Various agents and additives for cross-linking treatments are identified in disclosed studies. The characteristics of the various agents and additives may be advantageously employed in formulations applied in cross-linking treatments of the eye. In some embodiments, riboflavin is combined with Iron(II) to enhance the cross-linking activity generated by the riboflavin. In other embodiments, cross-linking treatments employ an Iron(II) solution in combination with a hydrogen peroxide pre-soak. In yet other embodiments, 2,3-butanedione is employed to increase the efficacy of corneal cross-linking with a photosensitizer, such as riboflavin. In further embodiments, folic acid is employed in combination with a photosensitizer, such as riboflavin, to enhance cross-linking activity. In yet further embodiments, 2,3-butanedione, folic acid, a quinoxaline, a quinoline, dibucaine, Methotrexate, menadione, or a derivative thereof is applied as a cross-linking agent.
