In general, grapefruit juice inhibits intestinal CYP3A4, and only slightly affects hepatic CYP3A4. This is demonstrated by the fact that intravenous preparations of drugs that are metabolised by CYP3A4 are not much affected, whereas oral preparations of the same drugs are, and their levels are increased by grapefruit juice. Some drugs that are not metabolised by CYP3A4 show decreased levels with grapefruit juice. This is probably because grapefruit juice is an inhibitor of some drug transporters, such as
P-glycoprotein. The active constituent of grapefruit juice is uncertain. However, grapefruit contains naringin, which degrades during processing to naringenin, a substance known to inhibit CYP3A4. Because of this, it has been assumed that whole grapefruit will not interact, but that processed grapefruit juice will. However, subsequently some reports have implicated the whole fruit. Other possible active constituents in the whole fruit include bergamottin and dihydroxybergamottin.
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