A container adapter for a medical delivery device is disclosed. The medical delivery device has a container seat for holding a container, a dosage chamber, a dosing mechanism and a lock mechanism adapted to prevent operation of the dosing mechanism when no container is held in the container seat and to allow operation of the dosing mechanism when the container is held in the container seat. The container adapter includes an adapter container seat arranged to hold a container, a joint structure arranged to be detachably positioned in the container seat of the medical delivery device and to connect the adapter container seat to the dosing mechanism, and an unlock member adapted to activate the lock mechanism. A delivery assembly and a method of delivering a liquid to a patient from different containers is also disclosed.