Half-life, dosing intervals, and what t½ actually means

Every peptide spec sheet quotes a half-life. It looks like a property of the molecule — and at the lowest level it is — but the number on the page is the result of a model fit to a specific experiment in a specific species at a specific dose. Treating it as a universal constant is the most common pharmacokinetics mistake in research-peptide work.

What t½ measures

The plasma half-life is the time required for the plasma concentration of a substance to drop by 50%, in the elimination phase. For most peptides, the elimination phase is well-approximated by first-order kinetics, which means the half-life is a constant — concentration drops by half in t½, then by half again in another t½, and so on.

The arithmetic that follows from this is unambiguous. After one half-life, 50% of the dose remains. After three half-lives, ~12.5%. After five, ~3%. After seven, ~1%. By convention, "fully eliminated" is usually taken at five half-lives.

This is the model. The model holds well for many peptides but can break for sequences with non-linear elimination, depot binding, or active metabolites. Read the half-life; don't worship it.

Two-compartment behavior is the rule, not the exception

Most peptides do not show single-exponential decay. They show two phases:

• A distribution phase (α-phase) — fast, within minutes. The peptide redistributes from the bloodstream into tissue compartments.
• An elimination phase (β-phase) — slower, hours to days. The peptide is cleared by proteolysis, renal filtration, or hepatic uptake.

The half-life on a spec sheet is almost always the β-phase value — the slower, larger half-life. Some sources report both ("α t½ = 12 min, β t½ = 4 h"); most report only the elimination half-life because it is what governs dosing intervals.

If you see a single number, assume it is the elimination half-life unless the source explicitly says otherwise.

Albumin binding extends half-life dramatically

Several research peptides — Semaglutide, Tirzepatide, CJC-1295 with DAC — carry a fatty-acid side chain or a maleimide linker that binds plasma albumin reversibly. The bound peptide is unavailable for proteolysis or renal clearance, and the unbound fraction is dosed slowly back into circulation as the equilibrium shifts.

The result: half-life jumps from minutes (the bare peptide) to days (the albumin-bound construct). Native GLP-1 has a half-life of ~2 minutes. Semaglutide, the same peptide with a C18 linker, has a half-life of ~7 days. Same molecular core, different pharmacokinetic regime.

When comparing peptides, check whether the half-life difference comes from sequence chemistry or from a deliberate albumin-binding modification. The two have very different research implications.

Steady state and dosing intervals

For peptides administered repeatedly, plasma concentration accumulates until the input rate equals the elimination rate — steady state. The time to reach steady state is approximately five half-lives, regardless of dose or interval.

The dosing interval relative to the half-life shapes the concentration profile:

• Interval = t½: peak-to-trough ratio of about 2. Reasonably flat plasma curve at steady state.
• Interval = 2×t½: peak-to-trough ratio of about 4. More variable, but still continuous coverage.
• Interval = 4×t½: peak-to-trough ratio of about 16. Each dose is mostly cleared before the next one. Effectively pulsed administration.

Research literature commonly reports dosing schedules in units that imply a target dosing interval. "Daily dosing" of a peptide with a 4-hour half-life produces a pulsed profile. "Weekly dosing" of a peptide with a 7-day half-life produces a flat profile. Knowing which regime you are in matters when interpreting study designs.

Half-life ≠ duration of action

This is the trap. Half-life is a pharmacokinetic property — it describes plasma concentration over time. Duration of action is a pharmacodynamic property — it describes how long the biological response persists.

The two can decouple in either direction:

• Action outlasts half-life: receptor occupancy or downstream signaling cascades persist after plasma levels drop. PT-141's behavioral effects in research models persist beyond its plasma t½.
• Action shorter than half-life: tachyphylaxis or receptor desensitisation blunts response while the peptide is still circulating.

A spec sheet that lists "half-life: 4 h" tells you about plasma kinetics. It does not tell you how long the peptide produces a measurable effect in any specific assay.

Why the same peptide can have different half-lives in different sources

Half-life is measured by sampling plasma at intervals after a known dose and fitting a curve. The reported value depends on:

• Species. Rodent and primate elimination kinetics differ substantially for many peptides. A "half-life of 30 min" measured in mice may correspond to "half-life of 90 min" in non-human primates.
• Dose. Non-linear PK shows up at high doses when clearance pathways saturate.
• Route. Subcutaneous depot effects give a longer apparent half-life than intravenous, because absorption from the depot becomes the rate-limiting step.
• Assay sensitivity. A more sensitive assay sees the late-phase elimination better and reports longer half-lives than a less sensitive one.

A spec-sheet half-life is a single number summarising all of these choices. The catalog page on this site notes the experimental context where it is known. When in doubt, treat the quoted value as an order-of-magnitude indication and consult primary literature for the specific research scenario.

A reading checklist

When a peptide page says "half-life: X hours":

1. Is X the α-phase, β-phase, or terminal half-life?
2. What species and route was the original measurement in?
3. Is there an albumin-binding or PEGylation modification driving the value?
4. Does the dosing schedule in the literature line up with the half-life as you'd expect?

If three of four answers are "I don't know", the number is not yet load-bearing for your study design. Read the source paper.