A method of preparing a pharmaceutical product in a single multiple chamber flexible bag. A pharmaceutical product is introduced in a liquid state into a first chamber of the flexible bag through a first port. The pharmaceutical product is lyophilized within the first chamber of the flexible bag to provide a lyophilized pharmaceutical product. The flexible bag has a second chamber and the first chamber and the second chamber are separated by a breakable seal. The second chamber further includes a reconstituting solution for reconstituting the lyophilized pharmaceutical product in the first chamber. A user may apply pressure to the flexible bag to break the seal and mix the lyophilized pharmaceutical product and the reconstituting solution to order to administer the pharmaceutical product to a patient.