Which insulin type has the fastest onset of action?

• A. Short-acting insulin
• B. Rapid-acting insulin
• C. Intermediate-acting insulin
• D. Long-acting insulin

Answer: (B)

The fastest onset is seen with rapid-acting insulin. These analogs (such as lispro, aspart, and glulisine) are formulated to be absorbed quickly from the subcutaneous tissue, so they start lowering glucose within about 10 to 20 minutes after injection. They reach their peak effect in roughly 1 to 3 hours and have a relatively short duration of about 3 to 5 hours. This makes them ideal for controlling glucose around meals, as they can be given shortly before eating and act promptly to manage the postprandial rise. In comparison, short-acting regular insulin takes longer to begin (about 30 to 60 minutes) and lasts longer, while intermediate-acting and long-acting insulins have slower or more prolonged action profiles with different clinical uses.