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.

Used with permission. For further information, see Johns Hopkins Medicine 2008 Prescription Drugs White Paper, www.johnshopkinshealthalerts.com/white_papers/prescription_drugs_wp/digital08_landing.html, by Simeon Margolis, MD, PhD.