Stabilized pharmaceutical formulations comprising IFN-β and highly purified mannitol are provided. The highly purified mannitol stabilizes the compositions by reducing the formation of IFN-β adducts in comparison with IFN-β formulated with mannitol that has not been highly purified. Methods for increasing the stability of IFN-β or a variant thereof in a liquid or lyophilized composition and for increasing storage stability of such a composition are also provided.