Insulin is an important method to control blood sugar in diabetic patients. Insulin is mainly used for type 1 diabetes, type 2 diabetes with significant elevation of blood sugar, and type 2 diabetes with combined acute and chronic complications, and gestational diabetes. The most important adverse effect of insulin is hypoglycemia. Patients with diabetes treated with insulin are prone to hypoglycemia when they eat irregularly or exercise too much. Mild hypoglycemia will occur with symptoms such as panic, sweating, hand trembling and hunger, which can improve after eating. Severe hypoglycemia may lead to hypoglycemic coma, and persistent hypoglycemic coma can lead to irreversible brain damage. Therefore, diabetic patients receiving insulin therapy should review their blood glucose regularly to adjust the drug dose to avoid severe hypoglycemia. Other common adverse reactions of insulin are, causing sodium and water retention leading to swelling, insulin allergy or fat atrophy at the injection site can occur and other adverse reactions.