Several reports show that taking certain medications with grapefruit juice can cause problems by markedly increasing absorption of the medication.
Many drugs are broken down in your intestines by an enzyme called cytochrome P450 that is normally there, so these drugs have to be given in higher concentrations than normal to achieve adequate blood levels. Grapefruit juice contain bergamottin (2) that blocks cytochrome P450 so that these drugs are not broken down and much higher amounts pass into the bloodstream. High doses of bergamottin may permanently damage the enzyme.
So far, the following drugs should not be taken with grapefruit juice: calcium channel blockers such as felodipine, nitrendipine, nisoldipine, nifedipine, nimodipine, and verapamil; the anti-aids drug, saquinavir; the immune suppressant, cyclosporin; the tranquilizers, midazolam and triazolam; and the antihistamine, terfenadine.
The components of grapefruit juice which are the most probable causes are psoralens and flavinoids. In the future, grapefruit juice will probably be prescribed with several drugs to lower the dose required and make them less expensive, but at this time, doctors are unable to tell you how much of any drug to take with the juice. Until we know more, you should not take any drugs with grapefruit juice.
1) U Fuhr. Drug interactions with grapefruit juice: Extent, probable mechanism and clinical relevance. Drug Safety 18: 4 (APR 1998):251-272.
2) Chemical Research in Toxicology April. 1998.
3) T Kantola, KT Kivisto, PJ Neuvonen. Grapefruit juice greatly increases serum concentrations of lovastatin and lovastatin acid. Clinical Pharmacology & Therapeutics 63: 4 (APR 1998):397-402.