Question from: 12/9/96
Answer: It's true that grapefruit juice (but not orange juice) has the ability to increase the blood concentration of some, but not all, members of a certain class of blood pressure lowering medicines called Calcium Channel Blockers (CCB's). Some of these medications include nifedipine (Adalat; Procardia), felodopine (Plendil), and verapamil (Calan, Isoptin, and others). Grapefruit juice had no effect on diltiazem (Cardizem, and others).
Apparently, there is an as yet unidentified chemical in grapefruit juice that inhibits the liver's cytochrome P450 metabolism of certain medications. A similar effect of grapefruit juice on increasing the blood concentrations of terfenadine (Seldane), estrogens, and caffeine has also been found. The effect of grapefruit juice on other drugs metabolized by P450 isozymes remains to be determined.
Hmmmmm.....maybe I'll start having a little bit of grapefruit juice with my morning coffee!
Charles H. Booras, MD
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