Recent studies have shown that taking medication with grapefruit juice increases the concentration of many drugs in humans. This seems to be due to the ability of grapefruit juice to inhibit an enzyme system found in cells lining the small intestine. This happens to be the same enzyme system that metabolizes certain prescription drugs. Therefore, inhibition of this system can result in an increase in bioavailability and maximal plasma concentrations of these drugs. This can be a cause of concern because grapefruit juice can cause certain drugs to stay in the body longer at levels higher than normal. This can create many potential problems. Side effects of the drugs may be enhanced and serious toxicity can occur.
Various classes of prescription medications should not be taken at the same time with grapefruit juice. Such classes include some sedatives, antianxiety agents, calcium channel blockers, certain statins (for high cholesterol), and antidepressants. Drugs that have been found to have the most marked interaction with grapefruit juice are as follows: felodipine, nitrendipine, nisoldipine, and saquinavir. Less pronounced increases in concentration were found for nifedipine, nimodipine, verapamil, cyclosporin (an antirejection drug for transplant recipients), midazolam, triazolam, and terfenadine (an antihistamine). Grapefruit juice should also be avoided with certain antibiotics: erythromycin, azithromycin, and clarithromycin. These lists are by no means complete because many drugs have not been studied yet.
Methadone has been demonstrated to inhibit the same enzyme system as grapefruit juice. Caution should be exercised when utilizing this medication as well, especially with other drugs that are able to induce or inhibit this enzyme system, such as rifampicin, nifedipine, diazepam, and fluvoxamine.
The components of grapefruit juice that are responsible for interacting with these drugs have yet to be fully determined. However, nariginin, a bioflavenoid in grapefruit juice is suspected to be influencing drug metabolism.Furanocoumarins, also found in grapefruit juice, may even be more potent inhibitors than flavenoids. In any case, evaluating the need to avoid concomitant grapefruit juice intake or methadone with prescription medication is best done on an individual basis with your physician.