For the development of any formulation, techniques such as thermal and isothermal stress testing were used to assess the compatibility of drug with excepients. Differential scanning calorimetry (DSC) and FTIR were the common methods for the study of compatibility. Isothermal stress testing (IST) is also a method for the compatibility study during proto type formulation. In the present study drug excepient compatibility study of lamivudine was conducted with different controlled release polymers. The drug and polymer mixtures were stored at 50 °C for 2 weeks. The samples were then characterized using DSC, FTIR and UV spectrophotometric methods. The results show that lamivudine was compatible with the all the polymers used in the study. The polymers used in the present study were definitely incorporated in the extended release lamivudine formulation.