The combat drugs administered to reapers are a potent cocktail of cell stimulants and growth hormones derived from the stimpack base. They boost the regenerative properties of the human body by 800%, which allows reapers in the field to quickly treat most types of injury. The healing process starts a few seconds after administering a dose of the substance, which is often credited with saving more lives than some field medics.  However, these cytokenetic drugs have been known to cause growth-related side effects.
Reapers heal for 2.8 HP every second if the Reaper is not attacked for 7 seconds.
Note that this ability is exclusive to Heart of the Swarm.