As the Wellness Letter has warned for years, grapefruit and its juice can interact with many commonly prescribed medications, including some cholesterol-lowering statins, calcium channel blockers, antihistamines, antidepressants and HIV drugs. And the list of troublesome drugs is “markedly increasing,” according to a review paper published in the Canadian journal CMAJ in November, by the researchers who discovered the interactions more than 20 years ago. More than 85 drugs are potentially affected by grapefruit, they reported. In particular, the number of drugs that can have serious adverse effects when combined with grapefruit more than doubled since 2008, from 17 to 43, due to the introduction of new products.
Substances in grapefruit—furanocoumarins and various flavonoids (like naringin, which provides the tart taste)—inhibit an enzyme, CYP3A4, in the intestines and liver that helps metabolize the drugs, so you end up with a higher level of the active ingredient in your bloodstream. In some cases, blood levels of the drugs triple or more, as if you took multiple doses. Depending on the drug, this can lead to such adverse effects as acute kidney failure, respiratory problems, heart arrhythmias and gastrointestinal bleeding. On the other hand, grapefruit juice can also lower blood levels of a few drugs by reducing absorption.
The interaction occurs quickly and can last 24 hours or longer—the time it takes for your body to make new enzymes. But the effects vary from person to person, and even from grapefruit to grapefruit. Older people are at higher risk because they tend to take more medications and consume more grapefruit, plus they are more sensitive to the effects of higher drug concentrations. The danger is greater for drugs that have a narrow therapeutic window (meaning you can get in trouble if your blood level is even a little too high or too low), such as immunosuppressants.
Studies examining the interaction have mainly used large quantities of grapefruit juice, but according to the CMAJ paper, as little as one whole grapefruit or about 6 to 8 ounces of juice, consumed hours before or after the medication, can be problematic in some cases, especially when such amounts are consumed more than once a day or daily. Moreover, it’s not just grapefruit that can be a problem. Seville oranges (sour like grapefruit, used in marmalade), pomelos and limes also have furanocoumarins, the researchers noted, and thus may have similar effects. Though the evidence is inconclusive, tangelos (a tangerine/grapefruit hybrid), cranberries and pomegranates may also interact with certain drugs.
Bottom line: If you take prescription medications, check with your health care provider or pharmacist before consuming grapefruit or the fruits mentioned above. Many drugs carry a warning label if there is an interaction with grapefruit—but the absence of a warning does not mean there is no interaction. The CMJ paper contains a list of interacting drugs.
Dodging the Interaction
Do you take a medication that can interact with grapefruit? A few smart steps may help keep you out of trouble.