Whether or not it is useful to take insulin after eating needs to be determined by the type of insulin, which is a very effective way to control blood sugar in diabetic patients. There are many types of insulin, and depending on the duration of action, insulin can be divided into rapid-acting insulin analogs, short-acting insulin, intermediate-acting insulin, long-acting insulin analogs, and premixed human insulin and premixed insulin analogs. The insulins that need to be injected before meals are mainly rapid-acting insulin analogues, short-acting insulins, premixed human insulins and premixed insulin analogues, among which rapid-acting insulin analogues and premixed insulin analogues have fast onset of action and short peak time, and can be injected subcutaneously before and during meals, or immediately after meals. Short-acting insulins and premixed human insulins have an onset of action 20-30 minutes after injection. Therefore, if these two insulins are injected after eating, the peak of insulin will lag behind the peak of meal blood sugar, which will not only fail to effectively reduce postprandial blood sugar, but also may lead to preprandial hypoglycemia in the next meal, if it is a rapid-acting insulin analogue or premixed insulin analogue, subcutaneous injection immediately after meal is useful.