A cartridge for use with a beneficial agent delivery device, said delivery device having a fillable reservoir, said cartridge comprising: a reconstitution unit dose package, comprising: a first compartment containing at least a first constituent of the beneficial agent; a second compartment containing at least a second constituent of the beneficial agent; wherein at least one of the first compartment and the second compartment comprises a flexible wall having a pre-formed structure; a first frangible seal 19' separating the first compartment and the second compartment, the first frangible seal being rupturable to allow the first constituent and the second constituent to merge to form a joined compartment; and a fitment (12, 257) disposed on said reconstitution unit dose package and configured to interface said reconstitution unit dose package with said fillable reservoir; and a coupler configured to establish fluid communication with the reconstitution unit dose package by engagement of a fluid passageway of the coupler and the fitment of the reconstitution unit dose package, the coupler supporting at least part of the reconstitution unit dose package when the fluid passageway and the fitment are in fluid communication, characterised in that the coupler comprises a needle assembly comprising a Luer Lock hub and a cannula attached to the hub; the needle assembly is accommodated in a cap formed in the coupler having a rubber sheath disposed in said coupler to form an aseptic seal between the needle hub of the needle assembly and the coupler, the sheath forming a fluid tight closure of the fluid passageway, the fluid tight closure being pierced by the cannula of the needle assembly for establishing fluid communication between the needle assembly and the reconstitution unit dose package. a reconstitution unit dose package, comprising: a first compartment containing at least a first constituent of the beneficial agent; a second compartment containing at least a second constituent of the beneficial agent; wherein at least one of the first compartment and the second compartment comprises a flexible wall having a pre-formed structure; a first frangible seal 19' separating the first compartment and the second compartment, the first frangible seal being rupturable to allow the first constituent and the second constituent to merge to form a joined compartment; and a fitment (12, 257) disposed on said reconstitution unit dose package and configured to interface said reconstitution unit dose package with said fillable reservoir; and a first compartment containing at least a first constituent of the beneficial agent; a second compartment containing at least a second constituent of the beneficial agent; wherein at least one of the first compartment and the second compartment comprises a flexible wall having a pre-formed structure; a first frangible seal 19' separating the first compartment and the second compartment, the first frangible seal being rupturable to allow the first constituent and the second constituent to merge to form a joined compartment; and a fitment (12, 257) disposed on said reconstitution unit dose package and configured to interface said reconstitution unit dose package with said fillable reservoir; and a coupler configured to establish fluid communication with the reconstitution unit dose package by engagement of a fluid passageway of the coupler and the fitment of the reconstitution unit dose package, the coupler supporting at least part of the reconstitution unit dose package when the fluid passageway and the fitment are in fluid communication, characterised in that the coupler comprises a needle assembly comprising a Luer Lock hub and a cannula attached to the hub; the needle assembly is accommodated in a cap formed in the coupler having a rubber sheath disposed in said coupler to form an aseptic seal between the needle hub of the needle assembly and the coupler, the sheath forming a fluid tight closure of the fluid passageway, the fluid tight closure being pierced by the cannula of the needle assembly for establishing fluid communication between the needle assembly and the reconstitution unit dose package.