The embodiments herein provide a nano-carrier system for delivering a long-acting injectable drug of buprenorphine and a method of synthesizing the same. The buprenorphine entrapped nanoparticles are prepared using a lipid/phospholipid core which is coated by a polymer. The lipid and phospholipid are dissolved in organic solvent. This solution is transferred into an aqueous phase consisting of distilled water or a buffer. A solution of polymer is added drop wise. The drug entrapped nanoparticle formation is achieved by diffusion of the organic solvent within the aqueous solvent to obtain the nanoparticles. The drug gets entrapped within the nanoparticles via the anti-solvency effect of the aqueous matrix. The resulting drug nanocarriers are capable of releasing the drug in a slow rate upon injection. The synthesized drug carrying nanoparticles are cryopreserved stored for future administration. For better storage, the nanodispersion is dried to form a powder.