Overall, grapefruit is a very nutritious fruit and is commonly consumed during breakfast. It is estimated that about 20 percent of Americans eat grapefruit or drink its juice daily. However, some compounds within grapefruit juice can directly or indirectly affect how oral medications are absorbed and metabolized. These compounds often increase the rate of absorption of certain drugs, which in turn increases the risk of overdose. Conversely, some drugs are destroyed by grapefruit juice so that not enough gets into the bloodstream to be therapeutic.
Components of Grapefruit
Grapefruit is a rich source of vitamin C, fiber and various other beneficial nutrients, but several compounds found in grapefruit juice exert inhibitory actions on the breakdown of medications by blocking enzymes in the intestines. A group of compounds called furanocoumarins is responsible for this interaction, which specifically include bergamottin, bergaptol, bergapten and 6-7-dihydroxybergamottin. Some flavonoids, such as naringin, may also interact with metabolizing drugs; grapefruit contains naringin and a few other flavonoids.
Modes of Action
The furanocoumarins within grapefruit, especially its juice, block enzymes in the wall of the small intestine that leads to destruction of many medications and prevents absorption into the bloodstream. As such, smaller amounts of the medication get into the body than anticipated, which greatly weakens the therapeutic affect. Conversely, the same compounds can block the actions of other enzymes needed to metabolize drugs and remove them from the body. When these enzymes are blocked, more biologically active chemicals from the drugs are absorbed into the blood, and the effects of the medication are magnified, which thenincreases the risks of toxic side effects. This magnifying effect was discovered in a 1989 study published in "Clinical Investigative Medicine." It was later found that the grapefruit juice mixed with the alcohol was responsible for increasing levels of the drug felodipine in the blood beyond expectations.
Medications Affected
A number of medications are affected by the compounds in grapefruit juice, and it is strongly advised not to mix them because of the unpredictable outcomes. How the drug might be affected by the juice is not easily predicted. According to the "PDR Guide to Drug Interactions, Side Effects, and Indications," these comprise all cholesterol-reducing statin-type drugs, some antihistamines, all calcium channel blockers, most psychiatric drugs for depression or anxiety, some immune suppressants, erectile dysfunction drugs containing sildenafil, such as Viagra, and a few others. Consulting your primary health physician is recommended before you mix any medication with grapefruit juice. Toxic blood levels of biologically active chemicals can occur, which could cause organ damage.
Considerations
One standard glass of grapefruit juice is considered enough to cause significant impact to the metabolization of certain medications, especially if taken within four hours of each other, but the effect may last up to 24 hours. This interaction is particularly dangerous if the medication has a low therapeutic index, which means that small increases in dosage quickly become toxic. Grapefruit juice only affects the enzymes in the intestines and not those in the liver, so injected drugs are not a concern.
References
- "Contemporary Nutrition"; Gordon M. Wardlaw; 2010
- "Clinical Investigative Medicine"; Ethanol Enhances the Hemodynamic Effects of Felodipine; D.G. Bailey et al; December 1989
- "PDR Guide to Drug Interactions, Side Effects, and Indications"; PDR Staff; 2009
- "Nutritional Sciences"; Michelle McGuire; 2007
- "Biochemistry of Human Nutrition"; George Gropper; 2000
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