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 is between 10 and 30 micrometers , d is between 30 and 70 micrometers , d is between 50 and 1 10 micrometers .