A Chinese Herbal Medicine for IBS-C

Inclusion Criteria: * 18-65 years adults * Fulfill the clinically diagnosed with IBS-C based on the ROME IV criteria * Abdominal pain intensity: weekly average of the worst daily (in the past 24 hours) abdominal pain score of ≥3.0 on a 0 to 10 point scale (based on the 2-week E-diary in screening) * CSBM ≤ 2 times/week (based on the 2-week E-diary in screening) * Understand and be able to follow written and oral instructions in Chinese * Provide informed consent * Able to use and complete 2-week E-diary for screening Exclusion Criteria: * Drug-induced or secondary causes of constipation * Use of medications and/or supplements that may confound the study outcomes (including but not limited to antibiotics, steroid, analgesic drug, neuromodulator, prebiotics, symbiotics, or probiotics) * Clinically significant colonoscopy or sigmoidoscopy examination findings after the onset of symptoms for patients who (1) is ≥50 years old; and/or (2) has family risk of colon-rectal cancer or familial polyposis syndromes; and/or (3) has alarm symptoms within 6 months before screening (unintentional weight loss: \>10% in 3 months, blood in the stools not caused by hemorrhoids or anal fissures, fever of unknown origin, anemia) * Clinically significant laboratory or imaging findings within 6 months before screening * Recent history of mushy or watery stools within one month * Allergic to Chinese herbal medicine or G6PD * Abdominal surgeries within the past year (except laparoscopic appendectomy) * Pregnancy, breastfeeding or plan to become pregnant with the study timeframe * Any disease(s), condition(s) or habit(s) deemed by the investigators that would compromise the individual's ability to complete the study, e.g. serious psychiatric conditions, at the discretion of the investigators * Any disease(s), condition(s) or habit(s) deemed by the investigators that would prevent participation in the study, e.g. participation in past or active clinical research, at the discretion of the investigators