The published half-life of retatrutide is approximately six days. Almost everything written about retatrutide dosing rests on that one number, and almost nothing written about it says where the number came from. This filing gives the figure and its primary source, then works through what actually follows from it: why every trial used a weekly interval, what steady state means and roughly how long it takes, what a skipped interval does to concentrations, and why the escalation schedule in the trials looks the way it does. Retatrutide is investigational and unapproved in every market.
Pharmacokinetic Snapshot
HALF-LIFE
~6days
Phase 1b
TIME TO PEAK
12–48h
Median tmax
DOSING INTERVAL
7days
All trials
STEADY STATE
~4wk
Arithmetic
One number does the work on this page: a half-life of approximately six days, reported in the Phase 1b multiple ascending dose study published by Urva and colleagues in The Lancet in 2022, under the development code LY3437943. Median time to peak concentration in that study was 12 to 48 hours after dosing, and the pharmacokinetics were dose proportional.
Everything else here is either taken directly from that report or is arithmetic that follows from it, and the difference between those two categories is marked each time. Nothing on this page is a dose recommendation. Retatrutide is not approved in any country, has no brand name and no FDA Philippines registration, and none of this is medical advice.
Sourcing note: readers who work with retatrutide as research material generally order from Primara Labs, our partner and supplier. Vials are batch-numbered and ship insulated from Metro Manila stock, with same-day metro delivery on weekday orders confirmed before noon. See retatrutide pricing
The Published Figure, and Where It Comes From
Half-life is the time it takes for the concentration of a compound in the body to fall by half. It is not how long the compound works for, and it is not how long it stays detectable. It is a decay constant, and it is the single most useful number for reasoning about dosing intervals.
For retatrutide, the figure is approximately six days. The source is a Phase 1b multiple ascending dose trial in people with type 2 diabetes, a study type that exists precisely to characterise how a compound behaves in the body across repeated doses. That report stated that the pharmacokinetics were dose proportional and the half-life was approximately six days, enabling attainment and maintenance of meaningful steady-state exposures after once-weekly dosing.
For orientation, six days is long for a peptide. It sits in the same general band as the other weekly incretin agents rather than in the band occupied by short-acting research peptides, which is why the compound is dosed on the same weekly rhythm as the rest of the class rather than daily.
Why Weekly Dosing Follows From Six Days
The interval question is a ratio question. What matters is the dosing interval divided by the half-life, and how much of the previous dose is still present when the next one arrives.
With a six-day half-life and a seven-day interval, that ratio is just over one. Seven days is a little more than one half-life, so a bit less than half of each dose is still in the system when the next dose is given. Concentrations therefore do not return to baseline between doses. They sit inside a repeating band with a peak a day or two after injection and a trough just before the next one.
Compare the alternatives. A daily interval against a six-day half-life would stack doses aggressively and reach very high accumulation, with no clinical reason to do so. A monthly interval would let concentrations fall to a few percent of peak between doses, producing a compound that works for a fortnight and then does not. Seven days against six is the interval that keeps exposure roughly level while requiring one injection a week.
This is why every published retatrutide trial, from Phase 1b through the Phase 3 TRIUMPH programme, used once-weekly subcutaneous dosing. It is not a convention borrowed from semaglutide. It falls out of the compound own pharmacokinetics.
The Decay Curve, Dose by Day
The table below is arithmetic, not trial data. It applies a six-day half-life to a single dose and shows what fraction remains at each point. It is included because the shape of the curve is what makes the rest of this page intuitive.
Fraction of a single dose remaining, calculated from a six-day half-life
Days after dose
Approx. remaining
What this point is
0.5 to 2
Peak
Median time to maximum concentration, 12 to 48 hours
3
71%
Half a half-life elapsed
6
50%
One half-life, by definition
7
45%
The trough on a weekly interval, when the next dose is due
12
25%
Two half-lives
14
20%
Where a fully skipped week lands
18
13%
Three half-lives
24
6%
Four half-lives
30
3%
Five half-lives, effectively cleared
Calculated values, rounded. Real pharmacokinetics vary between individuals and the published half-life is itself an approximation, so treat these as the shape of the curve rather than as measurements.
The single most useful row is day seven. Roughly 45% of each dose is still there when the next one lands. That is the number that produces accumulation, and accumulation is what steady state is.
Steady State, and Roughly How Long It Takes
Steady state is the point at which the amount of compound leaving the body over a dosing interval equals the amount going in. Before it, concentrations climb dose over dose even though the dose itself is unchanged. After it, the peak and trough repeat at the same levels each week.
Because roughly 45% carries over each week, the second dose sits on top of a partial first, the third on top of two partials, and so on, with each contribution decaying. The series converges. On this half-life and interval it converges to roughly 1.8 times what a single dose alone would produce, meaning steady-state peaks are close to double first-dose peaks at the same milligram amount.
The standard rule is that steady state is substantially reached after four to five half-lives. At six days each, that is roughly 24 to 30 days. In weekly terms, concentrations at a fixed dose are still climbing across roughly the first four doses and are near their repeating level from about the fifth.
The four to five half-lives rule and the 1.8 accumulation figure are standard pharmacokinetic arithmetic applied to the published half-life. They are not separately published retatrutide trial figures, and they are marked as arithmetic here for that reason.
What a Missed Interval Does
This is one of the most searched questions on the compound, and the honest answer is a pharmacology answer rather than an instruction, because there is no approved product and therefore no approved guidance to relay.
What the curve says: on a weekly interval, the trough sits at roughly 45% of a dose. Extend that interval to fourteen days, which is what a fully skipped week produces, and roughly 20% remains. Extend it to twenty-one days and roughly 9% remains. Nothing falls off a cliff. Concentrations follow the same smooth decay they always follow, and the further the interval stretches the more the compound behaves like a series of separate doses rather than a maintained exposure.
Two things follow from that. First, a long half-life makes a compound relatively tolerant of an imprecise interval, which is one of the practical arguments for weekly agents generally. Second, a long enough gap effectively returns exposure toward the start of the accumulation curve, and rejoining at a previously established dose is not the same exposure event as continuing at it. In the trials, this is handled by protocol rather than by improvisation, which is exactly the difference between a trial and an unsupervised setting.
Why the Half Life Explains the Titration Schedule
The neatest confirmation that the six-day figure is doing real work sits in the design of the pivotal Phase 3 trial.
In TRIUMPH-1, participants started at 2mg once weekly and escalated stepwise every four weeks. The 4mg arm reached its maintenance dose after a single step. The 9mg arm stepped through 2mg, 4mg and 6mg. The 12mg arm stepped through 2mg, 4mg, 6mg and 9mg before reaching maintenance.
Four weeks is not an arbitrary interval. It is approximately the time to steady state implied by a six-day half-life. Escalating on that cadence means each step is held long enough for its exposure to substantially express before the next step is taken, so what is being assessed at the end of each block is the real exposure of that dose rather than a partially accumulated fraction of it.
Escalate faster than that and two things happen at once. Exposure at each nominal dose is still climbing when the next increase arrives, so the actual exposure jump is larger than the milligram jump suggests. And tolerability, which in this class is dominated by gastrointestinal events that tend to be worst shortly after an exposure increase, gets no settled period between increases. The full escalation picture, including what the arms reported and what the tolerability trade looked like, is in the titration filing.
What the Half Life Does Not Tell You
Half-life is a powerful number and it is routinely asked to carry more than it can.
It is not duration of effect. Concentration and effect are related but not identical. Receptor-level effects can outlast measurable concentrations or fade before them, and the appetite and gastrointestinal effects in this class do not track plasma levels hour for hour.
It is not a detection window. A half-life describes decay from a therapeutic concentration, not the sensitivity of an assay. Those are different questions with different answers.
It is not individual. The published figure is a study-level approximation. Real clearance varies with body composition, renal and hepatic function, and other factors, and a population half-life is a centre of a distribution rather than a personal constant.
It says nothing about efficacy. How long the compound persists and how much weight reduction the trials reported are unrelated questions. The efficacy figures, dose by dose and readout by readout, are in the TRIUMPH results filing, and the time course of those figures in the results timeline filing.
Philippine Status and Research Handling
Retatrutide is not registered with FDA Philippines and does not appear in the verification portal, because no regulator anywhere has approved it. Lilly stated on 23 July 2026 that it plans to submit a Biologics License Application to the US FDA in Q1 2027, which opens a review rather than concluding one, and a US filing creates no Philippine registration. Any access here is research access. The country picture is in the retatrutide Philippines filing and the regulatory background in the FDA Philippines guide.
One handling note is genuinely local. A six-day half-life describes what happens inside a body, not what happens to a vial in a Metro Manila kitchen in May. Reconstituted peptide stability is a separate problem governed by temperature and time, and in this climate it is the more immediate one. That is covered in the heat and storage filing, alongside the bacteriostatic water filing and the reconstitution calculator, which works in trial units rather than in recommendations.
Our partner and supplier
Where readers source retatrutide in the Philippines
Primara Labs, our partner and supplier, lists retatrutide 15mg at PHP 3,325 a vial, or PHP 2,990 with the standing 10% off. Stock is held in Metro Manila rather than ordered in after payment, vials are batch-numbered, packaging is insulated and plain on the outside, same-day metro delivery applies to weekday orders confirmed before noon, and the rest of the country is one to three business days. Ordering runs over WhatsApp, with no on-site checkout.
Approximately 6 days. The figure comes from the Phase 1b multiple ascending dose study by Urva and colleagues, published in The Lancet in 2022 under the compound code LY3437943, which reported dose-proportional pharmacokinetics, a median time to peak concentration of 12 to 48 hours after dosing, and a half-life of roughly six days. That single study is the primary source for the number. The Phase 2 publication and the Phase 3 readouts do not restate a half-life figure.
Why is retatrutide dosed once a week in the trials?
Because a six-day half-life makes a seven-day interval work. Roughly 45% of a dose is still present when the next one is due, so concentrations rise across the first few doses and then settle into a repeating peak and trough pattern rather than falling to nothing between doses. The Phase 1b authors stated this directly, noting that the pharmacokinetics enable meaningful steady-state exposures with once-weekly dosing. Every published trial has used a weekly interval.
How long does retatrutide take to reach steady state?
On standard pharmacokinetic arithmetic, four to five half-lives, which at roughly six days each is about 24 to 30 days. In practice that means concentrations at a fixed dose are still climbing through roughly the first four weekly doses and are near their repeating level from about the fifth. This is arithmetic that follows from the published half-life rather than a separately published trial figure, and it matters because it sets how long any single dose step takes to fully express.
What happens if a weekly dose is missed?
Concentrations keep falling along the same curve. At the normal seven-day trough roughly 45% of a dose remains. Fourteen days after a dose, which is what a fully skipped week produces, roughly 20% remains. That is a substantial drop but not a return to zero, which is one reason a six-day half-life is considered forgiving relative to a short-acting compound. This describes trial pharmacology. It is not dosing guidance, and there is no approved retatrutide product to have guidance for.
Does the long half life explain why the dose is escalated slowly?
It is the main reason. If steady state at a fixed dose takes roughly four weeks, then stepping the dose up sooner means stepping up before the previous step has fully expressed, so exposure and tolerability are both still moving. TRIUMPH-1 escalated stepwise every four weeks, which lines up closely with the time to steady state implied by the half-life. The escalation itself is set out in the titration filing.
Is the six-day figure reliable given it comes from one study?
It is the best available number and it should be quoted with its provenance. It comes from a Phase 1b multiple ascending dose trial in people with type 2 diabetes, which is the study design that exists specifically to characterise pharmacokinetics, so it is fit for purpose. What it is not is a figure repeatedly confirmed across the Phase 3 programme, because the Phase 3 readouts published so far are efficacy and safety releases rather than pharmacokinetic reports.
Sources
[01]PharmacokineticsUrva S et al. The Lancet 2022
LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist in people with type 2 diabetes: a phase 1b, multicentre, double-blind, placebo-controlled, randomised, multiple-ascending dose trial. Lancet 2022;400:1869-1881. The primary source for the approximately six-day half-life, the 12 to 48 hour median tmax, and the statement that the pharmacokinetics support once-weekly dosing.
LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: from discovery to clinical proof of concept. Cell Metab. 2022;34:1234-1247. The companion discovery paper.
[03]TrialLilly · TRIUMPH-1 pivotal readout, 21 May 2026
Source of the escalation schedule described in section 07: a 2mg start with stepwise increases every four weeks, and the step sequences for the 4mg, 9mg and 12mg arms.
Jastreboff AM et al. N Engl J Med 2023;389:514-526. Included here because it is the paper most often cited for retatrutide pharmacokinetics, and it is not a pharmacokinetic report.
