Many people do not realize how many potential reactions there are between common foods and pharmaceutical drugs. Some of these interactions may just cause the drug to be less effective, but others can be dangerous. If you’ve ever been prescribed a drug and been warned to avoid grapefruit or grapefruit juice, it is because grapefruit juice is known to have a dangerous effect on the metabolism of many commonly prescribed drugs. In fact, it interacts with over 95 medicines.
Why Does Grapefruit Juice Interact with Drugs?
Grapefruit juice alters the metabolism of many drugs in the GI tract, causing more of the drug to be absorbed and enter the bloodstream. To understand how this occurs we must first understand a bit about drug metabolism in general.
1. Aschenbrenner, Diane S., and Samantha J. Venable. Drug Therapy in Nursing. Wolters Kluwer Health/Lippincott Williams & Wilkins, 2012.
2. Commissioner, Office of the. “Consumer Updates – Grapefruit Juice and Some Drugs Don’t Mix.” U S Food and Drug Administration Home Page, Office of the Commissioner, www.fda.gov/ForConsumers/ConsumerUpdates/ucm292276.htm.
3. “Common Grapefruit Juice Drug Interactions.” Drugs.com, Drugs.com, www.drugs.com/article/grapefruit-drug-interactions.html.
Enzymes (what is an enzyme?) called P-450 isoenzymes are responsible for drug metabolism. These enzymes are found in the liver, but also in the intestinal wall. Grapefruit juice blocks one of these enzymes in the gut, called CYP3A4. The blockage of this enzyme causes more of the drug to be absorbed through the intestinal wall which means that blood levels of the drug increase much more than if no grapefruit juice were consumed.
Grapefruit juice can also affect certain transporters responsible for moving the drug into the body’s cells. When these transporters are blocked, the amount of drug in the body is decreased. The juice even affects common OTC allergy drugs like Allegra (fexofenadine) in this way.
Fexofenadine, in fact, is also affected by apples and orange juice, both of which cause lower levels of the drug so that its effectiveness is reduced.
What Drugs are Affected?
Grapefruit’s action on this important enzyme affects the metabolism of many types of drugs, including:
- calcium channel blockers for hypertension
- statins for blood lipid levels
- corticosteroids for Crohn’s disease, ulcerative colitis (e.g. Entocort EC and Uceris)
- antihistamines for allergies
- drugs for abnormal heart rhythms like Pacerone and Nexteron (both contain amiodarone)
- some organ transplant rejection drugs, like Sandimmune and Neoral (both contain cyclosporine)
The specific compound in grapefruit juice that causes this interaction is still unknown. It is also important to know that the levels of CYP3A4 are not constant in all people. Some people have more of the enzyme than others. Therefore, some people will be less or more affected by consuming grapefruit or drinking the juice while taking these drugs than others. It is possible for one person to experience only a very small effect while another is greatly affected.
Your doctor can work with you to determine if you can safely consume grapefruit juice while taking any of the affected drugs. The amount of grapefruit consumed can increase or decrease the effect. Also, if you already habitually consume grapefruit before beginning drug therapy, the dose of medication can be adjusted to get a proper therapeutic effect. If you start consuming grapefruit only after beginning a drug therapy and only consume it now and again, this can cause a big problem. Therefore, if you begin taking one of these drugs, you should thereafter avoid grapefruit.
Grapefruit juice does not necessarily affect all possible drugs in the categories listed above.
The drug facts label on your OTC drug or the patient information sheet that comes with your prescription drug will tell you whether grapefruit juice affects the drug. Also be aware that other fruit juice flavored drinks may contain grapefruit, so check the labels of any juices you drink.
Seville Oranges, Pomelos, and Tangelos
You may also want to avoid Seville oranges, pomelos, and tangelos. These may have the same effect as grapefruit juice. Tangelos, in fact, are a cross between tangerines and grapefruit or tangerines and pomelos.
For more information, see the FDA consumer update on grapefruit juice drug interactions, and consult Drug.com where you can also find a list of common medications affected, as well as a means to research grapefruit drug interactions.