Grapefruit juice -

Grapefruit juice - encyclopedia article about Grapefruit juice.: "Grapefruit juice is the fruit juice coming from grapefruits. It is rich with Vitamin C. It is slightly sour. Variations include pink grapefruit juice. Pharmacological EffectsGrapefruit 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. 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. [1]

These cytochrome P450 enzymes, ordinarily responsible for limiting drug metabolites from entering the bloodstream, are inhibited. As a result, more medicine reaches the bloodstream. This may result in a harmful overdose. 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 (Versed), cyclosporin (Sandimmune, Neoral), lovastatin (Mevacor), simvastatin (Zocor), pravastatin (Pravachol), and caffeine, as well as a number of antihistamines including astemizole (Hismanal) and terfenadine (Seldane, Seldane-D). [2]

An easy way to tell if a medication may affected by grapefruit juice is by researching whether another known CYP3A4 inhibtor 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).

Orange juice does not contain naringin in as high a concentration, instead containing hesperetin. It is recommended as a substitute.
References
USDA Database of Flavonoid content of food
Pharmacological Effects