According to the U.S. Department of Agriculture, grapefruits are Americans' second favorite citrus fruit, with only oranges being more popular among consumers. However, per capita grapefruit consumption in the U.S. has fallen 50 percent since the late 1970s, mainly due to revelations that grapefruit interferes with the metabolism of many medications. This grapefruit-drug interaction has been demonstrated in several classes of medications, including some cardiac agents; your blood levels of these medications must be carefully regulated. Ask your doctor if your medication is affected by grapefruit.
Mechanism
Medications are foreign to your body, which must metabolize them before ultimately eliminating from your tissues. Your body must convert some drugs to active forms before they can exert their beneficial effects. Enzymes in your intestine and liver are largely responsible for the conversion and detoxification of many drugs. One such intestinal enzyme is called CYP3A4. Grapefruit interferes with this enzyme's ability to degrade certain medications. A September 2010 review in "Cardiovascular Journal of Africa" notes that grapefruit also affects proteins in your intestinal wall that transport drugs into your bloodstream.
Calcium Channel Blockers
Grapefruit's ability to block CYP3A4 was first demonstrated in 1989 during a clinical trial designed to evaluate alcohol's interactions with felodipine, a medicine used to treat high blood pressure. Since then, this same grapefruit-drug interaction has been demonstrated with nicardipine, nifedipine, nimodipine, nisoldipine and verapamil. In most cases, grapefruit's actions lead to dramatically elevated blood levels of these calcium channel blockers, which can cause dangerous decreases in your blood pressure or trigger heart rhythm abnormalities.
Anti-arrhythmics
Drugs that regulate your heart rhythm must be carefully monitored because the difference between beneficial and toxic blood concentrations is frequently small. By slowing the intestinal metabolism of amiodarone and quinidine and increasing their blood levels, grapefruit enhances your risk for dangerous side effects, including dizziness, liver damage, low blood pressure, worsening heart rhythm abnormalities and cardiac arrest. An August 2006 "American Family Physician" review reports that another anti-arrhythmic, disopyramide, is also a potential target for grapefruit's effects.
Considerations
Grapefruit interacts with dozens of medications, either by increasing their bioavailability and elevating your blood levels or by interfering with their conversion to active forms and decreasing their effectiveness. Cardiac drugs, cholesterol-lowering agents, sedatives, anti-convulsants and immune-suppressing medications are among the pharmaceuticals affected by grapefruit. If you take any prescription drug or regularly use an over-the-counter medication or herbal supplement, ask your doctor if you can safely consume grapefruit products.
References
- U.S. Department of Agriculture Economic Research Service
- "Cardiovascular Journal of Africa"; The Grapefruit: An Old Wine in a New Glass? Metabolic and Cardiovascular Perspectives; P.M. Owira, et al.; September/October 2010
- The People's Pharmacy: Graedon's Guide to Grapefruit Interactions
- "American Family Physician"; Management of Grapefruit-Drug Interactions; A.L. Stump, et al.; August 2006
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