Ever since the “grapefruit effect” was discovered more than a decade ago, this fascinating food and drug interaction has confused patients, physicians and pharmacists. People still don’t understand why it’s important, how long it lasts and how to avoid problems.

The controversy began in Canada when researchers were trying to determine whether alcohol would interact with a blood pressure medicine called Plendil (felodipine). The investigators chose double-strength grapefruit juice to mask the flavor of alcohol. This way the subjects could not tell if they were getting alcohol or not.

Although the alcohol interaction was not very interesting, the experiment did produce unexpected results. Grapefruit juice alone raised blood levels of felodipine dramatically, leading to adverse effects in some cases.

This chance discovery has led to a whole new appreciation of how foods can affect drug metabolism. Grapefruit, for example, can alter dozens of medicines. And a new report in the journal Neurology (Feb 24, 2004) suggests that some of these interactions can be potentially life threatening.

A 40-year-old German woman was admitted to the hospital complaining of severe weakness in both legs. This was rather puzzling, because she had been exercising vigorously until 10 days earlier, and enjoyed the sport of skydiving.

Laboratory tests suggested a serious muscle breakdown reaction as a result of her cholesterol-lowering medicine. The drug was stopped, and doctors intervened and prevented kidney failure.

She had been taking the drug, Zocor, for two years before this reaction (called rhabdomyolysis) developed, so doctors asked her what she had done differently. Two weeks before admission, she had followed the advice in a health magazine and begun eating a grapefruit for breakfast every day.

The scientists point out that grapefruit can interfere with the body’s normal way of handling Zocor, leading to unusually high levels of the drug. They recommend that patients on Zocor not eat grapefruit.

Zocor (simvastatin) is not the only cholesterol-lowering medicine that might interact badly with grapefruit. Lipitor (atorvastatin) and Mevacor (lovastatin) could also have unpleasant reactions. But grapefruit lovers can still control their cholesterol. Pravachol (pravastatin) is unaffected by grapefruit, and so is Zetia, the medicine prescribed for the German woman after she left the hospital.

Many patients wonder whether they can eat grapefruit or drink its juice at breakfast time and take their medication in the evening. This tactic seems logical, but it may be misleading. The grapefruit effect can last for more than 24 hours, so drugs that interact dangerously with grapefruit might be too risky even at bedtime. In such cases, it is worthwhile to ask the doctor if a medicine that doesn’t react with grapefruit might do the job.

We list medicines that don’t mix with grapefruit in our Guides to Grapefruit Interactions and Cholesterol Lowering Drugs. Anyone who would like copies, please send $3 in check or money order with a long (no. 10) stamped (60 cents), self-addressed envelope: Graedons’ People’s Pharmacy, No. JL-97, P. O. Box 52027, Durham, NC 27717-2027.

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