1. An implantable drug delivery system for the treatment of erectile dysfunction, comprising: a passively pressurized drug infusion pump containing a therapeutic, one or more catheters, one or more microfluidic valves, a microcontroller, and one or more patient input mechanisms, wherein one of the one or more patient input mechanisms comprises an implantable neural interface, wherein the one or more microfluidic valves are under operative control of the microcontroller, wherein the microcontroller opens the one or more microfluidic valves to dispense a discrete dosage of the therapeutic in response to a signal from the one or more patient input mechanisms, wherein the implantable neural interface is in electrical communication with nerves associated with a subject's penis, and wherein the microcontroller is programmed to interpret signals registered by the implantable neural interface, the signal interpretation further comprising requiring that the signals registered by the implantable neural interface exceed a selected threshold stimulus frequency for a defined duration within a predetermined time window prior to release of the therapeutic.