This is a very well-conducted study done into the use of Chinese herbal medicine for IBS, and published in one of the most respected medical journals in the world, the Journal of the American Medical Society JAMA. It was a randomised, double-blinded, placebo-controlled trial which is considered to yield the highest standard of evidence for the efficacy of a clinical intervention- in other words, it is a rigorous study design. 116 patients were assigned to 1 of 3 treatment groups, the first of which was given individually tailored herbal prescriptions, the second were all given the same herbal prescription, and the third were given a placebo (which was designed to look, smell and taste like a herbal formula) Patients in both of the treatment groups experienced significant improvement in symptoms compared to those in the placebo group. During the treatment period the individually tailored prescription group got the same results as the group that was all given the same prescription, but on followup assessment 3 months after the end of the study, only the group who had been given the individually tailored prescriptions maintained the improvements they had gained.
Out of 116 patients, two withdrew from the study because of adverse effects, one experienced upper gastrointestinal discomfort while taking the standard prescription, and the second developed headaches (though they had a strong history of headaches and therefore this cannot definitely be ascribed to the herbal treatment). However this is a good demonstration of why individualised prescriptions (rather than standard ones) are the standard of care in professional Chinese medicine- because it is sometimes necessary to make alterations to the basic formula, in case side-effects arise, or in case the patients health fluctuates during the course of treatment, which is not at all unusual.
In summary, this is probably the best-conducted study into the use of Chinese medicine that I have ever seen. The link to the full text is here.