Effect of Grapefruit Juice on the Urinary 6β-hydroxycortisol/cortisol Ratio

The effect of grapefruit juice on the urinary 6β-hydroxycortisol/cortisol ratio was investigated in healthy volunteers. The urinary concentration of cortisol was virtually unaffected by the administration of grapefruit juice whereas the concetration of 6β-hydroxycortisol in the urine decreased when grapefruit juice was given every 3 hours. The administration of grapefruit juice was thus considered to result in a decrease in the urinary 6β-hydroxycortisol/cortisol ratio. In addition, both diurnal and individual differences were also observed in the extent of the inhibitory effect of grapefruit juice.