An overview of interactions between grapefruit juice and drugs

Because grapefruit contains fibre, vitamin C, antioxidants and phytochemicals, the fruit and its juice are consumed widely to help meet daily nutritional requirements. However, in the past 15 years, studies have shown that grapefruit juice can induce a several-fold increase in the plasma levels of particular drugs that can result in increased therapeutic or even toxic effects. The effect seems to be mediated mainly by inhibition of the cytochrome P450 enzyme CYP3A4 in the wall of the small intestine. This results in a decreased intestinal first-pass metabolism of drugs metabolised by this enzyme leading to higher bioavailability and increased maximum plasma concentrations of the drug. The effect is most pronounced in drugs which experience a high firstpass effect. The components of grapefruit juice which are the most probable causes of the interaction are the furanocoumarin derivatives. Concomitant grapefruit juice intake does not generally decrease the variability of drug pharmacokinetic parameters. Therefore, it is recommended that patients abstain from drinking grapefruit juice when they are taking a drug that is extensively metabolised, unless a lack of interaction has already been demonstrated for that drug. It is also recommended that drugs possibly interacting with grapefruit juice should be appropriately labelled. The purpose of this article is to examine the cause of this food-drug interaction, discern the list of affected medications and to equip the pharmacist with this knowledge and specify their role in counselling patients on options to avoid the possibility of a grapefruit-drug interaction. Peer-reviewed. (Submitted: 2011-01-26. Accepted: 2011-07-25.) © Medpharm S Afr Pharm J 2011;78(8):40–45