Grapefruit juice increases antihypertensive effect of
calcium channel blockers
Arch Fam Med
(Jul) 5:413-416 1996
Grapefruit juice appears to cause a significant range of drug interactions.
In March of last year
(Clin Pharmacol Therap 57:318-324, 1995) Hollander et al found that a small
glass of grapefruit
juice given every 3 hours for 3 hours caused substantial increase (by 185
ng/ml) in the peak
concentration of cyclosporin. The postulated mechanism was inhibition of
cyclosporin metabolism via
cytochrome P450 enzymes in the gut wall and liver.
In the present paper, Pisarik reports an augmented anti-hypertensive effect
of a patient taking a
calcium channel blocker in a single patient. He cites other studies in which
several calcium channel blockers appeared to be markedly increased by
grapefruit juice. In this
case, the mechanism appears to be due to the presence of flavonoids in
grapefruit juice, which may
inhibit metabolism of certain calcium channel blockers.
Comment: The scope of drug interactions due to grapefruit juice may be
quite large and
certainly deserves further study.
(John T. Daugirdas, M.D., University of Illinois at Chicago)