Auto-injector having a median window with one triangular end

An autoinjector comprising a body that is adapted to receive a reservoir containing fluid and including a piston (P) and a needle, such as a pre-filled syringe, said body comprising a distal portion, a middle portion, and a proximal portion, said middle portion of said body including at least one middle window configured, before use, to permit viewing through the middle window 100% of the fluid contained in said reservoir, and configured, after injection, to permit viewing through the middle window said piston (P), wherein said at least one middle window has an external shape in the shape of an arrow that points towards the proximal portion of said body, a proximal portion of said at least one middle window including a triangular end portion, forming the point of the arrow, a base of said triangular end portion, remote from said point, forming a widest portion of said middle window.