The invention relates to a syringe (10) for medical applications comprising: - a syringe body (20) having an inner wall (21) and comprising a first stop (32), located along said inner wall, designed to leave at least one first untouched surface (22) on the inner wall of the syringe body, and - a piston (100) sliding along the inner wall of the syringe body along a longitudinal axis (A1) of the syringe, the piston comprising a piston head (110) and an actuating rod (120), connected to the piston head, comprising, on its outer surface, at least one rack (122) arranged longitudinally along the actuating rod, the rack comprising protruding notches (122A). According to the invention, the first stop (32) and the protruding notches (122A) are also shaped and constituted so as to prevent the piston (100) from lowering again due to pressure on the actuating rod (120), unless the first rack (122) is disengaged by rotating the piston (100) within the syringe body (20) around the longitudinal axis so that the first rack (122) faces the first untouched surface (22) of the inner wall (21) of the syringe body (20).
