PPT Slide
A Closer Look at Emax Models
The Emax “model” which is so widely used falls in the class of curve fitting approaches. Its general behavior of going to a finite limit coincides with the behavior of many natural systems. It does not address underlying biological factors. However, it can be modified to take such factors into account (such as modifying parameters to account for competitive antagonism). The message is that care must be taken when extending such a model,which works for each of two drugs individually. It can not be applied blindly to the two drugs jointly.
The Correct Equation for non-interacting additive effect doses "a" and "b" is
E=(EMa*Da/Xa+EMb*Db/Xb)/(1+Da/Xa+Db/Xb)