The psychopharmacology of herbal extracts: issues and challenges

Abstract

Over the past decade or so, there has been a substantial increase in research into the effects of herbal extracts, including their potential to modulate behaviour. Unlike mainstream pharmaceuticals, which have generally developed from the isolation or synthesis of active agents aimed at specific targets, herbal extracts may contain dozens of active components. Indeed, the philosophy of herbal medicine is that whole plant extracts exert multiple subtle effects. Their components may act individually either positively or negatively, and in concert may affect multiple neuronal, metabolic and hormonal systems. Since behavioural processes are themselves modulated by such systems, the effects of herbal extracts may particularly depend upon complex interactions within and between physiological systems which, even more than mainstream drugs, may be further modulated by dispositional and situational factors. Additionally, the interactions between these actives may be synergistic, resulting in complex doseand time-dependent effects. Clearly, this presents a challenge for the psychopharmacologist, having the twofold effect of making some positive effects fragile and negative findings sometimes difficult to interpret. Nevertheless, the herbal extract market represents a multibillion dollar industry which is poorly regulated globally. Millions of individuals self-medicate daily with herbal products in the unwarranted belief that because they are ‘natural’ they must be safe. One widely used claim for a number of herbal extracts is that they improve cognitive abilities and psychological ‘wellbeing’. This is often in the absence of any evidence base regarding efficacy. Given these factors, it is imperative that herbal extracts are subject to the same level of scrutiny as other psychopharmacological agents. Several recent articles have reporting negative findings from studies into the potential cognition-enhancing effects of two widely used herbal extracts, ginseng and Ginkgo biloba (Nathan et al. 2002; Solomon et al. 2002; Persson et al. 2004). Such reports may cast doubt on the efficacy of such products in the behavioural domain.While we applaud any attempt to examine potential behavioural effects of herbal extracts, we believe that the conclusion that ginseng and Ginkgo biloba do not improve cognitive performance may be premature. Firstly, the results should be placed into a wider research context and, secondly, the methodologies employed should be subjected to proper scrutiny. Below, we highlight some of the issues which we feel are pertinent to this area. A number of authors have called for a set of criteria for assessing any behavioural effects of herbal extracts (CurtisPrior et al. 1999; Vogler et al. 1999; Kennedy and Scholey 2003; Wesnes et al. 2004). We have endeavoured to use these in a series of studies which have revealed positive and negative modulation of mood and cognitive performance by herbal extracts (see http://www.hcnu.com). Where possible, such investigations should be adequately powered and include the use of (i) standardised extracts, (ii) a placebo control, (iii) assessment of several doses, (iv) doubleblind methodology, (v) a crossover design, (vi) a baseline (pre-dosing) assessment and (vii) standardised (preferably computerised) tasks with known bidirectional sensitivity. Finally, given that the paucity of human research in this area, where possible studies should include tasks which have previously been reported as being sensitive to the extract under investigation. Solomon et al. (2002) reported a study into the effects of a standardised extract of Ginkgo biloba on established penciland-paper tests of cognitive performance. Older (over 60 years) participants took 120 mg (3×40mg) of Ginkgo, or a placebo, daily for 6 weeks. Outcome measures included memory subscales from the Weschler Adult Intelligence Scale and the Boston Naming Test, and a global evaluation A. Scholey (*) . D. Kennedy . K. Wesnes Human Cognitive Neuroscience Unit, Northumbria University, Newcastle-Upon-Tyne, NE1 8ST, UK e-mail: a.scholey@unn.ac.uk Tel.: +44-191-2274468 Fax: +44-191-2274800