Grapefruit juice is the fruit juice from grapefruits. It is rich with 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 enzyme CYP3A4, which can impact the metabolism of a variety of drugs, increasing their bioavailability. In some cases, this can lead to a fatal interaction with drugs like Astemizole. 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.
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. 
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 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 sometimes recommended as a substitute. Juice of limes and Seville oranges can also inhibit drug metabolism, however, as can apple juice with some drugs. 
A recent study published in the July 2007 edition of the British Journal of Cancer reported that eating grapefruit every day could raise the risk of developing breast cancer by almost a third. The Study found that in the test subjects, 50,000 post-menopausal women, eating just a quarter of a grapefruit daily raised the risk by up to 50%. It is believed that the fruit boosts the levels of estrogen, which in turn increases the risk of developing the disease. 
- The World's Healthiest Foods; Grapefruit. The George Mateljan Foundation. Article
- Fellers PJ, Nikdel S, Lee HS. Nutrient content and nutrition labeling of several processed Florida citrus juice products. J Am Diet Assoc. 1990 Aug;90(8):1079-84. Abstract
- He K; Iyer KR; Hayes RN; Sinz MW; Woolf TF; Hollenberg PF. Inactivation of cytochrome P450 3A4 by bergamottin, a component of grapefruit juice. Chem Res Toxicol. 1998 Apr;11(4):252-9. Abstract
- Bailey DG, Malcolm J, Arnold O, Spence JD. Grapefruit juice-drug interactions. Br J Clin Pharmacol. 1998 Aug;46(2):101-10. Comment in: Br J Clin Pharmacol. 2001 Aug;52(2):216-7. Abstract
- Garg SK, Kumar N, Bhargava VK, Prabhakar SK. Effect of grapefruit juice on carbamazepine bioavailability in patients with epilepsy. Clin Pharmacol Ther. 1998 Sep;64(3):286-8. Abstract
- Bailey DG, Dresser GK. Interactions between grapefruit juice and cardiovascular drugs. Am J Cardiovasc Drugs. 2004;4(5):281-97. Abstract
- Bressler R. Grapefruit juice and drug interactions. Exploring mechanisms of this interaction and potential toxicity for certain drugs. Geriatrics. 2006 Nov;61(11):12-8. Abstract
- Bakalar, Nicholas. Experts Reveal the Secret Powers of Grapefruit Juice. New York Times. Published: March 21, 2006. Article
- Bakalar, Nicholas (2006-03-21). "Experts Reveal the Secret Powers of Grapefruit Juice". The New York Times. p. F6. Retrieved 2006-11-21.
"http://abello.dic.uchile.cl/~hechavez/articulos/grapefru.htm". External link in
- BBC NEWS | Health | Grapefruit link to breast cancer