An assembly for adding a medication or like substance to a liquid solution in a flexible bag (10) is provided. The flexible bag (10) contains a liquid solution, such as dialysate. An adapter (14) extends from the flexible bag (10) and includes a skirt (20) having an array of resilient sections (22) for engaging and snapping onto a neck finish (106) of a separate vial (100) containing a medication or like substance. A locking device (16) is adapted to be secured about the skirt (20) in a locking position that prevents outward movement of the array of resilient sections (22) and thereby prevents disengagement of the vial (100) from the adapter (14). A method of transferring the liquid solution from the bag (10) to the vial (100), mixing the medication with the liquid solution, and returning the mixture to the bag (10) is also provided.
