The invention relates to a method of making a solution for injection from a solvent and an active ingredient using a disposable injector with a housing, an injector-side, first cylinder-piston unit—at least intermittently fillable—arranged thereon and an upstream detachable container adapter, wherein the container adapter bears a likewise at least intermittently fillable second cylinder-piston unit. A solvent is stored in the first cylinder-piston unit. In the second cylinder-piston unit there is a freeze-dried active pharmaceutical ingredient. Immediately before use of the disposable injector the solvent is conveyed to the second cylinder-piston unit to the active pharmaceutical ingredient, where a solution forms. This solution is transfer-pumped into the first cylinder-piston unit to then be administered.