Medical Discovery News
By Drs. David Niesel and Norbert Herzog
Millions of Americans sit down every morning with a cup of coffee, milk, or juice and take their daily medications. The problem? Depending on what’s in that cup, they may be putting themselves at risk for serious side effects or even death.

It’s not the coffee or the milk – it’s the juice, specifically grapefruit juice. A recent study in the Canadian Medical Association Journal identified 85 drugs that interact with grapefruit juice, of which 43 can cause serious problems. Many of these drugs are commonly prescribed for important medical conditions: Lipitor for lowering cholesterol, Nifediac for lowering blood pressure, Xanax for treating anxiety, oxycodone for pain relief, and even Viagra. 

When drugs are taken orally, they move through the stomach to the small intestine, where the majority of absorption to the bloodstream occurs. Once in the bloodstream, the drug begins to take effect and start working.

Then the body starts to inactivate and break down drugs, first in the intestine and continuing in the liver, in order to remove them from the body. A superfamily of enzymes called cytochrome P450 is responsible for this. An important member of this enzyme family is CYP3A4, which processes about half the drugs in use today, including acetaminophen, codeine, diazepam and erythromycin.

However, in addition to grapefruits, Seville oranges, tangelos, limes and pomelos can also disrupt this process. They all contain furanocoumarins, which reduce the amount of CYP3A4 in the intestine. Without enough of this enzyme to start inactivating and breaking down the drug, more of the substance enters the bloodstream instead.

Too much of a drug in circulation can be toxic, causing damage to the liver and kidneys, gastrointestinal tract bleeding, respiratory failure, bone marrow suppression and even death. For example, levels of the blood pressure medication filodipine in the blood are five times higher when it is taken with grapefruit juice instead of water.

Ironically, while it can be toxic when taken with some drugs, grapefruit can dampen the effect of others. For example, it reduces the absorption of the allergy medication Allegra, making it less effective. This may be because it blocks the specific proteins that transport Allegra cross the membranes of the gastrointestinal tract.

The influence of a single glass of grapefruit juice on a drug’s metabolism can last for up to 24 hours. And if someone drinks grapefruit juice more than once a day, the effect is amplified. However, since reactions differ among individuals, some people don’t react at all. Those on prescription medications should ask their doctor or pharmacist how to properly take them. To see a list of the 85 medications that interact with grapefruit, visit 

Medical Discovery News is a weekly radio and print broadcast highlighting medical and scientific breakthroughs hosted by professor emeritus Norbert Herzog, and professor David Niesel, biomedical scientists at the University of Texas Medical Branch at Galveston. Learn more at