Patient instructed Humalog (insulin lispro) 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. It is used to improve blood sugar control in patients with DMII. Most people who take Humalog use a sliding scale or take a certain dose routinely throughout the day. If you are taking this medication with meals, use it within 15 minutes before or just after you eat. Do not take Humalog if your blood sugar is below 60 or you are experiencing s/s of hypoglycemia.