Merck Manual

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Some Possible Dietary Supplement–Drug Interactions

Some Possible Dietary Supplement–Drug Interactions

Medicinal Herb

Affected Drugs

Interaction

Anticoagulants (drugs that prevent blood clots, such as warfarin)

Chamomile taken with anticoagulants may increase the risk of bleeding.

Sedatives (such as barbiturates and benzodiazepines)

Chamomile may intensify or prolong the effects of sedatives.

Chamomile may reduce iron absorption.

Drugs that can damage the liver (such as amiodarone, anabolic steroids, ketoconazole, and methotrexate)

Echinacea taken for more than 8 weeks may damage the liver. When echinacea is taken with another drug that can damage the liver, the risk of liver damage may be increased.

Immunosuppressants (drugs that intentionally suppress the immune system, such as corticosteroids and cyclosporine)

By stimulating the immune system, echinacea may negate the effects of immunosuppressants.

Ephedra*

Stimulant drugs (such as caffeine, epinephrine, phenylpropanolamine, and pseudoephedrine)

Ephedra contains ephedrine, which is a stimulant that increases the stimulant effects of other drugs, increasing the risk of irregular or rapid heart rate and high blood pressure.

Monoamine oxidase inhibitors (MAOIs, a type of antidepressant)

Ephedrine may intensify the effects of these drugs and increase the risk of side effects, such as headache, tremors, irregular or rapid heart rate, and high blood pressure.

Anticoagulants (such as warfarin)

Feverfew taken with anticoagulants may increase the risk of bleeding.

Feverfew may reduce iron absorption.

Feverfew may increase heart rate and blood pressure when it is taken with drugs used to manage migraine headaches.

NSAIDs reduce the effectiveness of feverfew in preventing and managing migraine headaches.

Anticoagulants (such as warfarin)

Garlic taken with anticoagulants may increase the risk of bleeding.

Antiplatelet drugs (such as aspirin or clopidogrel)

Garlic may increase risk of bleeding.

Antihypertensives

Garlic may increase antihypertensive and BP-lowering effects.

Isoniazid

Garlic may reduce absorption of isoniazid.

Protease inhibitors (such as indinavir or saquinavir), which are used to treat human immunodeficiency virus (HIV) infection

Garlic decreases blood levels of protease inhibitors, making them less effective.

Anticoagulants (such as warfarin)

Ginger taken with anticoagulants may increase the risk of bleeding.

Antiplatelet drugs (such as aspirin or clopidogrel)

Ginger may increase risk of bleeding.

Anticoagulants (such as warfarin), and antiplatelet drugs (such as aspirin, and other NSAIDs)

Ginkgo taken with anticoagulants or with aspirin or other NSAIDs may increase the risk of bleeding.

Anticonvulsants (such as phenytoin)

Ginkgo may reduce the effectiveness of anticonvulsants in preventing seizures.

Monoamine oxidase inhibitors (MAOIs, a type of antidepressant)

Ginkgo may intensify the effects of these drugs and increase the risk of side effects, such as headache, tremors, and manic episodes.

Anticoagulants (such as warfarin), and antiplatelet drugs (such as aspirin, and other NSAIDs)

Ginseng taken with anticoagulants or with aspirin or other NSAIDs may increase the risk of bleeding.

Ginseng may intensify the effects of these drugs, causing an excessive decrease in blood sugar levels (hypoglycemia).

Ginseng may intensify the side effects of corticosteroids.

Digoxin

Ginseng may increase digoxin levels.

Estrogen therapy

Ginseng may intensify the side effects of estrogen.

Ginseng can cause headache, tremors, and manic episodes when it is taken with MAOIs.

Opioids (narcotics)

Ginseng may reduce the effectiveness of opioids.

Anticoagulants (such as warfarin)

Goldenseal may increase the effects of anticoagulants and may increase the risk of bleeding.

Antihypertensives

Berberine content may increase antihypertensive effects, possibly decreasing blood pressure too much.

Antihyperglycemics

Berberine may increase hypoglycemic effects

Warfarin

Green tea may cause warfarin to be less effective.

Sedatives (such as barbiturates and benzodiazepines)

Kava may intensify or prolong the effects of sedatives.

Licorice (Glycyrrhiza glabra)†

Licorice may increase salt and water retention and increase blood pressure, making antihypertensives less effective.

Licorice may increase the risk of an abnormal heart rhythm, making antiarrhythmic therapy less effective.

Digoxin

Because licorice increases urine formation, it can result in low levels of potassium, which is excreted in urine. When licorice is taken with digoxin, the low potassium levels increase the risk of digoxin toxicity.

Licorice may intensify the effects of most diuretics, causing increased, rapid loss of potassium. Licorice may interfere with the effectiveness of potassium-sparing diuretics, such as spironolactone, making these diuretics less effective.

Licorice may intensify the effects of these drugs and increase the risk of side effects, such as headache, tremors, and manic episodes.

Milk thistle may intensify the effects of these drugs, causing an excessive decrease in blood sugar levels.

Protease inhibitors (such as indinavir or saquinavir), which are used to treat HIV infection

Milk thistle decreases blood levels of protease inhibitors, making them less effective.

Warfarin

Milk thistle may increase risk of bleeding by increasing effects.

Estrogen therapy and oral contraceptives

Saw palmetto may affect hormone levels.

Antiplatelets and anticoagulants 

Saw palmetto may increase effects and may cause bleeding.

Cyclosporine

St. John’s wort may reduce blood levels of cyclosporine, making it less effective, with potentially dangerous results (such as rejection of an organ transplant).

Digoxin

St. John’s wort may reduce blood levels of digoxin, making it less effective, with potentially dangerous results.

Iron

St. John’s wort may reduce iron absorption.

St. John’s wort may intensify the effects of MAOIs, possibly causing very high blood pressure that requires emergency treatment.

St. John’s wort increases the metabolism of these drugs, reducing their effectiveness.

St. John’s wort increases the metabolism of these drugs, reducing their effectiveness.

Photosensitizing drugs (such as lansoprazole, omeprazole, piroxicam, and sulfonamide antibiotics)

When taken with these drugs, St. John’s wort may increase the risk of sun sensitivity.

Protease inhibitors (such as indinavir or saquinavir), which are used to treat HIV infection

St. John’s wort may reduce blood levels of protease inhibitors, making them less effective.

Selective serotonin reuptake inhibitors (SSRIs, such as fluoxetine, paroxetine, and sertraline)

St. John’s wort may intensify the effects of these drugs.

Tricyclic antidepressants

St. John's wart may decrease the effects of these drugs.

Warfarin

St. John’s wort may reduce blood levels of warfarin, making it less effective and clot formation more likely.

Sedatives (such as barbiturates and benzodiazepines)

Valerian may intensify or prolong the effects of sedatives, causing excessive sedation.

* Sale of supplements containing ephedra is banned in the United States.

† True, natural licorice, not the more common, artificially flavored licorice candy.