We study the influence of viscosity on DNA dynamics. By employing the nonlinear Peyrard-Bishop-Dauxois (PBD) model, it is shown that the DNA dynamics can be explained by a solution of a complex nonlinear Schrodinger equation (CNLSE). This is the nonlinear Schrodinger equation (NLSE) with a nonlinear parameter being a complex number. We compare real and imaginary parts of this nonlinear parameter and show that the latter one should not be negligible, which means that the CNLSE should be solved numerically.