An injection device for injecting selectable amounts of a substance includes an activating mechanism including an axially moveable operating button which can rotated in a resting position only, a tubular drive element non-rotatably connected to the operating button, an axially fixed and non-rotatable guiding element, and an axially moveable driven member held non-rotatably in the device. A rotational grid is operationally between rotatable and non-rotatable components of the injection device such that the operating button can be moved two rotational directions when in the resting position whereby a selected amount of the substance to be injected can be selectively varied. For example, a dosage inadvertently selected too large by rotating the operating button too far in one direction can, in accordance with the present invention, be reduced by rotating the operating button in the opposite direction. In some embodiments, the rotational grid is adapted to produce a clicking sound when the operating button is rotated.