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Drug-drug or drug-herb interactions can occur in several different ways:

• Pharmacodynamic interactions occur when the objects drug's effect is altered by the interfering drug or herb.  This interaction is not due to an alteration in the plasma concentration of either drug.  The net effect can be additive, synergistic (together the two drugs can achieve better results than the sum of their two actions alone) or antagonistic.  These adjectives can refer to alteration in the object drug's intended therapeutic effect, or can refer to the change in the toxicity levels and adverse, side-effects as well.
• Pharmacokinetic interactions denote changes in the absorption, distribution, metabolism or elimination of the object drug due to the presence of the interfering drug.  Unlike pharmacodynamic interactions, these interactions do result in changes in the plasma concentration of the object drug, and as a result, the toxic or sub-therapeutic levels occur more frequently.
• A good example of pharmacokinetic interactions include the cytochrome P450 system.  The interfering drug may act as an inducer, inhibitor and/or substrate of the same P450 enzyme that is responsible for the metabolism of the object drugs.  For instance, in people taking both St. John's wort and the indinavir, St. John wort induces the production of the P450 enzyme CYP3A4.   As CYP3A4 metabolizes indinavir, plasma levels of indinavir are decreased,   More CYP3A4 equal more metabolism of indinavir, as thus, lower plasma levels.  The concentration of indinavir may drop to sub-therapeutic levels.

Unfortunately, very little is known about herb-drug interactions.  So far, only St. John’s wort has been specifically shown to cause problems.  There are several reasons for the paucity of information, including_:13

• Research on Chinese medicinal herbs is not as advanced as research on pharmaceutical drugs.  Herbal products are not studied or monitored for safety and potential interactions to the same extent as prescription, FDA-approved drugs. 
• Also stemming from the above reason, the studies of prescription drugs rarely consider potential interactions with herbs
• Since herbal products are not regulated by the FDA, the ingredients of herbal products can vary greatly between manufacturers and even between batches from the same manufacturer
• Most health care practitioners do not have clinical experience combining herbs and drugs.

Because there isn't much research, information about herb-drug interactions is usually pieced together from several sources, such as:

• Anecdotal information
• Clinical experience
• Reports of potential interactions--based on knowledge of how specific drugs and herbs work together, their metabolism, the metabolic response to their administration (e.g. CYP450 induction, inhibition)
• Practictioners' own individual suspicions of interactions, based on a "reasonable likelihood," considering the metabolism of both drugs and herbs.  (see below for an example of how this might work)
• Pharmacokinetic studies that measure fluctuations in the herb plasma concentrations
• In vitro studies

The "reasonable likelihood" means of predicting an interaction, mentioned above, can be exemplified as follows.  If one knows that, for instance, the HIV Protease Inhibitors(PI's) (indinavir, ritonavir, saquinavir) are metabolized by the CYP450 enzymes 3A4,5,7, then one would expect any herb that is also metabolized by these enzymes to effect the plasma levels of the PI's.  This guess is only a "reasonable" one--not unequivocal.

The best way to avoid any herb-drug interaction is to be open and honest with your physician about which herbs you are taking.  He or she has access to databases that can look up the metabolic pathways for your drugs and herbs, as well as any known interactions that occur by this means or any others that were described at the top of this page.