A medical injector comprising: a body (12) having a proximal end and a distal end； a reservoir (20) located in said body (12), said reservoir (20) including at least first (22) and second (24) mixable components, at least one stopper (26, 28) being associated with said reservoir (20) , causing a distal advance of said plug (26, 28) over a predetermined distance the mixing of said mixable components (22, 24)； a plunger (40) located in said body (12)； a rod (42) of the button located in said body (12)； a button (46) fixed to said rod (42) of the button so that it can rotate with it； a bushing (44) located telescopically above said rod (42) of the button and configured to engage with said piston (40), said rod (42) having said button and said bushing (44) formed on them interacting elements each other, which selectively allow the rotation of said rod (42) of the button to be transmitted to said bushing (44) so that said rod (42) of the button and said bushing (44) can rotate together； a spring (52) for pushing said bushing (44) distally； and a releasable retention element (58) for non-permanently retaining said bush (44) in a first position against the force of said spring (52), including said releasable retention element (58): a fastener (60) formed in one of said body (12) and said bushing (44)； and, a channel (62) characterized in that the channel (62) formed in the other between said body (12) and said bush (44), said channel (62) having a first part (64) partially located circumferentially around a longitudinal axis (66) of said bushing (44), and a second part (68) extending from said first part (64) and located generally parallel to said longitudinal axis (66), said channel (62) being shaped to accommodate the aforementioned guarantor (60) and to allow a sliding movement of said guarantor (60) along it； wherein, said bushing (44) is held in said first position when said catch (60) is located in said first part (64) of said channel (62), and, in which, the rotatio
