Actrapid Penfill 100 IU/ml Solution for Injection is a rapid-acting insulin used to control blood glucose levels in individuals with diabetes mellitus. Manufactured by Novo Nordisk, this insulin formulation is administered subcutaneously, usually 20–30 minutes before meals, to facilitate glucose uptake by muscle and fat cells and to inhibit glucose production by the liver. The onset of action occurs within 30 minutes, with peak effects between 2 to 4 hours, and a duration of action up to 8 hours. It is often used in combination with intermediate or long-acting insulins to provide comprehensive blood sugar control. Common side effects may include hypoglycemia, injection site reactions, and weight gain. Patients should consult their healthcare provider before starting Actrapid Penfill, especially if they have a history of liver or kidney problems, or are pregnant or breastfeeding.