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Mary PaineMary F. Paine, R.Ph., Ph.D., is an associate professor in the Department of Experimental & Systems Pharmacology in the College of Pharmacy at Washington State University, Spokane, Washington.

Herbal products represent an ever-increasing share of the healthcare market and constitute a multibillion dollar industry that has been increasingly incorporated into Western medicine. Since passage of the Dietary Supplement Health and Education Act in 1994, sales in the United States have nearly tripled, reaching an estimated $5.6 billion in 2012. These products are marketed to a variety of niches, encompassing claims from enhanced libido to prevention of the common cold. Based on the perception that “natural” equates with “safe,” consumers often turn to these products as a means to decrease healthcare costs, alleviate self-diagnosed illnesses, or supplement prescribed therapeutic regimens.

Consequently, patients frequently take herbal products in conjunction with conventional medications, triggering potentially unwanted herb-drug interactions. Despite considerable investigation, clinical relevance and pharmacokinetic elucidation of these interactions remains inconclusive. Assessing the risk of herb-drug interactions typically is more challenging than that of drug-drug interactions, often due to relatively scant pharmacokinetic knowledge of individual herbal constituents that perpetrate such interactions. Comprehensive understanding of the dose-exposure-response relationship underpinning herb-drug interactions is essential for safe coadministration of herbal products with conventional medications.

Systematic evaluation of drug-drug interactions is routine during drug development, yet no such system exists for the evaluation of herb-drug interactions. This deficiency likely is due, in large part, to a number of limitations unique to the investigation and prediction of herb-drug interactions. Because herbal products are plant-derived, product variation can occur at multiple stages of production, from identification of the plant source to the manufacturing process. Rigorous characterization of herbal product composition rarely is conducted prior to clinical evaluation, resulting in herb-drug interaction studies that frequently yield conflicting results with questionable generalizability.

Even when herbal products are well characterized, adequate pharmacokinetic data rarely are available to describe herbal constituent disposition in a robust manner. The lack of pharmacokinetic data hampers the ability to extrapolate results beyond an individual study, compare results of independently conducted studies, and provide mechanistic insight into observed differences in study outcomes. Thorough assessment of herbal constituent pharmacokinetics is not trivial, being subject to numerous challenges not typically encountered when assessing the pharmacokinetics of drug molecules.

The April issue of the AAPS Newsmagazine highlights challenges in dealing with herb-drug interactions and suggests approaches to meeting these challenges. Read Predicting Pharmacokinetic Herb-Drug Interactions: Overcoming Hurdles That Extend beyond Drug-Drug Interactions from the  Pharmacokinetics, Pharmacodynamics, and Drug Metabolism section of AAPS and then participate in the discussion question below.

What are the current approaches for assessing pharmacokinetic herb-drug interactions, and how can they be improved?