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. 2001 Jul 20;1:5. doi: 10.1186/1472-6882-1-5

Systematic reviews of complementary therapies – an annotated bibliography. Part 2: Herbal medicine

Klaus Linde 1,2,✉, Gerben ter Riet 3,4, Maria Hondras 5, Andrew Vickers 6, Reinhard Saller 7, Dieter Melchart 1
PMCID: PMC37540  PMID: 11518548

Abstract

Background

Complementary therapies are widespread but controversial. We aim to provide a comprehensive collection and a summary of systematic reviews of clinical trials in three major complementary therapies (acupuncture, herbal medicine, homeopathy). This article is dealing with herbal medicine. Potentially relevant reviews were searched through the register of the Cochrane Complementary Medicine Field, the Cochrane Library, Medline, and bibliographies of articles and books. To be included articles had to review prospective clinical trials of herbal medicines; had to describe review methods explicitly; had to be published; and had to focus on treatment effects. Information on conditions, interventions, methods, results and conclusions was extracted using a pre-tested form and summarized descriptively.

Results

From a total of 79 potentially relevant reviews pre-selected in the screening process 58 met the inclusion criteria. Thirty of the reports reviewed ginkgo (for dementia, intermittent claudication, tinnitus, and macular degeneration), hypericum (for depression) or garlic preparations (for cardiovascular risk factors and lower limb atherosclerosis). The quality of primary studies was criticized in the majority of the reviews. Most reviews judged the available evidence as promising but definitive conclusions were rarely possible.

Conclusions

Systematic reviews are available on a broad range of herbal preparations prescribed for defined conditions. There is very little evidence on the effectiveness of herbalism as practised by specialist herbalists who combine herbs and use unconventional diagnosis.

Introduction

In this second part of our series on systematic reviews in complementary therapies we report our findings on herbal medicines. Herbal medicines (defined as preparations derived from plants and fungi, for example by alcoholic extraction or decoction, used to prevent and treat diseases) are an essential part of traditional medicine in almost any culture [1]. In industrialized countries herbal drugs and supplements are an important market. Some countries like Germany have a long tradition in the use of herbal preparations marketed as drugs and figures for prescriptions and sales are stable or slightly declining [2]. In the US and the UK herbal medicinal products are marketed as "food supplements" or "botanical medicines". In recent years sales of such products have been increasing strongly in these countries [3,4]. In the Third World herbs are mainly used by traditional healers [5].

Methods

A detailed description of the methods used in this review of reviews is given in the first part of this series [6]. For searches in Medline 50 single plant names and the 'exploded' term 'medicinal plants' were combined with the standard search strategy for systematic reviews. As a specific intervention-related inclusion criterion we required that reports reviewed prospective (not necessarily controlled) clinical trials of substances extracted from plants in humans. Reviews dealing with single substances (e.g., artemisin derivatives) derived from plants were excluded on the grounds that such agents are comparable to conventional drugs. Disease-oriented reviews including a variety of interventions were included only if they reviewed at least 4 herbal medicine trials.

Results

From a total of 79 potentially relevant reviews preselected in the literature screening process, 58 (published in 65 papers) met the inclusion criteria [7-71]. Eleven reports were not truly systematic reviews (not meeting inclusion criterion 2) [72-82], 5 dealt with isolated substances of plant origin [83-87] and 4 were excluded for other reasons (one disease- focused review with less than 4 herbal medicine trials [88], one review not on preventative or therapeutic use [89], two reviews not truly herbal medicine [90,91]).

More than half of the reports reviewed gingko, hypericum or garlic preparations. No less than 13 systematic reviews dealed with ginkgo (Ginkgo biloba) extracts (see table 1). Seven of these reviewed trials (total number of trials covered in any of the reviews 15) in patients with intermittent claudication [7-13]. Most of these reviews concluded that ginkgo extracts were significantly more effective than placebo in increasing measures like walking distance but the clinical relevance of the effects was felt to be moderate by some reviewers. The five reviews dealing with dementia and cerebral insufficiency (total number of trials included about 50) all draw positive conclusions [13-17]. However, many of the older trials were in patients with minor cognitive impairment and more evidence is needed to decide whether ginkgo extracts have clinically relevant beneficial effects in more severe forms of dementia. Finally, one review found that ginkgo extracts might be effective in the treatment of tinnitus [18] and another found insufficient evidence for efficacy in patients with macular degeneration [19].

Table 1.

Systematic reviews of clinical trials of ginkgo biloba extracts

Features
Author Year Indication Intervention Comparisons Studies 1/2/3/ Results Author's Conclusion
4/5
Ginkgo (Ginkgo biloba)
Pittler 2000 intermittent ginkgo placebo 8 RCT y/y/y/ Increase of pain-free walking Evidence for a modest benefit of
[7] claudication y/y distance over placebo after 12 uncertain clinical relevance
or 24 weeks 34 m (95%CI 26–
43 m)
Moher 2000 intermittent ginkgo* placebo 5 RCT y/y/y/ Increase of pain-free walking Inconsistent results from the few
[8] claudication n/y distance over placebo after 24 available small studies do not
weeks 32 m (95%CI 14–50 m) allow firm conclusions
Ernst 96 [9] intermittent ginkgo placebo, 10 p/ p/ n/ Most studies low quality. Available evidence promising but
claudication extract other drugs RCT/CCT n/n Increase of walking distance further high quality research
EGb761 compared to placebo 24 to 160 needed
m. At least similar
effectiveness compared to
other drugs.
Schneider 92 intermittent ginkgo placebo, 7 RCT/CCT ?/n/n/ mean effect size d = 0.75 Effectiveness over placebo clearly
[10] claudication other (vs. plac.), 2 y/y (95%CI 0.44–1.07) over shown
treatment RCT/CCT placebo
(other)
Letzel 92 intermittent ginkgo ginkgo vs. 5 RCT ?/p/n/ Pooled increase of walking Ginkgo extract EGb761 more
[11] claudication extract plac., ginkgo y/y distance: 45% over placebo for effective than placebo and
EGb 761 pentoxifyllin 9 RCT gingko and 57% for similarly effective as pentoxifyllin
vs. plac. pentoxifyllin pentoxifyllin
Kleijnen 91 intermittent ginkgo ginkgo vs. 15 y/y/y/ Many trials low quality. All trials Ginkgo seems effective for
[12] claudication plac., RCT/CCT n/n with positive results. Evidence intermittent claudication but further
pentoxifyllin (ginkgo), 5 similar as for pentoxifyllin high quality studies are needed
vs. placebo RCT/CCT
pentoxif.
Weiss 91 cerebral ins., ginkgo placebo 17RCT/CCT ?/p/p/ 10 of 12 interpretable trials on Effectiveness for both conditions
[13] intermittent extract (cerebral n/n cerebral insufficieny and all 4 biometrically shown
claudication EGb761 ins.), 8 interpretable trials on
RCT/CCT intermittent claudication with
significant positive results
Ernst 99 [14] dementia ginkgo placebo 9 RCT y/y/y/ Results collectively suggest Encouraging findings warranting
y/n that ginkgo is more effective for large scale trials
dementia than placebo
Oken 98 [15] Alzheimer ginkgo placebo 4 RCT y/y/n/ Significant effect over placebo Clinical relevance of the observed
dementia y/y for cognitive function (Hedges effects has to be confirmed in
g= 0.41, 95%CI 0.22–0.61) further research
Hopfenmüller cerebral ginkgo placebo 10 RCT, 1 n/ n/ n/ Global response (based on Ginkgo extract superior to placebo
94 [16] insufficiency extract LI CCT y/y symptom scores): OR 1.98
1370 (95%C11.39–2.57) in favour of
Ginkgo
Kleijnen 92 cerebral ginkgo ginkgo vs. 40 RCT/ y/y/y/ Many trials low quality. Virtually Ginkgo seems effective for
[17] insufficiency plac. CCT n/n all trials reported positive cerebral insufficiency but further
hydergine (ginkgo), 4 results. Evidence similar as for high quality studies are needed
vs. plac. RCT/CCT hydergine
(hydergine)
Ernst 99 [18] tinnitus ginkgo placebo, 5 RCT y/y/y/ 3 trials favour ginkgo over Results suggest that extracts of
other y/n placebo, 1 no difference, in one ginkgo biloba are effective in
treatment (1 trial ginkgo better than another treating tinnitus
trial) treatment
Evans 2000 macular ginkgo placebo 1 RCT y/y/y/ one small trial reporting Insufficient evidence to
[19] degeneration y/- improvement recommend ginkgo for age-related
macular degeneration
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Features: 1 = comprehensive search, 2 = explicit inclusion criteria, 3 = formal quality assessment, 4 = summary of results for each included study, 5 = meta-analysis; y = yes, p = partly, n = no, - = not applicable, ? = unclear review on all pharmacologic treatments for the respective condition RCT = randomized controlled trials, CCT = non-randomized controlled trials, CS = cohort studies, UCS = uncontrolled studies; OR = odds ratio, RR = rate ratio

The effectiveness of St. John's wort (Hypericum perforatum) extracts in depression was investigated in nine reviews [20-30] (total number of trials covered 29; see table 2). Mainly due to slight differences in the inclusion criteria (for example, restriction to trials with a minimum of 6 weeks observation or with a minimum quality score) the respective study collections differed to a considerable amount. However, the conclusions were very similar. Hypericum extracts have been shown to be superior to placebo in mild to moderate depressive disorders. There is growing evidence that hypericum is as effective as other antidepressants for mild to moderate depression and causes fewer side effects but further trials are still needed to establish long-term effectiveness and safety.

Table 2.

Systematic reviews of clinical trials of hypericum and garlic preparations

Features
Author Indication Intervention Comparisons Studies 1/2/3/ Results Author's Conclusion
Year 4/5
St John's wort (Hypericum perforatum)
Gaster depression hypericum placebo and 8 RCT p/y/p/ 4 placebo-controlled trials with Data suggest that hypericum is
2000 [20] antidepressants y/n positive results, in 4 trials superior to placebo, insuffcient
standard antidepr. tended to be evidence re equivalence with
slightly better antidepressants
Williams depression hypericum placebo and 14 RCT y/y/n/ Treatment response: RR 1.9 Data suggest that hypericum is
2000 & (and other antidepressants y/y (95%C11.2–2.8) vs. placebo and superior to placebo, insuffcient
Mulrow 98 drugs) 1.2 (1.0–1.4) vs. antidepressants evidence re equivalence with
[21,22] antidepressants
Kim 99 [23] depression hypericum placebo and 6 RCT p/y/y/ Treatment response: RR 1.48 Hypericum more effective than
antidepressants y/y (95%C11.03–1.92) vs. placebo placebo and similarly effective as
and 0.98 (0.67–1.28) vs. low dose antidepressants; quality
antidepressants problems
Stevinson depression hypericum placebo and 6 RCT y/y/y/ Only trials published after Linde Data confirm findings of earlier
99 [24] antidepressants y/n 96; trials show effects better trials, but still insuff. evidence to
than placebo/similar to assess equivalence with
antidepressants antidepressants
Linde 98 & depression hypericum placebo and 27 RCT y/y/y/ Treatment response: RR 2.47 Hypericum more effective than
96 [25,26] antidepressants y/y (95%C11.69–3.61) vs. placebo placebo. Inadequate evidence to
and 1.01 (0.87–1.16) vs. assess equivalence with
antidepressants antidepressants
Volz 97 depression hypericum placebo and 15 p/p/n/ Most placebo-controlled trials A therapy with hypericum of mild
[27] antidepressants RCT/CCT n/n positive; similarly effective as and moderate depression can be
(not adequately dosed) attempted. Further studies needed
antidepressants
Ernst 95 depression hypericum placebo and 11 RCT y/y/y/ Most of 8 placebo-controlled Hypericum is superior to placebo
[28] antidepressants y/n trials positive. 3 trials against and seems equally effective as
standard medication with similar standard medication
effects
Volz 2000 mild to hypericum fluoxetine 17+9 n/y/n/ No direct comparison of Response rates are similar;
[29] mod. CCT y/n hypericum and fluoxetine findings difficult to interpret
depression available. Mean depression because of the indirect comparison
score (HAMD) reduction in
hypericum trials 53%, in
fluoxetine trials 55%
Friede 98 anxiety in hypericum placebo, 8 RCT ?/y/y/ Trials collectively show reduction Hypericum is effective for
[30] depressed amitriptyline y/n of anxiety symptoms over depressed patients with anxiety
p. placebo. Only 1 trial vs
amitriptyline
Garlic (Allium sativum)
Lawrence cardiovasc. garlic mainly placebo; 45 RCT y/y/y/ 37 trials consistently show small Insufficient data to draw conclusion
2000 [31] risk factors no & other y/y short-term effects over placebo regarding clinical cardiovascular
treatment for cholesterol reduction. No outcomes. Garlic preparations may
consistent effects on blood have small, positive, short-term
pressure, promising effects re effects on lipids
platelet aggregation and
fibrionolytic activity
Stevinson hyperchol- garlic placebo 13 RCT y/y/y/ Pooled total cholesterol Available data suggest that garlic is
2000 [32] esterolemia y/y reduction over placebo 0.41 superior to placebo. The size of the
(95% Cl -0.66 to -0.15) mmol/l; effect is modest. The use of garlic
when analysis restricted to high for hyperchol. is therefore of
quality trials 0.11 (-0.30 to 0.08) questionable value
Silagy 94 & cholesterol garlic placebo 16 RCT y/p/y/ Pooled cholesterol reduction Meta-analysis suggests positive
Neil 96 lowering y/y over placebo 0.65 (95% Cl 0.53– effects but reviewers are sceptic
[33,34] 0.76) mmol/l (low quality; own replication
negative)
Warshafsky cholesterol garlic placebo 5 RCT p/y/y/ Pooled cholesterol reduction Available evidence supports the
93 [35] lowering y/y over placebo 0.59 (95%Cl 0.44– use of garlic as one modality to
0.74) mmol/l decrease cholesterol levels
Silagy 94 lowering dried garlic placebo, other 8 RCT y/p/y/ Pooled reduction over placebo: Garlic maybe of some clinical use
[36] blood (Kwai) treatment y/y SBP 7.7 (95% Cl 4.3–11.0), DBP in subjects with mild hypertension.
press. 5.0 (2.9–7.1) mm Hg Further research needed
Kleijnen 91 cardiovasc. garlic placebo 18 p/p/y/ Most studies with shortcomings. No clear conclusion drawn
[37] risk factors supplements RCT/CCT y/n The majority of trials with pos.
results but inconsistent effect
sizes
Kleijnen 89 cardiovasc. garlic & unclear 10 RCT, y/p/n/ All trials with severe Inadequate evidence to justify
[38] risk factors onions 8 CCT y/n shortcomings. Fresh garlic with supplementation, further research
beneficial effcts, onions and needed
commercially available
supplements yielded
contradictory results
Jepson 97 lower limb garlic placebo 1 RCT y/y/y/ Walking distance not Insufficient evidence
[39] atheroscler. y/- significantly different between
groups
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legend see table 1

Eight reviews have been performed on garlic (Allium sativum) for cardiovascular risk factors [31-38] (total number of trials covered about 50) and lower limb atherosclerosis [39] (see table 2). A modest short-term effect over placebo on lipid-lowering seems to be established but the clinical relevance of these effects is uncertain. Data from randomised trials on cardiovascular mortality are not available. Effects on blood pressure seem to be at best minor. The available results on fibrinolytic activity and platelet aggregation are promising but insufficient to draw clear conclusions. A specific problem in research on garlic is the great variety of garlic preparations used: the exact content of bioactive ingredients in these is often unclear.

Three reviews (covering a total of about 30 trials) have been performed on preparations containing extracts of Echinacea (Echinacea purpurea, pallida or angustifolia), two of which by the same study group [40-43]. The results suggest that Echinacea preparations may have some beneficial effects mainly in the early treatment of common colds. Similar to garlic a major problem is the high variaton of bioactive compounds between different Echinacea preparations. Cranberries (Vaccinium macrocarpon) for urinary tract infections [44,45], mistletoe (Viscum album) for cancer [46-48], peppermint (Mentha piperita) oil for irritable bowel syndromes [49,50] and saw palmetto (Serenoa repens) for benign prostate hyperplasia [51-53] have each been subject to two reviews. For saw palmetto there is good evidence for efficacy over placebo while for the other three the data are inconclusive (see table 3).

Table 3.

Systematic reviews of clinical trials of herbal medicines (at least 2 reviews per herb)

Features
Author Indication Intervention Comparisons Studies 1/2/3/ Results Author's Conclusion
Year 4/5
Echinacea (Echinacea purpurea, angustifolia and pallida)
Barrett upper resp. echinacea placebo 13RCT y/p/y/ Overall quality modest. All 4 Echinacea may be beneficial for
99 [40] infections (incl. y/n prevention studies show only early treatment of acute upper
combinations) minor trends, 8 of 9 treatment respiratory infections; little evidence
studies with generally positive to support the prolonged use for
results prevention
Melchart common echinacea placebo, no 16 RCT y/y/y/ Minor effects in prevention and Echinacea extract can be efficacious
99 [41] cold (incl. treatment y/p treatment, promising effects in for the common cold, but evidence
combinations) early treatment. Heterogen. insufficient for recommendations
preparations
Melchart immuno- echinacea placebo, no 18 RCT, 8 y/y/y/ Most studies low quality. Most Echinacea extracts can be
94 stimulation (incl. treatment CCT y/n studies show immunostimulating efficacious immunostimulators, but
[42,43] combinations) effects evidence insufficient for
recommendations
Cranberries (Vaccinium macrocarpon)
Jepson urinary cranberries placebo 4 RCT y/y/y/ In 3 of 4 trials cranberries effective Insufficient evidence, further research
98 [44] tract inf. y/n for at least one of the outcomes of needed
(prevent) interest
Jepson urinary cranberries O RCT y/y/-/ No trials meeting the inclusion No evidence available
98 [45] tract inf. -/- criteria
(treatm.)
Mistletoe (Viscum album)
Kleijnen cancer mistletoe placebo, no 11 y/y/y/ Most studies low quality. Most Insufficient evidence to recommend
94 [46] treatment RCT/CCT n/n studies show longer survival with mistletoe outside of clinical trials
mistletoe but not the best trial
Kiene 89 cancer mistletoe no treatment, 2 RCT, 33 y/n/n/ Most studies low quality. 9 of 12 Available evidence supports positive
[47,48] none CCT, 11 y/n interpretable studies suggest effects of mistletoe
other positive effects on survival
studies
Peppermint (Mentha piperita)
Jailwala irritable 1. peppermint placebo 1. 3 RCT p/y/y/ Chinese herbal therapy trial rated In both cases efficacy not clearly
2000* bowel oil 2. 1 RCT n/n as positive, one of three established
[49] syndr. 2. Chinese peppermint oil trials rated as
herbal positive
therapy
Pittler 98 irritable peppermint placebo, 8 RCT y/y/y/ Global improvement rates The role of peppermint oil for IBS
[50] bowel oil other y/y significantly higher compared to has not been established beyond
syndr. treatment placebo. Quality of trials doubtful reasonable doubt
Saw palmetto (Serenoa repens)
Boyle ben. Permixon® placebo, 11 RCTs, ?/n/n/ peak urine flow 2.20 (95% Cl 1.20– Despite some limitations strong
2000 [51] prostate (saw other 2 UCS y/y 3.20) ml/s increase over placebo; evidence that the extract tested has
hyperplasia palmetto) treatment significant decrease nocturia beneficial effects
Wilt 2000 ben. saw palmetto placebo, 14 RCT y/y/y/ Saw palmetto superior to placebo Evidence suggests that saw
&98 prostate other (plac), y/y for nocturia, self rating, peak urine palmetto improves urological
[52,53] hyperplasia treatment 5 RCT flow; similar effects as finasteride symptoms and flow measures.
(other) Further studies needed
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legend see table 1

Single systematic reviews have been published on aloe (Aloe vera) [54], artichoke (Cynara scolymus) leave extract [55], evening primrose (Oenothera biennis) oil [56], feverfew (Tanacetum parthenium) [57], ginger (Zingiber officinialis) [58], ginseng (Panax ginseng) [59], horse chestnut (Aesculus hippocastanum) seeds [60], kava (Piper methysticum) [61], milk thistle (Silybum marianum) [62], a fixed combination of three herbal extracts [63], rye-grass pollen (Secale cereale) extract [64,65], tea tree (Melaleuca alternafolia) oil [66], and valerian (Valehana officinalis) root [67] (see table 4). The only review which focused on a herbal intervention which is not marketed as a drug or food supplement was on cabbage leaves for breast engorgement and included a single small-scale trial [68]. Chinese herbal therapy for atopic eczema [69] and a variety of herbs for lowering blood glucose [70] and for analgesic and anti-inflammatory purposes [71] have also been reviewed. For some of these herbal preparations the evidence is promising but further studies are considered necessary to establish efficacy in almost every case.

Table 4.

Systematic reviews of clinical trials of herbal medicines

Features
Author Indication Intervention Comparisons Studies 1/2/3/ Results Author's Conclusion
Year 4/5
Vogler 99 various aloe placebo, other 6 RCT,4 y/y/y/ Positive results for genital Promising results, but overall
[54] & no treatment CCT y/n herpes, psoriasis, hyper- evidence insufficient
lipidemia, diabetes;
contradictory for wound healing
Pittler 98 cholesterol artichoke placebo 1 RCT y/y/y/ Effects over placebo only in the More trials needed
[55] lowering leave n/n subgroup of participants with
extract serum cholesterol > 210 mg/dl
Morse 89 atopic evening placebo 9 ?/n/n/ Epogam significantly better No conclusion drawn
[56] eczema primrose oil RCT/CCT y/y than placebo for most
(Epogam) outcomes
Vogler 98 migraine feverfew placebo 5 RCT y/y/y/ Majority of trials favor feverfew Effectiveness has not been
[57] y/n over placebo established beyond reasonable
doubt
Ernst 2000 nausea and ginger root placebo, 6 RCT y/y/y/ 2 of 3 trials on postoperative Evidence promising but insufficient
[58] vomiting metoclopramide y/p nausea positive (best to draw firm conclusions
negative), trials on
seasickness, morning sickness
and chemotherapy-induced
nausea positive
Vogler 99 various ginseng root placebo, other 16 RCT y/p/y/ Contradictory results re. The efficacy of ginseng root extract
[59] extract treatment (1 y/n physical performance (7 trials), is not established beyond
trial) psychological function (5), reasonable doubt for any of these
immunomodulation (2), indications
positive results in diabetes and
herpes simplex (1 trial
respectively)
Pittler 98 venous horse placebo, other 13 RCT y/y/y/ Significant effects over placebo horse chestnut seeds seem to be
[60] insufficieny chestnut treatment y/n and similar effects compared to effective; further tials needed
seeds other treatments (confirmation, long-term results,
combination)
Pittler 2000 anxiety kava placebo 7 RCT y/y/y/ All trials suggest superiority Available data suggest that kava is
[61] p/p over placebo; 3 trials with data a treatment option for anxiety.
for meta-analysis show sign. Further studies needed
superiority
Lawrence liver milk thistle placebo, other 33 RCT, y/y/y/ Variety of conditions studied, Efficacy is not established.
2000 [62] diseases & no treatment 1 CCT y/y studies often poor quality. Possible benefit shown most
Mixed and inconsistent findings frequently for aminotransferases.
Ernst 99 musculoskel. Phytodolor® placebo, other 10 RCT y/p/y/ Placebo-controlled trials show The data suggest that the
[63] pain populus, treatments y/n superiority over placebo and combination is effective in the
fraxinus, similar effects as NSAIDs symptomatic treatment of
solidago muskuloskeletal pain
MacDonald ben. prostata rye grass placebo, other 4 RCT y/y/y/ Signif. improvement over Available evidence suggests that
2000 & hyperplasia pollen therapy y/y placebo in subjective, but not Cernilton® is well tolerated and
Wilt 2000 extract objective symptoms; no modestly improves subjective
[64,65] differences compared to symptoms. Further studies needed
tadenan and paraprost
Ernst 2000 dermatologic tea trea oil placebo, other 4 RCT y/y/y/ 2 trials vs. placebo positive, 3 Data promising but insufficient
[66] conditions treatment y/n trials vs. other treatments
similar effects
Stevinson insomnia valerian root placebo 9 RCT y/y/y/ Highly heterogeneous studies Available evidence is promising but
2000 [67] y/n with sometimes contradictory not fully conclusive. Further,
and inconsistent findings rigorous trials needed
Renfrew breast cabbage usual care 1 RCT y/y/n/ fewer women stopping breast Further research desirable
84 [68] engorgement leaves y/n feeding among those receiving
cabbage leaves
Armstrong atopic Chinese placebo 2 RCT y/y/n/ 2 positive studies by the same Evidence encouraging but
99 [69] eczema herbal y/n insufficient given the potential of
therapy treat analysis relevant side effects
Ernst 97 hypoglyc. all plants no treatment, 7 RCT, 4 y/p/n/ Most studies low quality. Most Use of hypoglcemic plant remedies
[70] activity placebo, none CCT, 10 y/n papers report positive effects not supported by rigorous
UCS on a variety of plants research. Further studies required
Ernst 2000 analgetic or various placebo 18 RCT y/y/y/ Trials on evening primrose oil, The results suggest that several
[71] inflamm. y/n blackcurrant seed oil, borage herbal remedies have potential in
treatment oil, harpagophytum, willow alleviating the pain of rheumatic
bark, feverfew, and 3 diseases. More research urgently
combinations; almost all trials needed
positive
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legend see table 1

Discussion

Our overview shows that a considerable number of systematic reviews on herbal medicines is available. In the majority of cases the reviewers considered the available evidence as promising but only very rarely as convincing and sufficient as a firm basis for clinical decisions. The methodological quality of the primary studies has been criticized by many reviewers.

Our summary of the existing studies must be interpreted with caution. What we performed is a systematic review of systematic reviews which inherently bears a large risk of oversimplification. Readers who want to reliably assess the evidence for a given herb for a defined condition should read the respective reviews. Our collection – which to the best of our knowledge is complete up to summer 2000 – is aimed at facilitating the access and giving an idea of the amount of the available evidence. Based on the increase of herbal medicine reviews in recent years we expect that at least ten new publications will become available in the year 2001.

Most of the currently available systematic reviews address herbal preparations which are marketed and widely used in industrialized countries. However, the widespread traditional use of herbs in the Third World is rarely ever investigated and has not been subjected to systematic reviews. The many herbs used in folk medicine or other traditional uses of herbs (for example, hypericum is used for a variety of ailments other than depression including enuresis, diarrhoea, gastritis, bronchitis, asthma, sleeping disorders etc.) seem to be rarely investigated. Furthermore, practitioners of herbal medicine often combine different herbs and use unconventional diagnostic approaches to adapt prescriptions to single patients. It seems likely that these traditional forms of herbal medicine will remain underresearched relative to single herbal preparations due to the lack of financial incentive for sponsors and due to methodological problems.

Herbal medicines products are not, in general, subject to patent protection. This reduces the motivation for drug companies to invest in trials. Many of the existing herbal medicine manufacturers are comparably small companies, often with limited research resources and expertise. Maybe partly for these reasons, the quality of many older herbal medicine trials is low. Furthermore, negative trials which could threaten the company's survival might not become published.

A fundamental problem in all clinical research of herbal medicines is whether different products, extracts, or even different lots of the same extract are comparable and equivalent. This is a major issue in the expert research community and a major obstacle to a reliable assessment for the non-expert. For example, Echinacea products can contain other plant extracts, use different plant species (E. purpurea, pallida or angustifolia), different parts (herb, root, both), and might have been produced in quite different manners (hydro- or lipophilic extraction). Pooling studies that use different herbal products in a quantitative meta- analysis can be misleading. Health care professionals and patients considering to prescribe or take a particluar herbal product should check carefully whether the respective product or extract has been tested in the trials included in a review. On the health food store shelf the high quality, standardized products used in the trials might not be available. Only a herbal medicine expert can judge with some certainty whether the results can be extrapolated to the product of interest.

On the level of health care policies the available systematic reviews more often provide insight into the deficiencies of the evidence than guidance for decision making. Trials on hard endpoints are very rarely available and observation periods have generally been short. The clinical relevance of the observed effects is not always clear.

Herbal medicines are generally considered as comparably safe. While this is probably correct case reports show that severe side effects and relevant interactions with other drugs can occur. For example, hypericum extracts cause considerably fewer side effects than tricyclic antidepressants [92] but can decrease the concentration of a variety of other drugs by enzyme induction [93]. Several reviews summarizing side effects and interactions have been published [94-98].

In conclusion, the systematic reviews collected for this analysis are a good tool to get an overview of the available evidence from clinical trials in the area of herbal medicine. However, applying the findings to patients care is problematic for those who are not experts in herbal medicine. In this case it might be better to directly search the literature for clinical trials of the respective product.

Competing interest

KL, DM, GtR, and AV have been involved in some of the reviews analyzed. These were extracted and assessed by other members of the team.

Pre-publication history

The pre-publication history for this paper can be accessed here:

http://www.biomedcentral.com/content/backmatter/1472-6882-1-5-b1.pdf

Acknowledgments

Acknowledgements

KL's work was partly funded by the NIAMS grant 5 U24-AR-43346-02 and by the Carl and Veronica Carstens Foundation, Essen, Germany. We would like to thank Brian Berman for his support, his help to get funding and his patience in awaiting the completion of our work.

Contributor Information

Klaus Linde, Email: Klaus.Linde@lrz.tu-muenchen.de.

Gerben ter Riet, Email: GterRiet@EPID.UNIMAAS.NL.

Maria Hondras, Email: mhondras@interaccess.com.

Andrew Vickers, Email: vickersa@mskcc.org.

Reinhard Saller, Email: reinhard.saller@dim.usz.ch.

Dieter Melchart, Email: Dieter.Melchart@lrz.tu-muenchen.de.

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