Grapefruit juice inhibits CYP3A4 in the intestinal wall leading to a reduced intestinal first pass metabolism and thereby an increased oral bioavailability of certain drugs. For example, it has been shown that the oral bioavailability of midazolam, a CYP3A4 substrate, increased by 52% in healthy subjects after ingestion of grapefruit juice. However, this interaction has not been studied in patients with impaired liver function. Accordingly, the effect of grapefruit juice on the AUC of midazolam and the metabolite alpha-hydroxymidazolam was studied in patients with cirrhosis of the liver.
Andersen, V., Pedersen, N., Larsen, N-E., Sonne, J., & Larsen, S. (2002). Intestinal first pass metabolism of midazolam in liver cirrhosis: effect of grapefruit juice. British Journal of Clinical Pharmacology, 54(2), 120-124. https://doi.org/10.1046/j.1365-2125.2002.01615.x