Disclosed herein is a biocompatible, injectable intraocular drug delivery system comprising: (a) a plurality of microspheres with an average diameter between 5 about 8 microns and about 14 microns, and (b) an aqueous vehicle for the microspheres, wherein the microspheres consist essentially of: (1) a therapeutic agent which is an alpha 2 adrenergic agonist, such as a brimonidine, e.g. brimonidine tartrate, and (2) one or more biodegradable polymers, all of which biodegradable polymers are polylactic acid (PLA) polymers, such as poly(D,L)lactide polymer, and wherein the drug delivery system has a viscosity at 20°C of between about 15,000 cps and about 100,000 cps which permits the drug delivery system to be injected into an intraocular location through a 24 to 30 gauge syringe needle and the microspheres can release between 0.5 micrograms/day to about 20 micrograms/day of the brimonidine for a period of time between about 10 and about 100 days. Additionally wherein the biocompatible drug delivery system is for use in a method of treating an ocular condition, wherein the method does not result in significant ocular surface hyperemia, and wherein the method comprises injecting the drug delivery system onto an intraocular location through a 20 to 26 gauge syringe needle, or implanting the drug delivery system into an intraocular or intravitreal location, wherein the intraocular location is a sub-Tenon, subconjuctival, suprachoroidal, intrascleral or retrobulbar intraocular location. Additionally wherein the biocompatible drug delivery system is for use in a method for lowering intraocular pressure, the method comprising implanting the drug delivery system into an anterior sub-Tenons space, thereby lowering intraocular pressure.
