A cartridge (1) which is suitable to cooperate with the neck (3) of a container (5) in order to allow a product contained in the cartridge (1) to flow out into said container (5), said cartridge (1) including at least one reservoir (7); moveable sealing means (13) to close the opening (9) of the reservoir (7), and a socket (23) wherein the moveable sealing means (13) comprises at least one ring (19, 21) configured to cooperate with the neck (3) of the container (5) in order to allow the moveable sealing means (13) to slide along the lower wall (11) of the reservoir (7) by a pressure exerted on said ring (19, 21) by the neck (3) of the container (5) causing the opening of the one or more flaps (17) and further wherein at least one ring (19, 21) of the moveable sealing means (13) comprises holding lugs (25) configured to cooperate with an internal relief (27) of the socket (23) in order to maintain the moveable sealing means (13) in position inside the socket (23), closing the opening (9) of the reservoir (7).