Evidence for western herbal medicine

Evidence for efficacy and safety is a cardinal issue in all forms of natural and complementary medicine, as indeed it is also in mainstream medicine. Herbal medicine and other forms of natural medicine frequently stand accused of being supported by little or no evidence. In the case of western herbal medicine this can be comprehensively refuted as considerable evidence exists for its efficacy and safety. The issue of evidence is rather more complicated, and individual herbal medicines can claim different levels of evidence for their efficacy. Likewise, their use is associated with different levels of risk.

When assessed on the available evidence, western herbal medicine seems to be generally effective as well as safe. In terms of evidence in support of the efficacy of herbal medicine, it should be recognised that many different types of evidence may contribute to the overall picture. When assessing the evidence for a particular herbal medicine it is the totality of the different types of available evidence that should form the basis of the assessment. Different types of evidence relevant to the assessment of herbal medicines are listed in Table Types of evidence relevant to the therapeutic use of medicinal plants.

Table Types of evidence relevant to the therapeutic use of medicinal plants

Clinical evidenceNon-clinical evidence
Empirical data from traditional use (e.g. pharmacopoeial monograph)Data from in vitro (test tube) studies
Published case studiesData from animal studies
Uncontrolled trials
Randomised controlled trials
Systematic reviews of randomised controlled trials

A documented history of traditional use is an important type of evidence for many herbal medicines. Traditional use refers to the continuous use, over hundreds or sometimes thousands of years, of a remedy for a specific health problem. Traditional use normally occurs within a traditional medical system (such as western herbal medicine or Traditional Chinese Medicine) or an indigenous group. With few exceptions, all herbal medicines used in western herbal medicine are supported by evidence from traditional use. In cases where the same plant has been used in several traditional systems of medicine or by separate indigenous groups for the same ailment, the evidence of traditional use carries more weight.

In addition to traditional use many herbal medicines are now also supported by different types of clinical or scientific evidence, ranging from in vitro (test tube) investigations of biological activity to systematic reviews of randomised controlled trials. In vitro data can provide valuable information about the pharmacology and toxicology of herbal medicines, but cannot be extrapolated direcdy to use in humans. Important determinants of the clinical effects such as bioavailability and metabolism cannot be assessed in test tubes. However, data from in vitro studies are valuable when combined with information about traditional use or other clinical data. Table Types of evidence supporting the therapeutic uses of some herbal medicines gives examples of herbal medicines supported by different levels of evidence.

Table Types of evidence supporting the therapeutic uses of some herbal medicines

In vitro dataTraditional useRandomised controlled trial (RCT)Systematic review of RCTs
Burdock {Arctium lappa) in chronic skin disease
Echinacea {Echinacea spp.) in the common cold
St John’s wort {Hyperi cum perforatum) in mild-moderate depression
Cinkgo {Cinkgo biloba) leaf in peripheral arterial occlusive disease and cognitive deficit