The present invention is directed to a composition that can be used to deliver an antipsychotic drug such as risperidone, paliperidone or a combination thereof, as an injectable in-situ forming biodegradable implant for extended release providing therapeutic plasma levels from the first day. The composition is in the form of drug suspension on a biodegradable and biocompatible copolymer or copolymers solution using water miscible solvents that is administered in liquid form. Once the composition contacts the body fluids, the polymer matrix hardens retaining the drug, forming a solid or semisolid implant that releases the drug in a continuous manner. Therapeutic plasma levels of the drug can be achieved from the first day up to at least 14 days or more even up to at least four weeks.