Duration of Glucose-Lowering Effect After 5 Units of Humalog
The glucose-lowering effect of a 5-unit subcutaneous injection of Humalog (insulin lispro) ceases approximately 3 to 5 hours after administration, with most activity completed by 4 hours. 1
Pharmacokinetic Profile of Humalog
Humalog exhibits a rapid-acting profile that distinguishes it from regular human insulin:
- Onset of action: 0.25–0.5 hours (15 minutes) after subcutaneous injection 2, 1
- Peak effect: 1–3 hours, with maximum insulin concentrations typically occurring 30–90 minutes post-injection 2, 1
- Duration of action: Less than 5 hours, with glucose-lowering activity essentially complete by 4–5 hours 2, 1, 3
The rapid absorption and shorter duration result from insulin lispro's reduced capacity for self-association at the injection site compared to regular human insulin, allowing monomeric insulin to enter the bloodstream more quickly. 3
Clinical Implications for Glucose Monitoring
After a 5-unit dose, you should expect:
- Maximal glucose-lowering between 30–90 minutes post-injection 1
- Declining insulin activity after 2 hours 1
- Return to baseline insulin concentrations and cessation of glucose-lowering effect by 4–5 hours 1, 3
This shorter duration means that Humalog does not provide sustained basal insulin coverage—its effect is confined to the prandial (meal-related) period. 4
Important Clinical Caveats
Hypoglycemia risk: The most critical period for hypoglycemia is 1–3 hours post-injection when insulin activity peaks. 1 If a meal is delayed or carbohydrate intake is insufficient, hypoglycemia can occur during this window even though the total duration is short. 4
Residual beta-cell function: In patients with residual pancreatic beta-cell function (early type 1 diabetes), endogenous insulin secretion may extend glucose control beyond the 5-hour window of exogenous lispro, but this does not reflect continued activity of the injected Humalog itself. 5
Dose-independent kinetics: The 3–5 hour duration applies regardless of whether the dose is 5 units or a larger amount—the duration remains constant while the magnitude of glucose-lowering scales with dose. 1, 3
Practical Monitoring Recommendations
- Check blood glucose 1–2 hours post-injection to assess peak effect 4
- Recheck at 3–4 hours if hypoglycemia is suspected, as this represents the tail end of insulin activity 4
- Do not expect glucose-lowering beyond 5 hours from a single Humalog dose 1
- If glucose control is needed beyond 5 hours, basal insulin (glargine, detemir, or degludec) must be added to the regimen 4