Why does putting medicine in the eye not invalidate the fast?
Applying Surma (kohl) or putting medicine in the eye while fasting does not invalidate the fast; because there is no direct passage between the eye and the throat. The effect of the Surma or medicine that is felt in the throat is through fine veins, and such an effect does not invalidate the fast. (Book: Kitab an-Nawazil / Vol: 6 / Pg: 358)