Subsequent to complete chemical characterization, a randomized, placebo-controlled, double-blind, phase II dose-escalation clinical trial will be conducted to find an optimally safe and efficacious dosage of this standardized 20-herb preparation in 104 patients aged 18 to 75 with all types of IBS. At each of two dosage levels, 52 participants will be randomized to treatment for 8 weeks with the herbal formula or placebo in a 1:1 allocation ratio, and will be assessed at baseline, weeks 0, 2, 4, 8, and 12 for the clinically important and reliable outcome of patient reported global symptom improvement. At the conclusion of the 1st dosage level, safety will be assessed prior to using the higher dosage in a new cohort of participants. We also will assess individual IBS symptoms, nature, severity, duration, and frequency of adverse events, quality of life, concurrent IBS medications and health care utilization, and will perform blood tests for safety purposes. Adherence to study medication will be verified by dose counts. Results of this dose-ranging study will help to identify the optimal dosage of the herbal formula to be used in future randomized placebo-controlled trials and in head-to-head comparisons with conventional pharmaceuticals.