OBJECTIVE: To evaluate the efficacy and safety of administration of the formulated Persian herbal syrup on improving the symptoms of patients with constipation-predominant irritable bowel syndrome (IBS-C). METHODS: This study was conducted in 70 patients with IBS-C, who were recruited from 3 medical centers in Mashhad, Iran, from November 2017 to August 2018. Seventy patients were randomly assigned to 2 groups including treatment and placebo groups by block randomization, 35 cases in each group. Patients in the treatment group received 15 mL of anti-IBS syrup, thrice daily for 6 weeks and followed up for 4 weeks. Placebo syrup was also prepared through similar instruction, BP syrup without plant extract was used. Primary outcome induding IBS Symptom Severity Scale (IBS-SSS) questionnaire and secondary outcomes in terms of Hospital Anxiety and Depression (HADS) questionnaires, the Bristol Stool Form Scale (BSFS) were completed and evaluated at weeks 6 and 10, respectively. Safety indices were collected at the end of the treatment and Common Terminology Criteria for Adverse Events v4.0 (CTCAE) was used to evaluate the adverse events. RESULTS: The response to treatment was 84.4% (27/32) in the treatment group and 46.4% (13/28) in the placebo group, respectively (P= 0.002). Compared with pre-treatment, a significant decrease was found on the IBS-SSS and BSFS scores after 6-week intervention in both groups (P<0.001). Moreover, IBS-SSS and BSFS scores in the treatment group were lower than the placebo group after the intervention (P=0.041). There was no significant difference in the anxiety and depression scores after treatment in both groups (P>0.05). Side effects reported in the treatment group included 2 cases of headache during the first week of the onset of the treatment, 1 case of drowsiness, 1 case of increase in menstrual bleeding, which did not result in discontinuation of the treatment. In the placebo group, 1 case of exacerbation of the disease was reported. CONCLUSIONS Anti-IBS syrup significantly reduced the severity of IBS symptoms compared to placebo. However, there was a need for further investigation regarding the anxiety and depression scores. (Registration No. IRCT2017061034446N1).