Insulin is used in the treatment of diabetes mellitus. The side effects of insulin include hypoglycemia, allergic reaction, insulin resistance and lipoatrophy. 1. Hypoglycemia: It is easy to have symptoms such as hunger, anxiety, sweating, rapid heartbeat, etc. In serious cases, headache, coma, shock and even death can occur. This side effect is caused by overdose of insulin. 2. Allergic reaction: Generally the reaction is mild, and occasionally it can lead to anaphylactic shock. It is common in insulin preparations of animal origin or low purity insulin preparations. 3. Insulin resistance: It means that the regular therapeutic dose of insulin is not enough to maintain blood glucose in the normal range, and it is necessary to increase the dose to control blood glucose. It can be divided into acute type and chronic type. (1) Acute type: It is mostly seen in the acute stress state such as concurrent infection, surgery, etc., and the transient anti-insulin substances in the body increase. After the elimination of stress, insulin resistance can be improved. (2) Chronic type: clinical daily insulin dosage needs to be more than 200U to control blood glucose. Mostly seen in patients with long-term insulin therapy, combined with obesity, hyperinsulinemia can also occur. 4. Fatty atrophy: skin depression occurs at the site of insulin injection. It is mostly caused by long-term injection in the same area. It is forbidden for those who are allergic to insulin, and patients who need insulin treatment can apply it correctly under the guidance of doctors to avoid applying it blindly on their own, which may cause serious consequences.