In this work, we study a tumor model under chemotherapy treatment in which the drug directly affects an isolated population of the tumor cells. By using a dimensionless version of the model, we analyze the behavior of the tumor when we increase the intensity of the chemotherapy in function of the recruitment parameter. Our research shows that, for a certain interval of parameter there is a region for which the tumor is stable under the action of the chemotherapy. Besides, we consider the dynamical behavior of the tumor for different scenarios characterizing the parts where there is chaotic dynamics by calculating the distribution of the largest Lyapunov exponents (LLE) in function of the drug intensity. Our findings show that chaotic behavior takes place when the drug treatment starts acting on the organism and it disappear when the chemotherapy intensity crosses a certain specific parameter value. Finally, we expect these results to be useful for a better understanding of the development of a tumor in presence of a chemotherapy treatment.