Assembly with connectable and disconnectable elements comprises a flexible bag containing liquid solution with a first valve connector, at least one vial with active substance and provided with a hermetic closing cap, a coupling and piercing device couplable to the cap of the vial and an adapter between the first connector and the coupling and piercing device. The adapter has a hollow external body with a first end fixable by screwing to the first connector, an axially bored, coaxial internal stem, the screwing causes the opening of the first connector and a second end including a second valve connector fixable to the coupling and piercing device and is screwable in the external body so that internal stem causes the opening of the second valve connector.