An assembly having an adaptor for coupling with a medical container having a septum. The adaptor includes a gripping member for securing the adaptor to the medical container a pierceable elastomeric piece which, in an inactive state of the adaptor, does not contact the septum outer surface with pressure, and, in an active state of the adaptor, contacts the septum outer surface with pressure a blister comprising a shell and a removable closing membrane forming a housing receiving the adaptor a retaining system having a first retaining portion on the adaptor and a second retaining portion on the shell, the first retaining portion adapted, in a locked position of the retaining system, to cooperate with the second retaining portion, when the adaptor is not in the active state and an unlocking portion adapted to transition the retaining system to an unlocked position, when the adaptor is in the active state.
