“The lens of the eye is like a sponge, Doherty says. “It gets saturated when you have high blood sugar levels for a long time.” The extra sugar irritates the blood vessels in the eye and it can cause serious retina damage. It is often reversible but it can take up to a month before your vision clears. Keep in mind that it will fluctuate, she adds. So don’t get new glasses and see an ophthalmologist.