The capsule comprises: an internal capsule (4), which inserts removably in a mouth (2) of a container (1) to be closed, having an external cover (5), creating a security opening of the container (1); a reducer (7) which is applied on the mouth (2) of the container (1) and which is provided with an upper wall (70), in which a hole (71) is afforded, in which a syringe doser (8) can be introduced. The reducer (7) comprises: at least a radial projection (72) in the upper wall (70) thereof, which inserts in an annular recess (40) internally afforded in the upper part of the internal capsule (4); a first annular cavity (73), delimited by an external wall (74) and an internal wall (75) which develop axially below the upper wall (70) of the reducer (7), in which upper wall (70) an overhang (20) developing axially from the upper end of the mouth (2) of the container (1) is anchored.