The disclosure relates to a buprenorphine sustained release delivery system capable of delivering buprenorphine for duration of about 14 days to about 3 months. The buprenorphine sustained release delivery system includes a flowable composition and a solid implant for the sustained release of buprenorphine. The implant is produced from the flowable composition. The buprenorphine sustained release delivery system provides in situ 1-month and 3-month release profiles characterized by an exceptionally high bioavailability and minimal risk of permanent tissue damage and typically no risk of muscle necrosis. Particularly disclosed is an injectable flowable composition comprising: (a) about 5 wt% to about 25 wt% buprenorphine in the form of a free base or a pharmaceutically acceptable salt； (b) about 15 wt.% to about 70 wt.% of a poly(DL-lactide-coglycolide) copolymer； and (c) about 10 wt.% to about 90 wt.% of an organic liquid selected from the group consisting of N-methyl-2-pyrrolidone, 2-pyrrolidone, N,N-dimethylformamide, dimethyl sulfoxide, propylene carbonate, caprolactam, triacetin, polyethylene glycol, or a combination thereof； wherein the flowable composition forms a solid implant upon contact with a body fluid, an aqueous medium, or water.
