Grapefruit juice
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Grapefruit juice is the fruit juice coming from grapefruits. It is rich with Vitamin C and is slightly sour. Variations include pink grapefruit juice.
Pharmacological effects
Grapefruit juice has been found to interact with some medicines. It is believed that flavonoids in the juice affect the activity of certain intestinal enzymes like CYP3A4 and CYP1A2.These cytochrome P450 enzymes, which metabolize many drugs, are inhibited. As a result, serum drug concentrations increase and may become toxic. This is particularly dangerous when the drug in question has a low therapeutic index, so that a small increase in blood concentration can be the difference between therapeutic success and toxicity.
Drugs that may be affected include midazolam, ciclosporin, lovastatin, simvastatin, pravastatin, Felodipine, Sildenafil (Viagra) and caffeine, as well as a number of antihistamines including astemizole and terfenadine. [link]
An easy way to tell if a medication may affected by grapefruit juice is by researching whether another known CYP3A4 inhibitor drug is already contraindicated with the drug 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 metabolized 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 recommended as a substitute.
References
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