The side effects of insulin injection include blurred vision, hypoglycemia, edema, allergic reaction, etc. Different side effects are caused by different reasons, for example, blurred vision is caused by the change of lens refraction, while edema is caused by sodium retention. Insulin is a glucose-lowering drug that can treat conditions such as type 1 diabetes mellitus, type 2 diabetes mellitus that is ineffective with oral glucose-lowering drugs or dietary control; insulin therapy is contraindicated in patients with insulin allergy, hypoglycemia, acute hepatitis, cirrhosis of the liver, hemolytic jaundice, and so on. Side effects such as blurred vision, hypoglycemia, edema, and allergic reaction may occur after insulin treatment, and different side effects are caused by different reasons; among them, blurred vision is generally caused by the change of lens refraction; edema is caused by water and sodium retention; hypoglycemia can be caused by excessive dosage of medication, reduced intake of food, and so on. Allergy occurs because insulin is a protein and an antigen, and when this antigen enters the body the organism may treat this antigen as a foreign invader, leading to an autoimmune response. Therefore, insulin can cause a series of side effects. Patients who need to use insulin treatment should use the drug under the guidance of a doctor, and when side effects occur after treatment with insulin if the symptoms are serious or continue unrelieved it is recommended to go to the hospital.