Insulin is a hormone that works by lowering levels of glucose (sugar) in the blood. Insulin aspart is a fast-acting insulin that starts to work about 15 minutes after injection, peaks in about 1 hour, and keeps working for 2 to 4 hours. Insulin aspart is used to improve blood sugar control in adults and children with diabetes mellitus. This medicine is sometimes used together with a long-acting or intermediate-acting insulin. Insulin aspart is used to treat type 2 diabetes in adults. Insulin aspart is also used to treat type 1 diabetes in adults and children who are at least 2 years old. Insulin aspart may also be used for purposes not listed in this medication guide.