The answer is blindness

Common diseases that cause blindness are onchocerciasis, syphilis, diabetes, glaucoma, trachoma. Many other less common diseases may cause blindness.

While insulin can have side effects (particularly when more than prescribed is used), it does not cause blindness. Diabetes can cause retinal (eye) problems, though, which could be the reason some people believe insulin to cause eye problems.

renal (kidney) failure, heart disease, stroke, and blindness. Approximately 17 million Americans have diabetes. Unfortunately, as many as one-half are unaware they have it.

Diabetes mellitus is a chronic disease that causes serious health complications including renal (kidney) failure, heart disease, stroke , and blindness.