Theoretically the drug interaction with a Fixed-Dose Congination can be antagonism, synergism, and potentiation. Therefore, assessing DDI potential for a FDC as a perpetrator practically can be challenge due to complicated mechanisms of interaction. The symposium will be offering the introduction of mechanisms of DDIs involving multiple inhibitors, the methods to assess the total inhibition effects, regulatory guidance on co-development of two or more new investigational drugs for use in combination, in vitro or preclinical data for human prediction and cases examples of DDI assessments to illustrate the strategy when developing FDCs.