Grapefruits are high in vitamin C, potassium, and dietary fiber. So they’re good for us, right?
Well, in 1989 there was a study conducted to see if grapefruit juice could mask the taste of alcohol. Through this study, researchers discovered that something in the grapefruit juice caused a rise in the blood levels of a particular blood pressure medicine. Since this study, other research has determined that grapefruits and grapefruit juice affect the blood levels of many other commonly used medications.
A drug taken by mouth must get absorbed by the intestines. It then enters the blood stream and travels throughout the body, affecting changes in the system. For example, a pill taken to lower cholesterol travels to the liver where it will block cholesterol production. A pill for blood pressure may work in the kidneys, arteries or veins.
However, the drug doesn’t work indefinitely. It is eventually metabolized or broken down so that it can be eliminated from the body. The enzymes that break down drugs are located mainly in the liver, but they are also present in the intestines and many other areas of the body as well.
Grapefruit juice contains a substance that prevents the intestinal enzyme from metabolizing the drug. Therefore, more of the drug is getting absorbed through the gut into the blood stream. This leads to a heightened effect of the drug, whether its lowering cholesterol, blood pressure or whatever it’s intended purpose. This substance has not yet been identified.
The effect of grapefruit can last up to 72 hours. This is a problem for most of the blood pressure and cholesterol medicines, because they need to be taken every day. The solution is to avoid grapefruits and grapefruit juice or to talk with your doctor about changing to a medicine that does not have this interaction.
There are several drugs that have been shown to interact with grapefruits or grapefruit juice. Some are commonly used medications such as cholesterol lowering drugs (e.g. Lipitor and Zocor); and blood pressure medicines (e.g. Plendil and Procardia).
Many other medicines may potentially interact with grapefruit, but the effects may not be clinically significant. These include Viagra, fexofenadine (Allegra), estrogens, and warfarin (Coumadin). Some over-the-counter medicines, supplements or herbal preparations may also be affected, but formal studies have not been done to determine their effects.
The safest approach is to be informed and be proactive. Examine what medications and supplements you are currently taking and talk with your doctor and pharmacist about potential interactions.
The content in this column is for informational purposes only. Consult your physician for appropriate individual treatment. Dr. Reynolds practices Family Medicine in Chesterfield.