Grapefruit and grapefruit juice are known to have interactions with many different drugs. In the intestinal wall, components in grapefruit called furanocoumarins inhibit a certain enzyme system, known as cytochrome P450 3A4. Suppression leads to significantly higher blood levels as less is presystematically cleared, greater bioavailability, and increased risk of potential adverse effects and toxicity. If you are taking a drug that is extensively metabolized, avoid grapefruit and its juice. Consult your doctor for a complete list of interactions.
Antiinfective Agents
Antiinfective agents, such as the antimalarial drug halofantrine and the antiretroviral drug indinavir, are affected when grapefruit is consumed. Administering indinavir with grapefruit may delay the absorption of the drug as well as increase the time required to reach its peak plasma concentrations. With antimalarial drugs, coadministration with grapefruit may result in increased concentrations of the drugs.
Immunsuppressants
Immunosuppresants, such as cyclosporine, tacrolimus and sirolimus, are another type of drug affected by grapefruit. These drugs work by lowering the immune system and are most commonly used after a kidney, liver or heart transplant to prevent organ rejection. They may also be used to treat autoimmune disorders including rheumatoid arthritis. Grapefruit interacts with cyclosporine by increasing blood levels of this medication and risk of side effects.
Psychiatric Drugs
Grapefruit has an effect on antidepressants and antianxiety medications, such as buspirone, clomipramine and sertraline, or Zoloft, which work by changing the chemicals or by reacting with specific chemical receptors in the brain. Consumption of large amounts of grapefruit juice is associated with significantly increased plasma concentrations and delays reaching peak drug concentration. The drugs also stay in the body for much longer before being totally eliminated.
Cholesterol-Lowering Statins
These drugs, also known as HMG-CoA reductase inhibitors or "statins," reduce levels of LDL, the "bad" cholesterol, and triglycerides in the blood, while increasing levels of HDL, the "good" cholesterol. These medications, such as lovastatin, simvastatin and atorvastatin, may also be used to lower the risk of heart attacks or other complications. In small studies, the consumption of grapefruit juice significantly increased the concentrations of these drugs and their active derivatives, and resulted in more cases of musculoskeletal toxicity.
Calcium-Channel Blockers
Calcium channel blockers, including amlodipine, diltiazem, felodipine, nicardipine, nifedipine, nimodipine, nisoldipine and verapamil, are also prone to an interaction with grapefruit. These drugs relax and widen heart and blood vessel muscles and are usually used to treat hypertension or chest pain. The most prominent interaction occurs with felodipine. Eating grapefruit may increase the amount of this drug in your blood, which may increase your risk of serious side effects.
Other Drugs
Other drugs, including antiarrhythmic and anticonvulsant medications, are also affected by grapefruit. Amiodarone is a antiarrhythmic that helps keep the heart beating normally in people with life-threatening heart rhythm disorders. Grapefruit may significantly increase the plasma concentrations of this drug. Antiseizure medications, such as carbamazepine, work by decreasing nerve impulses that cause seizures and pain. With concomitant intake of grapefruit, the plasma drug concentrations can increase up to about 40 percent.
