Which insulin type has the fastest onset after subcutaneous administration?

• A. Regular human insulin
• B. Intermediate-acting insulin
• C. Long-acting insulin
• D. Rapid-acting insulin analogs (lispro, aspart)

Answer: (D)

The key idea is how quickly an insulin preparation starts acting after subcutaneous injection, which depends on how quickly the insulin is absorbed into the bloodstream. Rapid-acting insulin analogs—lispro and aspart (and related analogs)—are engineered to be absorbed quickly, existing mainly as monomers rather than hexamers. This rapid absorption translates to onset of action within roughly 10 to 30 minutes, making them the fastest among common insulins and well-suited for eating around mealtimes.

In contrast, regular human insulin forms hexamers that must dissociate to active monomers, delaying onset to about 30 to 60 minutes. Intermediate-acting insulins (like NPH) have slower, more variable absorption and longer onset, and long-acting insulins have even slower onset with a prolonged, steady effect to provide basal coverage. So, the fastest onset after subcutaneous administration is the rapid-acting insulin analogs.