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Disclaimer -

There is still a lack of solid research around interactions between herbal medicines and prescribed drugs. While a few clinical studies and trials have been conducted, it should be noted that some recommended actions are also based on case reports (concerning individual patients) and even theoretical concerns.

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Andrographis -

Andrographis (Andrographis paniculata) is contraindicated for patients taking immunosuppressant medication, as it may reduce their effectiveness. A clinical study has also found that it may potentiate the effects of midazolam.

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Ashwagandha -

There is a theoretical concern that taking this herbal medicine may potentiate the effects of thyroxine.

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Cayenne (chili pepper) -

A clinical study has found that cayenne (capsium spp) may increase absorption and levels of theophylline.

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Celery seed -

Two case reports have shown that celery seeds may reduce serum levels of thyroxine.

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Chamomile -

Chamomile (matricaria recutit) may potentiate the effects of warfarin and cause bleeding.

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Cranberry -

Cranberry (vaccinium macrocarpon) may decrease levels of immunosuppressants in the blood.

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Echinacea -

Echinacea may decrease the effectiveness of immunosuppressive medication. The herb may also interact with antiretroviral drugs.

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Evening primrose oil -

Evening primrose oil (oenothera biennis) may potentiate the effects of antiplatelet and anticoagulant drugs. But according to clinical evidence, there is a low level of risk. In addition, a case report has shown a reduction in serum levels of lithium.

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Garlic -

Garlic (allium sativum) can interact with paracetamol and decrease blood concentrations of warfarin. It can also cause hypoglycemia when taken with chlorpropamide. Garlic may also decrease the blood levels of HIV protease inhibitors.

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Ginger -

Ginger (zingiber officinale) may increase the effectiveness of phenprocoumon and therefore increase the risk of bleeding when taken with warfarin. It may also interact with nifedipine, producing a synergistic antiplatelet effect.

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Ginkgo -

Ginkgo (ginkgo biloba) interactions include antiplatelet and anticoagulant drugs such as warfarin (it can cause bleeding). It can raise blood pressure when combined with thiazide diuretic, and can lead to coma when taken with trazodone.

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Ginkgo -

Other interactions include antipsychotic medication, antiretroviral drugs, benzodiazepines, hypoglycemic drugs, nifedipine, tadalafil, and talinolol. Many of these, however, have a low level of risk.

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Green tea -

The use of green tea (camellia sinensis) is contraindicated for patients taking isoniazid, as it may affect the liver. It may also reduce the bioavailability of sunitinib, and decrease the effectiveness of ziprasidone.

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Hawthorn -

Clinical studies have found that hawthorn may increase the effectiveness of digoxin.

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Horsetail -

Two case reports have shown that horsetail (equisetum arvense) may cause a virological breakthrough when taken with antiretroviral drugs.

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Kava -

Kava (piper methysticum) can interact with Parkinson medications, particularly levodopa (or L-Dopa). It can also interact with alprazolam and lead to a semicomatose state.

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Korean ginseng -

Ginseng (panax ginseng) can lower blood concentrations of warfarin, and induce mania if used with phenelzine. It may decrease the effectiveness of antihypertensive medications, including nifedipine.

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Korean ginseng -

Ginseng may also cause side effects such as headaches, sleeplessness, and tremors when taken with MAO inhibitors. Ginseng may also interact with hypoglycemic drugs, including insulin, and potentiate hypoglycemic activity of these drugs.

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Licorice -

Licorice (glycyrrhiza glabra) may decrease the effectiveness of antihypertensive medications (excluding diuretics).

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Licorice -

Licorice may also cause hypokalemia when taken with cilostazol, according to a case report. In high doses, licorice can actually cause pseudoaldosteronism and high blood pressure.

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Milk thistle -

A clinical study has found that milk thistle increases the levels of domperidone and its potential toxic side effects. Hepatically-impaired patients may want to avoid taking it with immunosuppressives such as sirolimus, as it may reduce drug clearance. Milk thistle may also increase blood levels of ornidazole and talinolol.

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Rhodiola -

Rhodiola (rhodiola rosea) may interact with SSRIs, affecting serotonin levels.

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Sage -

A case report showed that sage may potentiate the adverse effects of methadone.

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Schisandra -

Schisandra (schisandra chinensis) may increase drug concentrations of immunosuppressives.

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St. John's wort -

Research has shown that St. John’s wort can reduce blood levels of benzodiazepines, calcium channel antagonists, cancer chemotherapeutic drugs, clozapine, immunosuppressives, cyclosporin, amitriptyline, digoxin, indinavir, warfarin, phenprocoumon, and theophylline. This flowering plant also interacts with oral contraceptives.

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Turmeric -

Turmeric (curcuma longa) may potentiate the effects of antiplatelet and anticoagulant drugs. Turmeric may also increase drug levels of tacrolimus and talinolol. There is a case report of acute liver injury related to long-term use of the herb in combination with etoricoxib.

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Valerian -

There is theoretical concern, but not much clinical evidence to support it, that valerian (valeriana officinalis) potentiates the effects of CNS depressants or alcohol.

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Willow bark -

A clinical study has found that willow bark may potentiate the effects of warfarin.

Sources: (Standard Process) (PubMed) (International Society of Pharmacovigilance) 

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