Core annular flow is used to enable the subcutaneous delivery of a viscous fluid such as a protein therapeutic formulation. The high-viscosity fluid is surrounded by a low-viscosity fluid, and the low-viscosity fluid lubricates the passage of the high-viscosity fluid. This allows the use of protein formulations that have a higher concentration and a higher viscosity at comparatively reduced injection forces and reduced injection times. Several different embodiments of injection devices that provide core annular flow are described herein.