The present invention concerns an adapter (1) which is suitable to be mounted on and to cooperate with the neck (7) of a container (3) in order to allow a product contained in the adapter (1) to flow out into the interior of said container (3), said adaptor including: at least one tank (4) which is suitable to contain said product, said tank (4) having a first opening (2) which is suitable to transfer said product from the tank (4) into the interior of the container (3), a single-use or reversible movable means for sealingly closing (5) the first opening (2), a means for activating (25) the movable means for sealingly closing (5) so as to allow said movable means for sealingly closing (5) to open and said product contained in the tank to flow out into the interior of the container (3). The present invention also relates to a container comprising said adapter and to a method for preparing and/or serving a liquid.