Grapefruit, although part of a healthy breakfast for many people, is off the menu for people taking certain medications, including Nifedical XL, an extended-release drug used to treat high blood pressure and angina. Nifedical XL -- the generic name of which is nifedipine -- is classified as a calcium channel blocker, a drug that relaxes blood vessels by slowing the movement of calcium through the heart and blood vessel walls. Grapefruit interferes with the absorption of Nifedical XL in the intestines and can cause higher-than-normal drug levels to accumulate in the bloodstream.
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Chemicals in grapefruit bind to an enzyme in the intestine called CYP3A4. This enzyme normally reduces absorption of medications in the intestine. Doctors don't know exactly which chemical in grapefruit causes interference with absorption of medications, but they think furanocoumarin -- also found in some types of oranges such as Seville oranges and tangelos -- causes the problem.
Effects
Because grapefruit inactivates the enzyme CYP3A4, more medication enters the bloodstream than normally would occur. Medication also enters the bloodstream faster than it normally would. Drinking just one glass of grapefruit juice can inactivate as much as 47 percent of CYP3A4, according to the Harvard Medical School Family Health Guide. The effects of grapefruit juice can last a long time; after 24 hours, 33 percent of the effect is still present.
Risks
If a higher-than-normal dose of Nifedical XL enters your bloodstream, more severe side effects of the drug could occur. In the case of Nifedical XL, your heart rate could become dangerously slow or your blood pressure could fall too low. Other potential side effects of this drug that could worsen if you consume grapefruit include dizziness, drowsiness, nausea, constipation, water retention and headaches.
Considerations
If you take a low dose of this medication, you may be able to have grapefruit products occasionally, but only if your doctor okays it. Do not take grapefruit and Nifedical XL at the same time; the more time between the two, the less effect grapefruit will have. Take the same amount of grapefruit daily so that you have the same medication effect every day, if your doctor allows it. If you're already taking grapefruit with this medication, do not stop it abruptly without talking to your doctor; you may need to have your medication dose adjusted.
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