In the present review, 3 out of the 4 good quality studies[11,20,31
] demonstrated 4 different herbal interventions: one Chinese herbal medicine (standard formula), one Tibetan herbal formula (Padma Lax) and two complex extracts of herbs: STW5 and STW5-II which could potentially relieve abdominal pain, constipation, diarrhea and alternating constipation and diarrhea. Moreover, three[11,29,31
] out of these four studies showed that four interventions including one individualized formula and three mono-extracts of single herb (Bitter Candytuft, Curcama and Fumitory) were not effective in IBS. We recognized that some complex herbal formulas may improve IBS symptoms, whereas three mono-extracts of single herbs had no beneficial effect. A possible explanation for these findings is that the therapeutic effect may be enhanced by the synergic actions of compounds in a mixture of different herbs.
] out of the 18 poor quality studies showed that some Chinese herbs formulas, such as, Huatan Liqi Tiaofu decoction, Tongxie Yaofang modified and Tongxie Yaofang plus Sini San decoction, Geqin Shujiang Saocao decoction, Huanchang decoction, Congpi Lunzhi Formula, Xiangsha Liujunzi decoction, Shunji mixture, Gegan Qinlian Pellet, and Liyiting decoction were more beneficial than CMs in the treatment of IBS.
However, these studies revealed several methodological flaws. We found that the longest duration of treatment was 18 wk, while the shortest was just 2 wk, and only 5 studies lasted more than 8 wk. The most frequent treatment duration was 3-4 wk. With such a short period of treatment, it is hard to reach the therapeutic goal in IBS. Some studies reported the long-term effects of herbs and the rate of symptom recurrence. Thus, it is necessary to have a relatively long treatment duration with herbal medicines as well as the duration of the follow-up period; Funnel plot of inclusion trials indicated asymmetry, the major interpretation is the presence of publication bias and variable methodological quality. In the studies that we reviewed, 18 out of 22 were conducted in China and published in Chinese. Chinese studies more frequently showed favorable therapeutic results compared to articles in English, particularly those with a high rate of positive outcome (99%)[32
]. The large number of poor quality studies is another source of bias. Furthermore, the small size of studies and the variability of the control treatment may cause asymmetry of the funnel plot. We noticed that most of the studies with Chinese herbs were of poor methodological quality and would not provide strong evidence to confirm the efficacy of CHM. However, the lack of good evidence supporting the effectiveness of CHM does not mean that these preparations are not effective in the treatment of IBS, instead we need to improve the methodological quality of trials in order to verify the efficacy of CHM as a therapeutic approach. We agree with the opinion of Liu[33
] that the potential beneficial effects of CHM need to be confirmed in rigorous trials with well-designed, randomized, double blinded, placebo controlled studies. A good example is the study performed by Bensoussan and colleague[11
There is growing interest in placebo response in patients with IBS. A systematic review of RCTs showed that global improvement in IBS symptoms with placebo was 40.2% (range 16%-71.4%)[34
].Other investigators have reported placebo response rate of 57% in IBS[35
]. Placebo response rate correlated with factors such as frequency of intervention, methodological quality of study, duration of the study, the patient-practitioner interaction and the diagnosis treated[34,36–38
]. Since the number of studies using placebo were small, we did not explore the response effect of placebo in the present study.
In the 22 trials that we reviewed, there were only 15 adverse events associated with HM. These were abdominal distention, constipation, abdominal pain, diarrhea, dizziness and hypersomnia, headache and nausea. However, no serious side effects or abnormalities of laboratory parameters such as liver function, renal function or haematological tests were reported with the treatment. Studies conducted in the West reported more adverse events than those from China. It is possible that because of the lack of rigorous monitoring, several adverse effects including serious events may not have been reported. Similarly, because of publication bias, adverse events related to herbal medicine may not be reported properly.
In summary, the use of HM for treating IBS is increasing worldwide. Most of the studies included in our review showed a beneficial effect on IBS symptoms. However, the methodological quality of the studies was variable, with 82% being of poor quality which may have overestimated the effectiveness of treatment. Although adverse events arising from the use of herbs were mild and infrequent, HM should be used with caution because of the reasons discussed above. It is therefore necessary to conduct Level I studies in order to provide evidence for Grade A recommendations[39
] and clarify whether Chinese herbal medicines are reliable and safe therapy in IBS.