An automatic injection pen with forward housing (110) for holding a syringe and a rearward housing (130) for providing an actuated extension and injection of a needle is described. When the pieces are rotatably connected together (i.e. 165 and 166 are aligned), a switch, latch or other locking member is depressed, allowing the rearward piece to provide the spring loaded injection motion to the plunger of the syringe when a trigger or button (170) is pushed.
