Humalog is a fast-acting insulin that is used to control high blood sugar in adults and children with diabetes. It is an insulin analog, which means it is a man-made version of the natural insulin that the body produces. Humalog contains insulin lispro, which is a rapid-acting insulin analog that works very quickly. It begins working to lower blood sugar levels about 15 minutes after injection, reaches a peak in about 1 hour, and continues to work for 2 to 4 hours. Humalog is taken at mealtimes to help manage the surge in blood sugar that can occur after eating.