Insulin is a hormone that works by lowering levels of glucose (sugar) in the blood. Insulin aspart is a fast-acting insulin. Insulin aspart protamine is an intermediate-acting insulin. This combination insulin starts to work within 10 to 20 minutes after injection, peaks in 2 hours, and keeps working for up to 24 hours. Insulin aspart and insulin aspart protamine is a combination medicine used to improve blood sugar control in adults with diabetes mellitus (type 1 or type 2). Insulin aspart and insulin aspart protamine may also be used for purposes not listed in this medication guide.