Harvesting the best: evidence-based analysis of herbal handbooks for clinicians

INTRODUCTION

The recent increase and widespread use of herbs and dietary supplements (H/DS) extends across the lifespan from pediatric to geriatric patients [1–4]. Patients with chronic conditions are among the heaviest users as they constantly search for additional ways to relieve their symptoms and discomfort [5, 6]. At the same time, a parallel proliferation in the literature has been published on the subject: 405 books on medicinal herbs were published from 1986 to 1989, with more than 1,000 titles appearing between 2000 and 2004 [7]. Clinicians and patients have many sources of information, but selection of the most reliable references is a challenge. Lack of information on potential toxicities, adverse effects, and interactions with medications, other herbs, laboratory tests, and disease states becomes critical in providing health care. Due to time constraints during clinical encounters, many clinicians prefer the convenient presentation and concise entries afforded by handbooks [8].

The objective of this pilot project was to develop criteria for evaluating the quality of tertiary literature on medicinal herbs and dietary supplements and to apply these criteria to selected handbooks intended for clinicians. The tertiary literature is defined here to include textbooks, compendia, and full-text computer databases [9].

METHODS

Based on well-respected drug information standards [9–12], the authors established the following categories for evaluation of H/DS handbook entries: botanical names or synonyms, basic pharmacology, dosage forms with adult and pediatric doses, therapeutic uses (both popular and evidence based), contraindications (including pregnancy and lactation), adverse reactions, interactions with drugs, interactions with laboratory tests, and references to current primary literature. The WorldCAT database indicated that 56 handbooks on medicinal plants or herbal medicine were published in English between 2001 and 2005 [13]. The selection criteria for the handbooks included in this pilot study were: date of publication (2001–2005), known and respected medical or complementary and alternative medicine (CAM) publishers [14], and the authors' experience in providing CAM-related information.‡,§ Thus, a convenience sample of handbooks was selected from mainstream medical literature, authored by health care professionals, and published by large, well-known medical publishers. The project did not attempt comprehensive coverage of all herb and supplement handbooks for clinicians. The twelve titles are listed in Table 1.

Table 1 Handbooks evaluated

Three herbs were selected (garlic, black cohosh, and yohimbe) to test the proposed methodology. Garlic is one of the most commonly used supplements. Black cohosh is a gender- and population-specific therapy, and yohimbe is a relatively toxic, more obscure, and less commonly used herb.

Handbook entries for these herbs were compared to the comprehensive monographs from the tertiary “gold standard” resources: Micromedex–AltMedDex, Natural Medicines Comprehensive Database, and Natural Standard [15–18]. “Gold standard” was defined in the current study as the method, procedure, or measurement that is widely accepted as being the best available, against which new developments should be compared. These comprehensive monographs were selected based on reviews in the literature as well as the authors' experience in using them to answer questions [19, 20]. Based on the content in these monographs and the authors' evaluation categories, the content rating system was developed (Table 2).

Table 2 Evaluation criteria: ratings assigned to information categories in handbook monographs

RESULTS

Using the objective criteria created by the authors, twelve handbooks were evaluated. The highest possible rating for any handbook was forty-five points, indicating a rating of five in all nine categories. To demonstrate this process, Table 3 illustrates how the ratings were applied to information on garlic. The four handbooks that received the highest ratings were: Natural Standard Herb and Supplement Handbook: The Clinical Bottom Line, Mosby's Handbook of Herbs and Supplements and Their Therapeutic Uses, Mosby's Handbook of Herbs & Natural Supplements, and Professional's Handbook of Complementary & Alternative Medicine. The information presented in these four books was more descriptive, better referenced, and more complete than in the other handbooks. Table 4 shows the comparative ratings for the handbooks studied and illustrates their strengths and weaknesses.

Table 3 Example of rating of information on garlic

Table 4 Summary of handbook ratings

DISCUSSION

In this project, three experts in the fields of information and dietary supplements developed evaluative criteria that can provide preliminary guidance on the selection of H/DS handbooks. Handbooks were reviewed based on the developed criteria and compared to the “gold standard” references. The criteria used to evaluate the handbooks are described in detail in Table 2 and applied in Table 3.

Each category had a number of special considerations, including those relevant to the subject of dietary supplements. For the category “name,” the essential entry was a botanical binomial and common name. Experience indicated that numerous variations on both of these were possible; thus, an increasing number of entries counted toward a higher rating. Because herbs are often used as part of ethnic healing, the addition of non-English common names was helpful. Finally, brand names are useful to a harried clinician.

The compact size of handbooks dictates that the “pharmacology” category is described in less detail. The sources receiving higher ratings listed several active ingredients and provided short descriptions of mechanism of action. The highest-rated handbooks also described the pharmacology of individual constituents' actions and their therapeutic classification.

Herbal products are available in a variety of dosage forms ranging from prepared pharmaceuticals to recipes for teas, decoctions, and poultices. The handbooks rated more highly in this category described the available forms for administration and gave accepted doses for adults. The highest-rated entries also included doses for children and gave dosing for specific indications.

In the category “historic/popular/traditional uses,” handbooks provided insight for clinicians as to why patients self-medicate with a particular herb. The more extensive entries included mention of uses by ethnic groups, as a part of traditional healing or religious rituals and lore. The criteria were written based on the hope that discussion of ethnic uses would appear under the “historic/popular/traditional uses” category. However, none of the handbooks addressed this topic, and it was eliminated from the criteria.

Entries for “uses with scientific evidence” varied from mere mentions to specific descriptions of research related to an indication. The best sources rated or discussed the strength of the evidence.

As health care professionals learn more about herbal toxicities and adverse reactions, information on “contraindications” and “adverse reactions” becomes an essential feature in a quality reference. Evaluation of contraindications content revealed that information varied from precautions in the general population to the incorporation of precautions in pregnancy and lactation and teratogenic potential. The handbooks rated most highly for this category discussed dangers for special conditions such as patients with renal failure, liver failure, and other conditions that might affect herb metabolism. Better “adverse reactions” entries added information on prevalence and distinguished between theoretical versus documented adverse reactions. A detailed listing added one or more of the following, adverse reactions in animal studies versus human studies, mutagenic potential, adverse reactions related to various dosage forms, and lethal doses.

Increased knowledge of interactions between H/DS and pharmaceuticals, other herbs, and laboratory tests has focused attention on this category. Better handbooks listed interactions with other herbs, foods, disease states, laboratory tests, or diagnostic procedures. Because many interactions are strictly hypothetical, notes as to the prevalence of an interaction and whether it is theoretical or documented are the gold standard.

Because these handbooks are written for clinicians grounded in evidence-based medicine, all entries should have references to the primary scientific literature. The highest rating meant that the handbook had current and updated references to tertiary and primary resources integrated in the text. This category highlighted the advantage of the online “gold standard” databases, because they are easily and frequently updated as compared to print handbooks. Table 5 provides a description of strengths and weaknesses of the best titles; the best features of the other titles are described in Table 6. These tables can assist librarians and other health care professionals with selecting titles based on their information needs.

Table 5 Top-rated handbooks

Table 6 Strengths of other handbooks listed by category

Upon completion of this review, it was discovered that some of the categories needed revision for the sake of clarity or because the handbooks did not perfectly fit into the criteria, sometimes falling somewhere in between categories. For example, the criteria were based on the supposition that updating references and citing primary literature is common practice. As the evaluation process continued, it became apparent that few handbooks met this standard. Consequently, the criteria evolved so that lower ratings indicated citation of current tertiary sources. The criteria were also adjusted to reflect the fact that when primary references were used, often many were old and a recent literature review had not been done. The reviewers' standard was lowered to reflect the range of information offered by the handbooks and still be able to objectively differentiate between the titles.

Several handbooks intended for nurses were included. The overall impression was that these nursing titles offered more patient education material. In addition, they often integrated conventional and complementary medicine approaches. This finding would lead the authors to add an evaluation category addressing patient education to future, more comprehensive reviews.

One of the major liabilities of this evaluation was that it was based on the subjective judgment or interpretation of the evaluators. The authors attempted to make the criteria as objective as possible but there potentially was still room for subjective differences in interpretation of how some of the criteria should be applied. Also, two of the evaluators were pharmacists, so it was possible that a practitioner from another health care profession would evaluate the references differently.

CONCLUSIONS

Objective criteria for evaluating the content of herbal information references were developed and employed in this project. In this evaluation of commonly used herbal handbooks, the best resources of this kind for a busy health care practitioner were identified. This methodology may provide a useful tool for medical librarians and health care professionals in selecting handbooks on herbs and dietary supplements.