The present invention provides sustained release compositions that can be manufactured easily while providing long duration of the pharmaceutically useful protein being delivered. The compositions comprise insoluble complexes of the pharmaceutically useful protein and a precipitating agent at an appropriate ratio to achieve a desired sustained release profile. The precipitating agent can be selected from a group of various agents including divalent metal cations, Hofmeister series salts and pH adjusters. Particularly provided are compositions comprising insulin that are suitable for pulmonary delivery.