Injection devices and systems and methods for using them

Systems and methods are provided for injecting one or more agents into tissue within a patient's body. In one embodiment, an injection device is provided that includes an outer tubular member comprising a proximal end, a distal end sized for introduction into the patient's body, and a lumen extending between the proximal end and the distal end; a relatively small cross-section needle tip extending distally beyond the distal end; and an internal pressure release member slidably disposed within the lumen between a distal position and a proximal position to prevent viscous injectate from expressing from the needle tip.