A closure for dispensing one or more active agents into a container comprises a sealed or sealable chamber having a breakable wall and a hollow piston slidably mounted in a piston guide. Said hollow piston comprises an outer wall having an end in the chamber and at least one ventilation aperture. Said end has a cutting formation. Said hollow piston is slidable in the piston guide between a ventilation position in which the at least one ventilation aperture allows ventilation of the chamber and a sealed position in which the at least one ventilation aperture is sealed to prevent ventilation of the chamber and a deployed position in which the cutting formation has broken through at least a portion of the breakable wall. The outer wall has a retaining formation which engages with the piston guide to releasably resist sliding of the hollow piston between the ventilation position and the sealed position and the deployed position. The hollow piston may have an outer wall, and said outer wall has an end within the chamber, facing the breakable wall, wherein the cutting end has a first edge having a cutting formation and a gap in said cutting formation.