A disposable safety syringe in one embodiment includes a stepped syringe barrel comprising two opposite mating members on an inner surface proximate a distal end; a plunger comprising a connection member and a breakable section behind the connection member; a first needle assembly comprising a needle and a needle hub secured to the distal end of the syringe barrel; and a hollow second needle assembly shaped to lockingly secure to the first needle assembly in the syringe barrel, the second needle assembly comprising two opposite corresponding mating members on an outer surface, the corresponding mating members being lockingly engaged with the mating members. After dispensing, the plunger may clockwise rotate about the syringe barrel until the mating members and the corresponding mating members are unlocked lengthwise each other. The plunger, the second needle assembly, and the first needle assembly may move rearward to retract the needle within the syringe barrel.