An injectable pharmaceutical composition includes a solution of 3-acyl-buprenorphine, or a pharmaceutically acceptable salt thereof, in a biocompatible organic solvent, wherein the injectable pharmaceutical composition exhibits a steady release profile lasting over one week when injected into a patient. The acyl group is an alkylcarbonyl group, and an alkyl portion of the alkylcarbonyl group is a straight-chain, branched-chain, having 1-20 carbon atoms. The biocompatible organic solvent is N-methyl-2-pyrrolidone, ethyl acetate, ethanol, butanol, 2-butanol, isobutanol, ispropanol, glycerin, benzyl benzoate, dimethyl sulfoxide, N,N-dimethylacetamide, propylene glycol, dimethyl glycol, benzyl alcohol, or a combination of two or more thereof.
