Grapefruit juice increases blood concentrations of some dihydropyridine calcium-channel blockers, which are metabolised by the P450 enzymes that also metabolise cyclosporin. We evaluated, in a randomised cross-over study, the effect of grapefruit juice on blood cyclosporin concentrations in 14 healthy adults. Each subject was given oral cyclosporin 300 mg with 250 mL grapefruit juice, orange juice, or water. Area-under-the-curve (AUC) was significantly higher with grapefruit juice than with water or orange juice (means 7057, 4871, and 4932 ng h/mL, respectively; p < 0.0001). Thus grapefruit juice may provide a non-toxic and inexpensive alternative to drugs that are used to reduce cyclosporin dose.