Grapefruit is an excellent source of vitamin C, folic acid, potassium, antioxidants, pectin and lycopene. It has been touted as a dietary aid for people who want to lose weight, a claim that was reinforced in a 2006 "Journal of Medicinal Food" study. However, scientists have discovered that grapefruit interferes with an enzyme, called CYP3A4, that is responsible for metabolizing certain medications. This property has important implications for people who take prescription drugs.
Calcium Channel Blockers
In 1991, Canadian researchers first discovered grapefruit's potential for increasing blood levels of medications while working with a blood pressure-lowering drug called felodipine. Other drugs in this same class, called calcium channel blockers, are similarly affected by grapefruit. These blockers include nifedipine, nicardipine and verapamil. Elevations in blood levels of these drugs can lead to dizziness, severe falls in blood pressure and heart rhythm abnormalities.
Heart Rhythm Medications
Quinidine and amiodarone, two drugs that are used to control abnormal heart rhythms, compete with grapefruit for metabolization by the CYP3A4 enzyme complex. Although designed to prevent the development of dangerous electrical disturbances in your heart, these medications can themselves cause serious and, in the case of quinidine, even fatal heart rhythm disturbances when your blood levels are too high.
Statin Drugs
Statins, which are used to lower blood cholesterol, are among the most commonly prescribed pharmaceutical agents in America. Simvastatin, lovastatin, pravastatin and atorvastatin belong to this class. The consumption of grapefruit with a statin drug increases the likelihood of the usual side effects associated with statin use, which include liver damage, muscle injury and kidney failure.
Hormones
Oral contraceptives and agents used for hormone replacement therapy, such as estradiol, are among the drugs that are affected by grapefruit. As is the case with other medications, grapefruit increases your blood levels of these hormones, which raises the risk of side effects, such as blood clots, headaches, nausea and vomiting, breast tenderness and excessive vaginal bleeding.
Sedatives and Anticonvulsants
Among the sedatives and anti-anxiety medications affected by grapefruit are diazepam, zaleplon, buspirone and triazolam. Carbamazepine, a drug used to control seizures, interacts with grapefruit, too. Increased blood levels of these medications caused by ingestion of grapefruit will intensify their side effects, which include drowsiness or difficulty sleeping, nausea, headache, constipation, confusion, depression, weakness, and difficulty breathing or walking.
Others
Less frequently prescribed medications that interact with grapefruit include immunosuppresants, such as cyclosporine, sirolimus and tacrolimus, used by transplant patients; and anti-HIV drugs, such as saquinavir and ritonavir. Dextromethorphan, an over-the-counter cough remedy, is also affected by grapefruit. Dozens of other medications are metabolized by CYP3A4, but their interactions with grapefruit have not yet been characterized. If you take a prescription medication, check with your doctor to see if you should limit your grapefruit consumption.
References
- "Journal of Medicinal Food"; The effects of grapefruit on weight loss and insulin resistance: relationship to the metabolic syndrome; Fujioka K, et al.; 2006
- "Lancet"; Interaction of citrus juices with felodipine and nifedipine; Bailey D, et al.; 1991
- "Drug Safety"; Poisoning due to class IA antiarrhythmic drugs. Quinidine, procainamide, and disopyramide; Kim S, Benowitz N; 1990
