5 Common Food-Drug Interactions

Reviewed by Taylor Wolfram, MS, RDN, LDN
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You've probably heard the warnings not to drink grapefruit juice with cholesterol medication. However, that isn't the only combination of food and drugs to avoid. Grapefruit juice can interact with numerous other medications, both prescription and over-the-counter. And many other foods commonly interact with drugs, too.

Here are five foods that most commonly interact with medications.

Grapefruit Juice

One way grapefruit juice interacts with medication is by increasing the absorption of certain drugs — as is the case with some, but not all, cholesterol-lowering statins. MedinePlus recommends avoiding grapefruit juice if you are taking statins.

Grapefruit juice also can cause the body to metabolize drugs abnormally, resulting in lower or higher than normal blood levels of the drug. Many medications are affected in this way, including antihistamines, blood pressure drugs, thyroid replacement drugs, oral contraceptives, stomach acid-blocking drugs, and the cough suppressant dextromethorphan. It's best to avoid or significantly reduce intake of grapefruit juice when taking these medications.

But why is grapefruit juice of concern and not other citrus juices? Grapefruit juice contains a class of compounds called furanocoumarins, which act in the body to alter the characteristics of these medications. Orange juice and other citrus juices do not contain these compounds. There is some concern for Seville oranges and the pomelo, which are relatives of the grapefruit.

Green Leafy Vegetables

Blood-thinning drugs such as Coumadin® (warfarin) interfere with vitamin K-dependent clotting factors. Eating too many green leafy vegetables, which are high in vitamin K, can decrease the ability of blood-thinners to prevent clotting. But you don't have to give up greens altogether. Problems arise from significantly and suddenly increasing or decreasing intake, as it can alter the effectiveness of the medicine. So eat your greens in consistent amounts rather than avoiding them altogether.

Natural Black Licorice (Glycyrrhiza)

Glycyrrhiza — a natural ingredient used to make black licorice — can deplete the body of potassium while causing an increased retention of sodium. When the body is depleted of potassium, the activity of digoxin, a medication used to treat heart failure, can be greatly enhanced, resulting in the heart not beating properly.

Glycyrrhiza also can decrease the effectiveness of high blood pressure medicines. And people taking Coumadin® (warfarin) should beware that glycyrrhiza can break down the drug, resulting in an increase in the body's clotting mechanism.

Excessive amounts of natural licorice should be avoided when taking all of these medications. However, artificially-flavored black licorice doesn't contain glycyrrhiza and is not of concern.

Salt Substitutes

Consumers taking digoxin for heart failure or ACE inhibitors for high blood pressure should be careful with salt substitutes, which most often replace sodium with potassium. With an increased consumption of potassium, the effectiveness of digoxin can be decreased, resulting in heart failure. And those taking ACE inhibitors might see a significant increase in blood potassium levels, as these drugs are known to increase potassium.

If you have decreased kidney function, discuss the use of salt substitutes with your doctor.

Tyramine-Containing Foods

High blood levels of the amino acid tyramine can cause an increase in blood pressure. Several medications interfere with the breakdown of tyramine, including monoamine oxidase inhibitors, or (MAOIs, which treat depression, and drugs used to treat the symptoms of Parkinson's disease. Tyramine-rich foods includes, but is not limited to: chocolate, aged and mature cheeses, smoked and aged/fermented meats, hot dogs, some processed lunch meats, fermented soy products and draft beers (canned and bottled beers are OK).

When receiving a prescription for a new medication or taking a new over-the-counter drug, always read drug warning labels and ask your physician or pharmacist about which foods or other drugs you should avoid or be concerned about taking.

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