The purpose of the current paper is to elaborate the model for assessing cumulative synergetic effect in M&A (Mergers and Acquisitions) deals on the basis of a real options approach. The majority of papers on the synergetic effects of M&A deals typically focus on a particular type of synergy, while the current paper proposes a model that accounts for the cumulative simultaneous effect of different types of operating and financial synergies. The methodology of our research is loosely based on Datar-Mathews real option valuation model, which is flexible and intuitive for practitioners. Formulae for assessing eight types of synergy typically arising from M&A deals are developed. They are integrated into a single model to assess their cumulative effect on the M&A deal using a simulation modelling approach. The method was used ex post to find synergy values in two recent M&A deals in the pharmaceutical industry, and produced sound results. The proposed approach to value target companies could be used by firms before an M&A deal in the due diligence process. Using this tool a company can build a bidding strategy and define the maximum premium it can pay for the target.