The use of herbal medicines is widespread and growing. In developing countries, they often represent the main form of healthcare, and in developed countries the use of OTC and individualised herbal medicines has rocketed.
However, herbal medicine is associated with several myths, such as the 'natural equals safe' myth, the synergy myth and the evidence myth.
Herbal medicines in the form of whole-plant extracts vary greatly in composition and represent unquantifiable safety risks. Plants evolved their many diverse secondary metabolites, not for the good of mankind but in order to deter parasites, predators and other threats.
The chemical constitution of plant extracts vary with genetic and phenotypic differences arising in different environments, and from seasonal and developmental changes during the life cycle of the plant.
Standardisation of extracts typically controls the concentrations of only one or a small number of bioactive components or toxins thought to be important. Partial characterisation and standardisation of plant extracts means that potentially harmful compounds may remain undetected or occur at unpredictable concentrations.
For individually tailored herbal mixtures prepared from freshly collected plant material, this risk is amplified. The uncertainty about what is being ingested when herbal medicine is used is increased by the possibility of species misidentification, contamination or adulteration, unreliable standardisation and extract instability.
A disturbing example of deliberate adulteration was revealed when some Chinese herbal remedies were found to contain unlabelled prescription drugs, including steroids.
The risk of negative herb-drug interactions is also well documented. Examples include interactions between Hypericum perforatum (St John's wort) and ciclosporins, SSRIs and other drugs, and also between Ginkgo biloba and anticoagulants.
The concomitant use of several plant species in individualised herbal medicine increases the potential for negative herb-drug or herb-herb interactions. An individually tailored herbal remedy might contain as many as 20 different herbs.
The use of multiple self-prescribed, OTC extracts is also common. A British survey found it is common for people over 50 to take six herbs and supplements and several prescription drugs at the same time.
Of even greater concern was the fact that only around 30 per cent of herb and supplement use is reported to doctors, which may reflect the widespread belief in the 'natural equals safe' fallacy.
Synergy is the idea that the effect of one component in an extract is amplified by the presence of other components and that the increase in effect is more than what would be expected from simple addition of their separate effects. A synergistic effect might produce novel or greater therapeutic results, beneficial effects at lower dosages, or fewer or less severe adverse effects.
The evidence for synergistic effects in phytomedicines has been outlined in two reviews. However, neither of these reviews provides a single example of a proven synergistic effect between components of a whole-plant extract in vivo in humans or in a clinical context.
True synergy can only be demonstrated by first isolating the individual components and measuring their separate and combined effects. This requirement means that, even if a synergistic effect with a clinically relevant application was convincingly demonstrated, it would then be more logical, and safer, to use a therapeutically optimised combination of the active components, rather than revert to the uncertainty of a whole-plant extract.
Until truly synergistic effects with clinical relevance are convincingly proven, synergy in herbal medicine remains no more than a convenient, even if plausible, myth.
The variability in plant extracts means that evidence for efficacy and safety obtained from clinical trials or other scientific studies is usually specific only to the particular sample tested. In many studies the species being researched cannot even be identified with certainty.
Only a handful of herbs have been tested in multiple clinical trials using the same standardised extracts. The most well known of these are Ginkgo biloba, used in dementia and venous insufficiency, and St John's wort, used in depression. Even for these extensively studied herbs the evidence for efficacy is far from unequivocal.
The evidence supporting the use of individualised herbal medicine, in which combinations of several different herbs are tailored by the herbalist for the individual patient, is even more sparse. In a recently published systematic review, we were able to locate only three randomised clinical trials of individualised herbal medicine in any indication.
The results of these three studies led us to conclude that there is presently no convincing evidence to support the use of individualised herbal medicine in any indication.
Given the extensive use of individualised herbal medicine worldwide and its long history, these findings should be a cause for concern.
The claims made by herbalists that their practice is evidence-based are misleading because they are based on the results of studies with single standardised whole-plant extracts.
The findings of this review are particularly pertinent because section 12(1) of the UK's Medicines Act relates to the regulation of unlicensed herbal remedies made up to meet the needs of individual patients. The Medicines Act has been undergoing a review.
Plants undoubtedly contain many compounds with potentially important clinical applications, demonstrated by the development of aspirin from Salix (willow) and digoxin from Digitalis (foxglove).
The future of herbal medicine may well lay in the isolation and rational design of more of such drugs, rather than in the continued use of variable and poorly understood extracts and mixtures with uncertain effects and significant safety risks.
Dr Canter is a research fellow in complementary medicine at Peninsula Medical School, Plymouth.
- Herbal medicine is associated with several myths: 'natural equals safe', the synergy myth and the evidence myth.
- Risks associated with herbal medicines include toxicity, species misidentification, contamination, deliberate adulteration and negative herb-drug and herb-herb interactions.
- There are no proven examples of clinically relevant synergistic effects in whole-plant extracts.
- The evidence base for herbal medicine, and particularly for individualised herbal medicine involving herb mixtures, is extremely weak.
- Canter P H, Ernst E. Herbal supplements use by persons aged over 50 years in Britain: Drugs Ageing 2004; 21: 597-605.
- Guo R, Canter P H, Ernst E. A systematic review of randomised clinical trials of individualised herbal medicine in any indication. Postgrad Med J 2007; 83: 633-7.
- Williamson E M. Synergy and other interactions in phytomedicines. Phytomedicine 2001; 8: 401-9.
- Spinella M. The importance of pharmacological synergy in psychoactive herbal medicines. Altern Med Rev 2002; 7: 130-7.
- Canter PH. Does the synergy argument in herbal medicine add up? Evidence Based Integrative Medicine 2005; 2: 115-6.
- United Kingdom's Medicines Act www.statutelaw.gov.uk/content.aspx?activeTextDocId=1662209 .