Humalog (Insulin Lispro) is a man-made fast-acting insulin analog created using genetic engineering. It is very similar in structure to naturally occurring human insulin. The minor modifications in structure give Insulin Lispro a rapid absorption and shorter delay of onset compared to naturally occurring insulin. It starts to work about 15 minutes after subcutaneous (under the skin) injection, achieving peak blood levels 30-70 minutes after, and lasts about two to five hours. It should be taken 15 minutes before your meal and, if necessary may be taken right after a meal within 20 minutes of the beginning of the meal).