The effective dose of red yeast rice to lower cholesterol can doubled by taking it
with grapefruit juice. See the reference below. Grapefruit juice inhibits the metabolism of
HMG-CoA reductase inhibitors by the liver and gut P450 isoenzyme CYP3A4.

Kantola T, Kivisto KT, Neuvonen PJ. Grapefruit juice greatly increases
serum concentrations of lovastatin and lovastatin acid. Clin Pharmacol Ther
1998;63:397–402.