The synergy ratio (SR) could explain the synergy effect and the negative synergy effect of seven relationships between drug interactions. The SR could be numerically calculated by experimental and clinical data. Moreover, the ideal ratio of combination drugs could be calculated. In conclusion, RSM could provide a general and practical methodology to analyze herbal interactions and the optimization of prescriptions. Moreover, it could be used to find out the effectiveness of co-treatment of acupuncture or other treatments.