In vivo absorption of didanosine formulated in pellets composed of chitosan microspheres. The in vivo absorption of didanosine was studied, focusing on the performance of a novel pharmaceutical formulation for didanosine, composed of chitosan granules containing didanosine incorporated in chitosan microspheres. This novel formulation is aimed at oral administration in AIDS therapy. The experimental results in male adult dogs showed controlled delivery of didanosine along 36 h, with a 2-fold increase in the absorption time of didanosine compared to the commercial granules, gastro-resistant didanosine and tablets. The higher absorption is due to adhesion to the intestinal membrane, improving absorption through increase of residence time, permeation and release. Furthermore, the novel formulation facilitates handling and deglutition, especially in the elderly and children, as well as enhances the taste and reduces the frequency of doses and collateral effects associated with a high concentration of the buffer agents usually used in other formulations.