The short version
- A vial label (5 mg, 10 mg, 20 mg) states the total mass of lyophilised peptide in the vial; a trial dose (1 to 12 mg) states the mass injected once a week. They are different quantities on different axes.
- A 10 mg vial contains ten 1 mg measures, two and a half 4 mg measures, one 8 mg measure with 2 mg left over, and less than one 12 mg measure.
- Concentration after reconstitution is vial content divided by diluent volume: 10 mg in 2 mL is 5 mg/mL, so 4 mg occupies 0.8 mL and 1 mg occupies 0.2 mL.
- Only a batch-specific certificate of analysis (HPLC purity, mass by weight) links the label to the contents; the trial papers describe doses of pharmaceutical-grade product, not research vials.
Two numbers, two axes
The most common confusion around retatrutide numbers is between the mass in a vial and the mass in a dose. Research suppliers label vials by total peptide content: 5 mg, 10 mg, 20 mg. Clinical papers describe weekly doses: 1, 4, 8, 12 mg in phase 2, 4, 9 and 12 mg in phase 3.
Both are milligrams of the same molecule, but one is a container size and the other is a rate of exposure. A 10 mg vial is not "a 10 mg dose" any more than a litre bottle is a serving.
The trial product itself was never sold in vials of this kind. The registry entry for the obesity trial notes that escalation was performed "by increasing the volume of administered study drug", which tells you participants received a prepared solution at a fixed concentration, with the volume adjusted to deliver the assigned mass. The research vial is a different object: dry powder, reconstituted by the researcher, with a concentration the researcher chooses.
The number on a vial says how much powder is inside. The number in a trial says how much was injected each week. They are not the same thing.
The 5 mg, 10 mg and 20 mg vials
The three sizes exist for practical reasons. A 5 mg vial suits low-mass work or short experiments, where opening a larger vial would waste material once the solution's storage window runs out. A 10 mg vial is the most common listing and the easiest to reconstitute to a round concentration (5 or 10 mg/mL). A 20 mg vial lowers the cost per milligram and covers the higher trial doses in a single container. Larger sizes exist for bulk work but are outside this page.
| Vial | 1 mg measures | 4 mg measures | 8 mg measures | 9 mg measures | 12 mg measures |
|---|---|---|---|---|---|
| 5 mg | 5 | 1.25 | 0.625 | 0.56 | 0.42 |
| 10 mg | 10 | 2.5 | 1.25 | 1.11 | 0.83 |
| 20 mg | 20 | 5 | 2.5 | 2.22 | 1.67 |
The columns are the trial maintenance doses: 1, 4, 8 and 12 mg from the NEJM phase 2 trial and 9 mg from the phase 3 program. The cells are simple division. They are here to show how far a vial goes against a published dose, not to describe a use.
Vial size against the trial dose ladder
Read against the escalation ladder used in phase 2 (2, 4, 8, 12 mg), a 10 mg vial covers the first two rungs with room to spare, exactly one 8 mg rung, and falls short of 12 mg. Against the phase 3 ladder (4, 9, 12 mg), a 20 mg vial covers 4 mg five times, 9 mg twice, and 12 mg once.
The 12 mg dose, which produced the largest weight change in the published data (-24.2% at 48 weeks), is the only trial dose that a 10 mg vial cannot contain at any concentration. None of this is dosing guidance: it is arithmetic that shows what the published figures represent as quantities of peptide. The ladder itself is in the dosing schedule guide.
Concentration by diluent volume
Vial content only becomes a workable quantity once it is a concentration. The table gives the result for 1, 2 and 3 mL of diluent, with the volume of solution that then holds 1 mg. Any other mass scales from that figure by multiplication: at 5 mg/mL, 4 mg corresponds to 0.8 mL of solution and 8 mg to 1.6 mL; at 10 mg/mL the same masses correspond to 0.4 and 0.8 mL.
| Vial content | 1 mL diluent | 2 mL diluent | 3 mL diluent |
|---|---|---|---|
| 5 mg | 5 mg/mL 1 mg in 0.2 mL (200 µL) | 2.50 mg/mL 1 mg in 0.4 mL (400 µL) | 1.67 mg/mL 1 mg in 0.6 mL (600 µL) |
| 10 mg | 10 mg/mL 1 mg in 0.1 mL (100 µL) | 5 mg/mL 1 mg in 0.2 mL (200 µL) | 3.33 mg/mL 1 mg in 0.3 mL (300 µL) |
| 20 mg | 20 mg/mL 1 mg in 0.05 mL (50 µL) | 10 mg/mL 1 mg in 0.1 mL (100 µL) | 6.67 mg/mL 1 mg in 0.15 mL (150 µL) |
A calculator that does this for any vial size, diluent volume and target mass is in the reconstitution guide.
What a certificate of analysis tells you
Everything above assumes the label is true. A research vial has no regulator behind it, so the only evidence about its contents is a certificate of analysis issued for the batch by an analytical laboratory.
A useful COA reports at least two things: purity by HPLC, expressed as the percentage of the peak area that is retatrutide rather than related impurities, and identity by mass spectrometry, confirming the molecular weight. Some add a net peptide content or a quantity assay, which is the number that actually validates "10 mg". A COA that names a different batch, carries no date, or omits the method is decoration.
The trial doses were delivered as a pharmaceutical product with full release testing; a research vial with a batch COA is the nearest thing outside a trial to knowing what mass is in the container. Suppliers who publish a COA per batch make that check possible; one listing that does is at See it at OXpeptides (research use only).
A certificate of analysis is a lab test of the batch. Without it, the label is just a claim.
Storage and stability
Lyophilised peptide is the stable form: kept sealed, cold and dark it holds its content for a long time, and supplier data sheets typically state the window for their product. Reconstituted solution is the fragile form: refrigerated at 2 to 8 °C, upright and capped, it should be used within the period the supplier specifies, and never frozen and thawed repeatedly.
The 6-day half-life reported in the phase 1b study is elimination from the body and says nothing about shelf life; they are unrelated quantities. For a vial that will be drawn from more than once in a laboratory, the size chosen matches the volume of work planned inside that storage window, which is the practical argument for the 5 mg size in small studies. The complete dose picture that all of this arithmetic serves is on the retatrutide dosage guide.
Questions readers ask
Is a 10 mg retatrutide vial one dose?
No. 10 mg is the total peptide in the vial. The trial doses were 1 to 12 mg per week, so a 10 mg vial holds anything from less than one 12 mg measure to ten 1 mg measures, depending on which trial dose it is compared with.
How many 4 mg measures are in a 20 mg vial?
Five. 20 mg divided by 4 mg. At 10 mg/mL (20 mg in 2 mL) each 4 mg measure is 0.4 mL.
What does 'retatrutide 5mg' mean on a listing?
It means 5 mg of lyophilised retatrutide in the vial. It says nothing about concentration (that depends on how much diluent is added) and nothing about a dose (the trials used 1 to 12 mg weekly).
Which vial size matches the phase 3 doses?
The phase 3 doses are 4, 9 and 12 mg weekly. A 10 mg vial covers 4 mg or 9 mg once; a 20 mg vial covers 4 mg five times, 9 mg twice, or 12 mg once with 8 mg left. No vial size is a trial dose; the arithmetic connects them.
Sources
Reviewed against the primary sources on 21 Sep 2026. Every DOI resolves on CrossRef and every NCT number on ClinicalTrials.gov.
- Jastreboff AM, Kaplan LM, Frias JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial. New England Journal of Medicine. 2023;389(6):514-526. doi.org/10.1056/NEJMoa2301972
- Rosenstock J, Frias J, Jastreboff AM, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, parallel-group, phase 2 trial conducted in the USA. The Lancet. 2023;402(10401):529-544. doi.org/10.1016/S0140-6736(23)01053-X
- Giblin A, et al. Retatrutide for the treatment of obesity, obstructive sleep apnea and knee osteoarthritis: Rationale and design of the TRIUMPH registrational clinical trials. Diabetes, Obesity and Metabolism. 2025;28:83-93. doi.org/10.1111/dom.70209
- ClinicalTrials.gov. A Phase 2 Study of Once-Weekly LY3437943 Compared With Placebo in Participants Who Have Obesity or Are Overweight. Completed; 338 participants. clinicaltrials.gov/study/NCT04881760
- Urva S, Coskun T, Loh MT, et al. 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. The Lancet. 2022;400(10366):1869-1881. doi.org/10.1016/S0140-6736(22)02033-5