Inclusion Criteria:
1. Aged from 18 to 65 years old, regardless of gender.
2. Meeting the Western medicine diagnostic criteria for IBS-D (Rome IV criteria).
3. Conforming to the TCM syndrome differentiation criteria for Large Intestinal Damp-heat.
4. During the run-in period, the weekly average score of abdominal pain is ≥ 3 points based on the 11-point (0-10) Numerical Rating Scale (NRS); and there are at least 2 days per week with stool consistency classified as Type 6 or Type 7 according to the Bristol Stool Scale.
5. The total score of the Irritable Bowel Syndrome Symptom Severity Scale (IBS-SSS) at baseline is ≥ 175 points.
6. Colonoscopy performed within 12 months prior to the run-in period, and meeting one of the following conditions for enrollment:
1\) Normal colonoscopy results with no organic lesions; 2) Abnormal colonoscopy findings such as hemorrhoids and intestinal polyps (diameter ≤ 5 mm, number ≤ 3), and assessed eligible for enrollment by the investigator; 3) Previous intestinal polyps with diameter \> 5 mm or number \> 3, which were treated endoscopically more than 6 months ago, with residual polyps of diameter ≤ 5 mm and number ≤ 3, and assessed eligible for enrollment by the investigator.
(7) Participants are fully conscious, free from mental illness or cognitive impairment, capable of understanding and completing scale assessments. All participants voluntarily agree to participate in this clinical trial, sign the informed consent form, and comply with all study protocol requirements throughout the trial.
Exclusion Criteria:
1. Patients diagnosed with irritable bowel syndrome classified as IBS-C, IBS-M or IBS-U.
2. Patients with a confirmed diagnosis of digestive system diseases in medical history or at screening, including peptic ulcer (excluding those with healed ulcers assessed by the investigator to have no impact on efficacy and safety evaluation), inflammatory bowel disease, intestinal tuberculosis, intestinal obstruction, intestinal adhesion, eosinophilic gastroenteritis, tuberculous peritonitis, liver cirrhosis, etc.
3. Patients with acute diarrhea and/or severe abdominal pain caused by other diseases within 1 month prior to screening, or those suffering from other diseases assessed by the investigator to interfere with efficacy evaluation and increase safety risks.
4. Patients with a history of abdominal surgery who may present symptoms similar to IBS as judged by the investigator (appendectomy and cesarean section are excluded).
5. Patients with a confirmed diagnosis of systemic diseases affecting gastrointestinal function in medical history or at screening, such as diabetes mellitus, hyperthyroidism or hypothyroidism, chronic renal insufficiency, autoimmune diseases (e.g. allergic enteritis, Henoch-Schönlein purpura, Behçet's syndrome, systemic lupus erythematosus), etc.
6. Patients complicated with other painful conditions such as arthritis and migraine that may interfere with Numeric Rating Scale (NRS) pain assessment; female patients with severe dysmenorrhea requiring analgesic medication.
7. Patients with malignant tumors within the past 5 years or diagnosed at screening.
8. Participants presenting gastrointestinal alarm symptoms within 14 days prior to screening, including hematochezia, positive fecal occult blood test, anemia, ascites, nocturnal diarrhea, unexplained weight loss or fever, etc., who are deemed ineligible for enrollment by the investigator.
9. Patients with severe diseases involving cardiovascular, cerebrovascular, hepatic, renal or other systems.
10. Patients who have undergone surgical operation within 30 days prior to screening, which is assessed by the investigator to affect safety or efficacy evaluation, or those scheduled to receive surgery during the study period.
11. Participants who have received medications or treatments for IBS within 4 weeks prior to randomization (e.g. antispasmodics, antidiarrheals, prokinetic agents, neurotransmitter regulators, intestinal flora modulators, traditional Chinese medicine, acupuncture and tuina).
12. Participants who have taken other drugs affecting gastrointestinal function within 4 weeks prior to randomization (e.g. anticholinergics, 5-Hydroxytryptamine 3 (5-HT3) receptor antagonists, antacids, antidepressants, anxiolytics, opioid receptor agonists, long-term or high-dose non-steroidal anti-inflammatory drugs, traditional Chinese herbal decoctions, etc.), or those who cannot discontinue such drugs during the trial.
13. Participants with abnormal liver and renal function test results at screening and baseline:
1. Aspartate aminotransferase (AST) ≥ 1.5 times the Upper Limit of Normal (ULN);
2. Alanine aminotransferase (ALT) ≥ 1.5 × ULN;
3. Blood urea nitrogen (BUN) ≥ 1.5 × ULN; d) Serum creatinine \> ULN.
14. Participants with treatment compliance \< 80% or \> 120% during the run-in period, or those who completed the run-in period diary cards for fewer than 5 days per week.
15. Participants who took rescue medication (pinaverium bromide) continuously or intermittently for more than 2 days in the first week of the run-in period (Day -14 to Day -8), or used the rescue medication (pinaverium bromide) in the second week of the run-in period (Day -7 to Day -1).
16. Participants with a score ≥ 60 on the Self-Rating Anxiety Scale (SAS) and/or a score ≥ 63 on the Self-Rating Depression Scale (SDS) at screening.
17. Participants with allergic diathesis or known allergy to the study drugs.
18. Participants with a history of alcohol abuse or drug abuse within the past 1 year.
19. Pregnant or lactating women; participants or their partners who plan to become pregnant during the period from signing the informed consent form to the end of follow-up, or refuse to adopt reliable non-drug contraceptive measures.
20. Participants who have enrolled in and received treatment in any other clinical trial within the past 3 months.
21. Other participants deemed unsuitable for enrollment by the investigator.
