Johns Hopkins Health Alert
Chemical Culprit in Grapefruit-Drug Interactions Identified
People are discouraged from consuming grapefruits or grapefruit juice while taking certain medications because they can affect the way the medications are metabolized. Now scientists are closer to understanding why this dangerous interaction occurs.
Certain foods and drinks don’t mix well with certain medications. For example, grapefruits or grapefruit juice may interact badly with a number of medications, because natural grapefruit contains a substance that affects the activity of an enzyme in the intestines and liver that processes these medications. This could result in a dangerous increase in the level of the drug in your blood.
Another potentially dangerous interaction is between the blood thinner warfarin (Coumadin and generic brands) and vitamin K. The vitamin, present in many multivitamins and supplements, neutralizes or reduces the effect of the medication warfarin. This raises the risk of a blood clot, which the warfarin is intended to prevent.
Now scientists have identified the specific chemical in grapefruit juice responsible for many drug–food interactions, according to an article in the American Journal of Clinical Nutrition (Volume 83, page 1097).
Previous research implicated a family of chemical compounds called furanocoumarins (FCs) as the culprit in grapefruit juice. To confirm this suspicion, the scientists created FC-free grapefruit juice and compared its effects with those of whole grapefruit juice or orange juice.
Eighteen study volunteers drank 8 oz of whole or FC-free juice along with a dose of felodipine (Plendil), a blood pressure medication. (The blood level of Plendil is higher if taken with grapefruit juice, potentially causing dangerously low blood pressure.) The blood concentration of Plendil was nearly three times higher when people took it with 8 oz of whole grapefruit juice, compared with blood levels after subjects took it with the FC-free juice or orange juice. The researchers said their finding could assist in the study of other drug–food interactions.
Posted in Prescription Drugs on February 5, 2008
Reviewed September 2011
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I believe that there other medications that interact unfavorably with grapefruit juice. Can you tell me what they are? Thank You S. Golub MD
Posted by: rsgolubmd | February 9, 2008 11:07 AM
Two obvious questions remain. (1) How long does the FC effect last? (2) Can one lower the dose or frequency of taking a drug if one drings grapefruit juice?
Posted by: bobbob | February 10, 2008 9:38 AM
How do I get replies to comments?
Posted by: Casey | February 10, 2008 8:14 PM
Why not take advantage of grapefruit's tendency to amplify the affects of drugs, by prescribing lower doses that MUST be taken with grapefruit? Seems like a way to reduce drug costs.
Posted by: TRAPAA | February 11, 2008 12:16 PM
Why not take advantage of grapefruit's tendency to amplify the affects of drugs, by prescribing lower doses that MUST be taken with grapefruit? Seems like a way to reduce drug costs.
Posted by: TRAPAA | February 11, 2008 12:18 PM
this explains why some drugs can make the pro time higher or lower, I cannot find any pain medication except tylenol that will not interfere with my coumadin... I never could understand why the same medication could work two ways either make the reading higher or lower...
Posted by: junesandy | February 14, 2008 3:33 AM
Grapefruit juice can amplify the effects of a drug, but it can ALSO decrease the effect. The drug mentioned in this article is just one drug of many. It has not been exhaustively tested with all medications, but with what testing there has been, grapefruit juice was found to block absorption as well as increase it. So plain water is your best bet, and always following the instructions on the bottle.
Many people also don't get the best effects out of their medications because they don't take them as instructed. And many people get a prescription from the doctor, and don't ever even fill it. It could be cost-related, it is true. But for example, with antibiotics, people often stop taking the medication before they have taken the full course of treatment. So always read your instructions that should come with your medicines, or ask your pharmacist. Many bottles will even have warning stickers on them now, and I have seen grapefruit juice warning stickers on some medicines recently.
Posted by: Jo | February 15, 2008 8:33 AM
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Note from the editor:
The mention of orange juice was not an error. In fact, orange juice was the "control group" in the study.
The study looked at blood levels of felodipine (Plendil; a blood pressure drug) after participants consumed 1 of 3 beverages:
1) grapefruit juice 2) furanocoumarin-free grapefruit juice. Furanocoumarins are compounds believed to be responsible for the drug-food interaction between grapefruit juice and some medications OR 3) orange juice
The researchers found that blood levels of Plendil were similar in people who drank orange juice or furanocoumarin-free grapefruit juice.
But levels of the drug were 3 times higher in those who drank regular grapefruit juice.
Thus, orange juice is safe to drink with blood pressure medication. However, you may want to avoid Seville oranges, as they are the only type of oranges to contain furanocoumarins.
We hope this clarifies all our reader questions.
Posted by: Jo | February 7, 2008 8:47 AM