Thursday, February 18, 2010

Herbal Products Drug Interactions


Until 150 years ago, all medicines were derived from natural substances. Most of these early medicines were described under the broad heading ‘herbs’ although that term was misleading. Even though people often think of herbs as plant or plant-derived materials, several commonly used items were obtained from animals and minerals. Further, the term ‘herbs’ suggests something that is beneficial and has little potential for harm. A recent survey shows that 18% of adults in the United States use prescription drugs concurrently with herbal or vitamin products, placing an estimated 15 million people at risk of potential drug interactions. It is quite common for a patient to seek herbal treatment while taking several prescription medications and safety has become a major issue. The manufacturers of these herbal products are not required to submit proof of safety and efficacy to the USFDA before marketing. For this reason, the adverse effects and interactions associated with herbal remedies are largely unknown. Concurrent use of herbs may mimic, magnify or oppose the effects of drugs. The possibility of drug-herb interactions is very high. The outcome of the interactions may lead to adverse effects.

Types of herbal interactions:

Herbal interactions can be classified into pharmacokinetic and pharmacodynamic interactions as that of modern medicines. The former involves mainly the Cytochrome P450 drug metabolizing enzymes in the liver microsomes. For example, the constituent of garlic inhibits the activity of various CYP isoforms, including CYP3A4 in vitro, Silibinin the major constituent of milk thistle inhibits CYP3A4 and CYP2C9 activities in vitro. Peppermint oil and menthol (a constituent of peppermint oil) inhibits CYP3A4 activity in vitro. The pharmacodynamic interactions can be predicted if the phytochemical ingredients of a herb and the pharmacology of the interacting drugs are known.

The most common example for this type is the increased risk of bleeding due to warfarin when taken with Garlic. Some of the clinically significant drug-herb interactions are summarized in the table below.

Herbal drug


Interacting drugs



(Capsicum annum)

Carminative, Antirheumatic and in Neuralgia

ACE inhibitors, Theophylline, Sedatives, Antidepressants

Increases drug absorption and effect; possibility of cough with ACE inhibitors


(Allium sativum)

Antihyperlipidemic and antihypertensive

Aspirin, Clopidogrel Dipyridamole, Warfarin Cyclosporine, Saquinavir

Risk of bleeding with anticoagulants. Decreased effect of immunosuppressants and protease inhibitors

Ginger (Zindiber officinale)

Antiemetic and antivertigo antivertigo

Aspirin, Ticlopidine, Clopidogrel, Dipyridamole, Warfarin, H2-blockers and Proton Pump Inhibitors

Increased risk of bleeding. May counteract antiulcer medications.

Ginkgo (Ginkgo biloba)

Increasing blood circulation, oxygenation and for improving memory and mental alertness

Aspirin, Ticlopidine, Clopidogrel, Dipyridamole, Warfarin, Antidepressants, Antipsychotics, Insulin

May increase the antiplatelet and anticoagulant effect and increased risk of bleeding. Increased risk of seizures with antipsychotics and affects insulin levels.

St.John's wort (Hypericum perforatum)

Depression, enlarged prostate and urinary inflammations

Antidepressants, Protease inhibitors, Non-nucleoside reverse transcriptase inhibitors, Digoxin, Theophylline, Cyclosporine, Oral Contraceptive Pills (OCPs)

Known to cause serotonin syndrom. May reduce the effectiveness of OCPs and bioavailability of digoxin, theophylline and cyclosporine




Nervine tonic


Reduced Anti-epileptic activity

Aloe (Aloe vera)

Carminative, Purgative

Digoxine, Thiazide

Increased cardiac toxicity due to electrolyte imbalance.


Historically, herbs and drugs have been two very different treatment modalities, which have rarely been used together. The line that separates herbs and drugs, however, has been blurred in recent decades with the increased accessibility to the lay public of different treatment modalities. Even though herbal products are available without prescription, medical guidance is necessary because of the adverse effects of these products and the potential for drug interaction.

The solution to this situation lies in the understanding of drug-drug and drug-herb interaction in detail with scientific evidence. With understanding of these mechanisms, one can recognize potential interactions and take proper steps to prevent their occurrence.


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