We present a formalism to calculate the nucleon-nucleon interaction with contributions from the NN-pi channel in the isobar approximation for the N-pi system. In order to write dispersion relations for the T = 1, J = 0 amplitudes, we study the complex singularities that are present in the Born amplitude for the production process. We use the N/D coupled channels method for the helicity partial wave amplitudes and write the explicit equations for the T = 1, J = 0 amplitudes taking into account, through a simplified model, the contributions of the complex singularities.