An autoinjector includes a body (1; 1010), a reservoir containing fluid product, a piston, a needle, a piston rod (5 5; 1005) that is displaceable between a loaded position and an injection position, and an injection spring (8; 1008) to stress 10 the piston rod towards the injection position. The autoinjector also includes an injection lock having a control sleeve (4; 1004) containing the piston rod and the injection spring. The piston rod includes a radial recess (50; 1050) receiving a substantially spherical blocking element (7; 1007) mobile between a blocking position and an unblocking position, stressed radially towards the exterior by the piston rod, and held in the blocking position by a blocking member (23; 1004) that is displaceable relative to the piston rod to release the blocking element.