Objectives: To examine the efficacy and safety of personalized tongxie formulas; to decrease type II errors to minimum. Methods: Patients were randomized (1:1:1) into three groups given tongxie, placebo, or pinaverium 3 times daily for 4 weeks. Patients in the tongxie group were treated with personalized formulas based on TCM differential diagnosis, i.e., basic type of IBS, IBS due to liver depression and qi stagnation, excess heat in the liver, deficient spleen function, deficient kidney function, and others (groups 1-6). Primary endpoints were significantly greater reductions in abdominal pain and Bristol stool score. Secondary endpoints were reductions in pain and stool frequencies and abdominal discomfort and its frequency. Results: There were significantly more patients whose stool consistencies were improved than pains were relieved in the entire population (p < 0.001), but there was no significantly difference in subpopulation group 3 (p > 0.05). There were significantly more patients whose stool frequencies were reduced than pain frequencies were reduced in the entire population (p < 0.001), but there were no significantly difference in the subpopulation Groups 1, 3, 4, and 6 (p > 0.05). Multiple active ingredients and their mechanisms of actions to relieve IBS symptoms were identified. Conclusion: The outcomes in subpopulations may be different from those of the entire population, indicating that personalized formulas are important to achieve optimal outcomes; the active ingredients and innovative mechanisms identified in this study can be the candidates for developing new IBS drugs, and used to manage IBS, respectively.