringin, naringenin and bergapten in grapefruit juice were assayed by specific HPLC methods. Naringin was found to be the most abundant flavonoid in grapefruit juice products, followed by naringenin and bergapten. The content of naringin varied among the products, ranging from 104 mg/l (Tropicana ruby red) to 628 mg/l (Ralphs white frozen concentrate). The mean contents of naringin in ruby red (158 +/- 66 [SD] mg/l) and pink (279 +/- 123 mg/l) grapefruit juice products were significantly lower than white (481 +/- 94 mg/l) (p <0.005) grapefruit juice products. Content of naringenin also varied from brand to brand and ranged from 3.9 mg/l (Vons white frozen concentrate) to 31.2 mg/l (Tree Sweet pink). Bergapten content was very low in grapefruit juice products ranging from 0 (not detectable) to 5.5 mg/l. There were no significant differences in naringenin and bergapten contents among the three types of grapefruit juice products.

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What is the duration of the inhibition?

Inhibition of CYP3A4 by grapefruit juice can last a number of hours. Increased felodipine concentrations have been observed even when the drug was taken 24 hours after drinking a glass of juice.1 In one study the extent of the increase in felodipine AUC and Cmax was maximal when grapefruit juice was given four hours before, or simultaneously with, the drug. The half-life of the effect of grapefruit juice on CYP3A4 was estimated in one study to be 12 hours.30

Does the quantity of juice matter?

Several studies appear to show that one glass of regular strength juice has a similar effect on the concentrations of felodipine as 2-3 glasses of double strength juice. However, consumption of very large quantities of grapefruit juice (6-8 glasses per day) may lead to inhibition of hepatic CYP3A4, whereas lower quantities (up to three glasses per day) seem to have intestinal activity only.15

All the studies, which investigate the effects of grapefruit on the metabolism of drugs, use grapefruit juice (either single or double strength) to study its effects. The effects are best seen when 240 mL of double strength reconstituted grapefruit juice is taken simultaneously with the drug, or if 240 mL of regular strength grapefruit juice is drunk three times a day.4

Is the effect of ingesting a whole grapefruit the same as ingesting the juice?

At the present time there are no studies which indicate that eating either a half or a whole grapefruit will have any effect on a drug's metabolism. However, one study has reported that blended grapefruit segments and an extract from grapefruit peel caused a similar interaction as the juice, with felodipine.15

Do other citrus fruits have the same effect?

Sweet orange juice does not inhibit CYP3A4. Seville (or bitter) orange juice does inhibit CYP3A4 but, unlike grapefruit juice, does not influence cyclosporin disposition.32 It is not known whether other drugs which interact with grapefruit juice may also interact with this type of orange juice. Tangelos (a hybrid between grapefruit and tangerine) have not been tested for drug interactions.

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