Grapefruit juice and the fruit itself may be delicious and packed with vitamin C and potassium, but consuming them when taking certain medications can be problematic.
Darrell Willm, senior director of pharmacy services at Tidelands Health, says the thick-skinned citrus fruit has a property that can block the actions of CYP3A4, an enzyme that helps metabolize some medications. When that happens, more of the medication can enter your bloodstream and stay in your body longer, which can lead to potentially serious side effects.
Conversely, some studies have shown grapefruit having the opposite effect with some medications by limiting the amount of medication that enters the bloodstream – thus rendering it less effective. For example, that situation can occur with Allegra (fexofenadine).
As a result, the FDA has required warnings on some prescription and over-the-counter medications because of potentially dangerous interactions with grapefruit and grapefruit juice.