Regular insulin (Humulin R, Novolin R) is a short-acting insulin that mimics the body's natural insulin response to meals. It facilitates glucose uptake into cells by binding to insulin receptors on cell membranes, activating glucose transporter proteins (GLUT4) that move glucose from the bloodstream into skeletal muscle, adipose tissue, and other cells. Regular insulin is the ONLY insulin that can be administered intravenously, making it critical for emergency management of diabetic ketoacidosis (DKA) and hyperkalemia. When given subcutaneously, onset is 30 to 60 minutes with a peak at 2 to 4 hours. When given IV, onset is approximately 15 minutes with a much shorter duration, requiring continuous infusion and close monitoring.