Abstract: The invention relates to a closure for a container having an adapter part which can be fastened to the container connecting piece forming the container opening and on the external thread of which a hood-shaped part is screwed which forms the upper removal opening, wherein a capsule which is filled with an active substance and the contents of which can be moved into the interior of the container lies on the upper edge of the container connecting piece, wherein the capsule can be cut open by blades which protrude on a blade ring which can be rotated by rotation of the hood-like part and can be moved downwards, wherein the blade ring can be moved downwards during a first rotational range of the hood-shaped part until the blades stab into the capsule, and wherein the blade ring can be rotated by the hood-shaped part during the subsequent second rotational range of the hood-shaped part, in particular without a further movement downwards.