The precise determination of dose-effect curves and the combination effect of drugs is of crucial importance in the development of new therapies for the most dreadful diseases. We have found that the current implementations of the theory of Chou et al. are not accurate enough in some circumstances and might lead to erroneous predictions of synergistic or antagonistic behaviour. We have identified the source of inaccuracies and fixed it thereby improving the accuracy of those methods. Here we explain the main features of our approach and demonstrate its higher accuracy as compared to the standard methods. Therefore, this new implementation might have a huge impact in the reliability of future research on new Combination Therapies.