A Note of Caution: Be Careful When Mixing Grapefruit Juice With Rxs

April 06, 1999

New Study Finds Grapefruit Juice Could Decrease the Absorption of Many Drugs

Alexandria, VA -- Drinking a glass of grapefruit juice with medications has long been known to help with the body's absorption of certain drugs. However, scientists at the University of California at San Francisco (UCSF) have now found that grapefruit juice may actually have a negative impact on the body's absorption of many widely-prescribed medications.

Grapefruit juice is known to improve the oral absorption of several important medications on the market by decreasing levels of an intestinal enzyme, known as CYP3A4, that would otherwise breakdown drug molecules before they reach the blood stream. These drugs include antihypertensive calcium channel blockers (i.e., felodipine, Plendil and HIV-protease inhibitors (i.e., saquinavir, Invirase).

In a study* published in the April 1999 issue of Pharmaceutical Research, an American Association of Pharmaceutical Scientists (AAPS) publication, Dr. Andrea Soldner and other scientists at UCSF demonstrate that grapefruit juice can actually inhibit the body's absorption of certain drugs including: Vinblastine (for combating cancer)
Cyclosporine (for supressing organ rejection following transplant)
Losartan (for controlling high blood pressure)
Digoxin (for treating congestive heart failure)
Fexofenadine (for alleviating allergy symptoms)

This inhibition occurs because an unknown substance in grapefruit juice activates one of the body's naturally-produced drug efflux mechanisms, known as P-glycoprotein, located in the intestinal tract. When grapefruit juice interacts with P-glycoprotein, the result is an increased likelihood that certain drugs will be stopped from entering the bloodstream.

These findings help to clarify some major discrepancies we've noticed in the impact of grapefruit juice on various types of medications," said Dr. Leslie Benet, founder of the American Association of Pharmaceutical Scientists, professor of Biopharmaceutical Sciences at UCSF and director of the study. Patients already taking grapefruit juice with their medications can continue to do so. However, for certain drugs we studied, such as immunosuppressives and HIV protease inhibitors, patients may get a further increase in absorption by taking their drugs a couple of hours after a glass of grapefruit juice.

According to Dr. Benet, Patients who have not previously taken their drugs with grapefruit juice should be very cautious in doing so, since we now recognize, depending on the drug, that grapefruit juice may either increase or decrease levels of drug in the blood, leading to potential concerns for toxicity or lack of efficacy.
AAPS is a professional, scientific society of more than 9,000 members employed in academia, industry, government and other research institutes worldwide. Founded in 1986, the goal of AAPS is to improve human health through the development of better pharmaceuticals. For more information about AAPS, visit our Web site at http://www.aaps.org.

*The results of the in vitro experiments are only preliminary and have yet to be proven in animal and human studies.

American Association of Pharmaceutical Scientists

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