Also known as: Humulin N · Novolin N · isophane insulin · intermediate-acting insulin
Insulin NPH (neutral protamine Hagedorn, isophane insulin) is an intermediate-acting insulin suspension with an elimination half-life of approximately 4–6 hours following subcutaneous injection[1], based on FDA-approved prescribing information for Humulin N. Onset of glucose-lowering action occurs at 1–2 hours, peak effect at 4–8 hours, and duration approximately 12–18 hours. NPH is not administered intravenously; it is a suspension that requires thorough mixing before each injection.
| Parameter | Value (SC) | Source |
|---|---|---|
| Elimination Half-Life | ~4–6 hours | FDA Humulin N Prescribing Information[1] |
| Onset of Action | 1–2 hours | FDA Humulin N label[1] |
| Time to Peak Effect | 4–8 hours | FDA Humulin N label[1] |
| Duration of Action | 12–18 hours | FDA Humulin N label[1] |
| Bioavailability (SC) | ~70% (estimated) | Lepore et al. 2000 (PMID 11118018)[3] |
| Full Clearance (5 × t½) | ~20–30 hours | Calculated from label PK data |
| IV Administration | Not compatible — suspension only | FDA Humulin N label[1] |
| Primary Clearance Route | Receptor-mediated degradation; liver (~60%), kidney | FDA Humulin N label[1] |
| Standard Dosing Frequency | Once or twice daily (basal coverage) | FDA-approved labeling |
| Data Quality | Human RCT — FDA-approved prescribing information (Humulin N, Novolin N) | |
Insulin NPH has an elimination half-life of approximately 4–6 hours following subcutaneous injection, per FDA-approved prescribing information for Humulin N.[1] "NPH" stands for Neutral Protamine Hagedorn — named after Hans Christian Hagedorn, whose 1946 research team developed the protamine-zinc insulin suspension that became the basis for NPH formulations. The intermediate-acting profile results from the protamine–insulin complex forming a depot at the injection site that releases insulin gradually as the complex dissolves.
Like all insulin formulations, NPH pharmacokinetics are characterised through euglycaemic clamp studies. Serial plasma insulin measurements combined with glucose infusion rate data provide t½, Tmax, and action duration estimates. Because NPH involves an absorption depot, the apparent half-life reflects the absorption kinetics from the SC depot combined with systemic elimination — the depot phase dominates the overall kinetic profile.[3]
NPH insulin's measured plasma half-life (~4–6 hours) describes the terminal elimination phase once insulin is absorbed. However, the clinically relevant parameter is action duration (~12–18 hours), which reflects the extended absorption from the subcutaneous depot. The prolonged glucose-lowering duration arises from continued dissolution of the protamine–insulin depot over many hours, not from extended systemic circulation. This distinction explains why NPH behaves as an intermediate-acting (not short-acting) insulin despite a moderate half-life.
| Half-Lives Elapsed | Time After SC Injection | % Remaining | Clinical Note |
|---|---|---|---|
| 1 | ~4–6 hours | 50% | At or approaching peak glucose-lowering effect (peak 4–8 h) |
| 2 | ~8–12 hours | 25% | Still significant glucose-lowering activity |
| 3 | ~12–18 hours | 12.5% | Action duration boundary; effect largely resolved |
| 4 | ~16–24 hours | 6.25% | Near-complete action; overlap with next dose if twice-daily |
| 5 (threshold) | ~20–30 hours | ~3% | Pharmacologically negligible in plasma |
NPH's 12–18 hour duration of action supports once- or twice-daily dosing as a basal insulin. Typical regimens include: bedtime NPH (to cover overnight glucose, with peak hitting early morning when cortisol-driven glucose rise occurs — the "dawn phenomenon"); or morning plus evening NPH to provide all-day basal coverage. The pronounced peak at 4–8 hours makes overnight hypoglycaemia a clinically important risk with bedtime NPH — a limitation that long-acting peakless analogues (glargine, detemir, degludec) specifically address.
| Insulin Type | Examples | Half-Life SC | Peak | Duration | Overnight Hypo Risk |
|---|---|---|---|---|---|
| Rapid-acting analogue | Lispro, Aspart, Glulisine | ~1 h | 1–2 h | 3–4 h | Low (short duration) |
| Regular (short-acting) | Humulin R, Novolin R | ~2 h | 2–4 h | 5–8 h | Moderate |
| NPH (intermediate) | Humulin N, Novolin N | ~4–6 h | 4–8 h | 12–18 h | Higher (pronounced peak) |
| Glargine (long-acting) | Lantus, Basaglar, Toujeo | ~12–19 h | Peakless | ~24 h | Lower (peakless) |
| Route | Compatibility | Notes |
|---|---|---|
| Subcutaneous (abdomen) | Yes — standard route | Must mix suspension; abdomen slightly faster than thigh/arm |
| Subcutaneous (thigh/arm) | Yes | Slower absorption than abdomen; consistent site rotation recommended |
| Intravenous (IV) | Not compatible | NPH is a suspension — IV administration is contraindicated |
| Intramuscular (IM) | Not recommended | Suspension properties make IM inconsistent; not a standard route |
Insulin NPH is not detected by standard workplace immunoassay drug panels. WADA prohibits non-therapeutic insulin use in sport (S4 — Hormone and Metabolic Modulators).
NPH insulin's active component is structurally identical to human insulin. Detection of NPH-derived insulin in WADA anti-doping testing relies on C-peptide suppression testing — exogenous insulin suppresses endogenous pancreatic insulin secretion, reflected as a low C-peptide:insulin molar ratio in urine or blood samples collected within the action window.
NPH insulin is a suspension of human insulin complexed with protamine (a polycationic protein derived from salmon sperm) and zinc in an isophane (1:1 molar) ratio. The protamine–insulin complex is virtually insoluble at neutral pH, forming a white, cloudy suspension that must be mixed before injection.[1]
After subcutaneous injection, the protamine–insulin crystals form a semi-solid depot. Dissolution of this depot at the SC injection site is the rate-limiting step, governed by the relatively slow dissociation of the protamine–insulin complex at physiological pH and temperature. As insulin monomers dissociate from the depot and are absorbed into capillaries, the gradual dissolution produces the characteristic delayed onset (1–2 hours) and extended duration (12–18 hours).[3]
The pronounced peak at 4–8 hours reflects the kinetics of this dissolution process — absorption rate is not constant but increases as depot temperature equilibrates and dissolution accelerates, then slows as the depot is depleted. This peak-then-decline profile differs fundamentally from long-acting analogues (glargine, detemir, degludec), which use different physicochemical mechanisms to achieve flatter, more reproducible absorption profiles.
Substantial day-to-day variability (within-patient coefficient of variation ~68% for NPH vs ~26% for glargine) arises from inconsistent mixing of the suspension before injection, variation in injection site and depth, temperature effects on depot dissolution rate, and exercise-induced changes in SC blood flow.[3]
Log NPH injection times and model the 4–8 hour peak window and 12–18 hour action duration. Identify overnight hypoglycaemia risk windows. On-device. No data shared externally.
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