⏱️Insulin On Board (IOB)
IOB is the amount of active insulin still working in your body after a previous dose. Ignoring it when stacking correction or meal boluses is one of the most common causes of dangerous hypoglycemia.
What Is Insulin On Board (IOB)?
Insulin On Board (IOB) — sometimes called Active Insulin or Insulin Action Remaining — is the estimated amount of previously-injected insulin that is still biologically active and lowering blood glucose at any given moment.
When you inject a bolus of rapid-acting insulin, it doesn't act instantly and then vanish. Its effect builds over 15–30 minutes, peaks around 60–90 minutes, and then gradually trails off over 3–5 hours. During this entire window, that insulin is still working — still capable of lowering your blood glucose.
If you inject another correction dose without accounting for this remaining active insulin, you risk insulin stacking — combining the effects of multiple overlapping doses, which can cause severe and unexpected hypoglycemia.
Why Insulin Stacking Is Dangerous
A common scenario that leads to stacking:
- You check your blood glucose 2 hours after a meal bolus — it's still 220 mg/dL (high).
- You calculate a correction dose: (220 − 100) ÷ 36 = 3.3 units, and inject it.
- But your meal bolus from 2 hours ago still has active IOB remaining — perhaps 2–3 units still working.
- An hour later, your blood glucose crashes to 50 mg/dL as both doses peak simultaneously.
This is why clinicians always instruct patients: never correct again until you know how much active insulin is still working.
How IOB Decays
Insulin does not disappear at a constant linear rate. Its activity tapers off along a curve. The rate of decay is characterized by the insulin's Duration of Insulin Action (DIA) — the total time window over which a given dose has meaningful glucose-lowering effect.
Insulin Point uses this decay formula:
Active insulin falls steadily after a dose and tapers to zero at the end of the DIA. Real curves vary by insulin type.
Duration of Insulin Action (DIA) by Insulin Type
| Insulin Type | Typical DIA | IOB at 2 hr | IOB at 3 hr |
|---|---|---|---|
| Ultra-rapid (Fiasp, Lyumjev) | 3–4 hr | ~35% | ~10% |
| Rapid-acting (lispro, aspart, glulisine) | 4–5 hr | ~50% | ~25% |
| Short-acting (Regular U-100) | 5–8 hr | ~60% | ~40% |
Example: If you took 6 units of rapid-acting insulin 2 hours ago, approximately 3 units (50%) are still active and working. Any correction dose calculation must subtract this IOB:
How Insulin Point Calculates IOB
Insulin Point's Bolus & Correction Calculator includes a continuous IOB decay guard with a toggle switch. When switched on, entering the number of units taken and hours since the last dose applies the physiological decay model to deduct active IOB from the recommended correction dose. When switched off, stacking protection is bypassed for baseline calculations.
If the deducted IOB is large enough that no further correction is needed, the calculator displays a warning rather than recommending additional insulin — actively preventing stacking.
Practical Rules for Managing IOB
- Wait at least 2–3 hours after a rapid-acting bolus before correcting high blood glucose — unless you are dangerously hyperglycemic or your care team advises otherwise.
- Always use a calculator or CGM that tracks IOB. Manual mental estimates are unreliable.
- CGM systems with closed-loop algorithms (Omnipod 5, Tandem Control-IQ) automatically factor IOB into every dose decision.
- If in doubt, check a blood glucose first — then decide. A glucose trending down on CGM with significant IOB often requires no additional correction.