Twenty years ago, Canadian researchers discovered grapefruit interferes with the body's metabolism of certain drugs, including the immunosuppressant cyclosporine and at least one drug used to treat high blood pressure.

The same group of scientists reported recently the number of drugs on the market that react adversely with grapefruit has increased substantially in recent years - from 17 to 43. The list includes cholesterol-lowering statins such as Zocor and Lipitor and blood pressure medications such as Nifediac and Afeditab, the study notes.

Chemicals found in grapefruit change the way these medications are metabolized in the gastrointestinal tract, dramatically increasing concentrations of the drug in the bloodstream. Those chemicals, called furanocoumarins, are also present in other citrus fruits, including Seville oranges - the kind often used to make marmalade - and limes and pomelos.

The amount of grapefruit that can set off a reaction varies from drug to drug, the study notes, but in many cases, about eight ounces of juice or a whole grapefruit "has sufficient potency to cause a pertinent pharmacokinetic interaction."

The drugs in question have three common traits: They're all taken orally; they all have limited bioavailability (which means that only small percentages of the active drug make it into the bloodstream under normal circumstances); and they all interact in the GI tract with an enzyme called CYP3A4. The researchers have identified more than 85 drugs that might interact with grapefruit, but some of these interactions aren't likely to cause serious adverse effects.

High concentrations of the grapefruit-sensitive drugs can be toxic to the kidneys and can also lead to GI-tract bleeding, respiratory failure, bone-marrow suppression among people with comprised immune systems and sudden death.

These interactions can take place many hours after grapefruit or its juice is consumed, and even just a single grapefruit or glass of grapefruit juice a day can spell trouble if you're taking one of these drugs.

People older than 45 are at increased risk, the study notes, in part because they tend to eat more grapefruit than younger people do and also take more prescription drugs. Older people are more susceptible to the ill effects of high concentrations of drugs in their system, too, the study says.

To reduce your risk, ask your physician or pharmacist about whether the prescription drugs you take are likely to be affected by grapefruit consumption.