A controlled release pharmaceutical composition comprising a physiologically active agent dispersed in preformed porous polymeric microparticles is provided. The active agent concentration may be up to about 10% by weight to achieve controlled release. Each of the porous microparticles has a plurality of preformed pores into which active agent is loaded and from which the active agent is subsequently released to the environment of use. The compositions are capable of delivering physiologically effective amounts of active agent for at least about thirty days, which delivery may be reversibly controlled by exposure to ultrasound.