Some fruit juices and fruits can interact with numerous drugs, in many cases causing adverse effects. The effect is most studied with grapefruit and grapefruit juice, but similar effects have been observed with certain other citrus fruits.
The effect was first discovered accidentally in 1991, when a test of drug interactions with alcohol used grapefruit juice to hide the taste of the ethanol. A 2005 medical review advised patients to avoid all citrus juices until further research clarifies the risks. It was reported in 2008 that similar effects had been observed with apple juice.
One whole grapefruit, or a small glass () of grapefruit juice, can cause drug overdose toxicity. Fruit consumed three days before the medicine can still have an effect. The relative risks of different types of citrus fruit have not been systematically studied. Affected drugs typically have an auxiliary label saying "Do not take with grapefruit" on the container, and the interaction is elaborated upon in the package insert. People are also advised to ask their physician or pharmacist about drug interactions.
The effects are caused by furanocoumarins (and, to a lesser extent, flavonoids). These chemicals inhibit key drug metabolizing enzymes, such as cytochrome P450 3A4 (CYP3A4). CYP3A4 is a metabolizing enzyme for almost 50% of drugs, and is found in the liver and small intestinal epithelial cells. As a result, many drugs are affected. Inhibition of enzymes can have two different effects, depending on whether the drug is either
metabolized by the enzyme to an inactive metabolite, or
activated by the enzyme to an active metabolite.
In the first instance, inhibition of drug-metabolizing enzymes results in elevated concentrations of an active drug in the body, which may cause adverse effects. Conversely, if the medication is a prodrug, it needs to be metabolised to be converted to the active drug. Compromising its metabolism lowers concentrations of the active drug, reducing its therapeutic effect, and risking therapeutic failure.
Low drug concentrations can also be caused when the fruit suppresses drug absorption from the intestine.
History
The effect of grapefruit juice with regard to drug absorption was originally discovered in 1989 by a group led by pharmacologist David Bailey. Their first published clinical report on grapefruit drug interactions was in 1991.
Polyphenols
Citrus fruits may contain various polyphenols, including naringin and furanocoumarins, such as bergamottin, dihydroxybergamottin, and bergapten. Grapefruit, Seville oranges, and bergamot contain naringin. Furanocoumarins may have a stronger effect than naringin.
Mechanism
Organic derivatives of furanocoumarin interfere with liver and intestinal enzyme CYP3A4 and may be responsible for the effects of grapefruit on the enzyme. Cytochrome isoforms affected by grapefruit components also include CYP1A2, CYP2C9, and CYP2D6. Drugs metabolized by these enzymes may have interactions with citrus chemicals.
When drugs are taken orally, they enter the gut lumen to be absorbed in the small intestine and sometimes, in the stomach. In order for drugs to be absorbed, they must pass through the epithelial cells that line the lumen wall before they can enter the hepatic portal circulation to be distributed systemically in blood circulation. Drugs are metabolized by drug-specific metabolizing enzymes in the epithelial cells. Metabolizing enzymes transform these drugs into metabolites. The primary purpose for drug metabolism is to detoxify, inactivate, solubilize and eliminate these drugs. As a result, the amount of the drug in its original form that reaches systemic circulation is reduced due to this first-pass metabolism.
Many drugs are impacted by consumption of citrus juice. When the metabolizing enzyme is inhibited, less of the drug will be metabolized by it in the epithelial cells. This interaction 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 effect and toxicity. Citrus juice inhibits the enzyme only within the intestines if consumed in small amounts. When larger amounts are consumed they may also inhibit the enzyme in the liver. The hepatic enzyme inhibition may cause an additional increase in potency and a prolonged metabolic half-life (prolonged metabolic half-life for all ways of drug administration). The degree of the effect varies widely between individuals and between samples of juice, and therefore cannot be accounted for a priori.