Abstract: A medical device and system for the administration of a medical substance or for dosing body fluids comprising a rotor, rotating around an axis X-X, operatively connected to an injection device of said medical substance, and a rotor locking/unlocking device mechanically separate from the rotor and integral in rotation therewith. The locking/unlocking device comprises, in one piece, a disc, centered with respect to said axis X-X, from which a plurality of flexible fins radially unravel. The flexible fins are configured to flex around transverse hinges at a connection portion with the disc, and are angularly spaced so as to define gaps between adjacent pairs of fins which prevent interferences between adjacent fins in the respective flexing movement. The flexible fins are associated with ferromagnetic means for the operation of the same fins and locking/unlocking means of the rotation of the disc. The disc and fins are formed from a single plastic body.