The present invention discloses novel synergistic pharmaceutical co-crystals of Metformin or its pharmaceutically acceptable salts and oleoylethanolamide in fixed stoichiometric ratio at lower dosage, with improved residence time thereby increasing the bioavailability and efficacy, as anti- diabetic agent. The invention further discloses a pharmaceutical composition comprising co- crystals of Metformin or its pharmaceutically acceptable salts and oleoylethanolamide with other antidiabetic agents.