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Grapefruit can have a number of interactions with drugs, often increasing the effective potency of compounds. In the field of polyphenolic compounds, grapefruit contains the flavanone naringin, alongside the two furanocoumarins bergamottin and dihydroxybergamottin, which inhibit the protein isoform CYP3A4 predominately in the small intestine, but at higher doses, hepatic CYP3A4 inhibition is present as well.[18] It is via inhibition of this enzyme that grapefruit increases the effects of a variety of drugs by increasing their bioavailability.[19][20][21][22][23] 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.[24]

Grapefruit juice may be the first documented, but apple and orange juices have been also implicated in interfering with etoposide, a chemotherapy drug, some beta blocker drugs used to treat high blood pressure, and cyclosporine, taken by transplant patients to prevent rejection of their new organs.[25]

Unlike other fruits, grapefruit contains a large amount of naringin, and it can take up to 72 hours before the effects of the naringin on the CYP3A4 enzyme are seen. This is particularly problematic due to the fact that only 4 oz of grapefruit contain enough naringin to inhibit substrates of CYP3A4.

Grapefruit and grapefruit juice have the potential to interact with numerous drugs. A number of organic compounds, identified as furanocoumarin derivates, interfere with the hepatic and intestinal enzyme cytochrome P450 isoform CYP3A4 and are therefore primarily responsible for the interaction. However, bioactive compounds in grapefruit juice may also interfere with P-glycoprotein and organic anion transporting polypeptides (OATPs) either increasing or decreasing bioavailability of a number of drugs.