Pharmaceutical composition made of microparticles for the slow release of an active substance at least during a period covering the 6th month after injection of said composition, said composition comprising a group of microparticles made of a copolymer of the PLGA type which incorporate an active substance in the form of a water insoluble peptide salt; said copolymer furthermore comprising at least 75% of lactic acid and an inherent viscosity between 0.1 and 0.9 dl/g, as measured in chloroform at 25° C. and at a polymer concentration of 0.5 g/dL; said microparticles furthermore having a size distribution defined as follows: —D (v,0.1) is between 10 and 30 micrometers, —D (v,0.5) is between 30 and 70 micrometers, —D (v,0.9) is between 50 and 1 10 micrometers.