The short version
- Concentration equals the milligrams of peptide in the vial divided by the millilitres of diluent added: a 10 mg research vial with 2 mL gives a 5 mg/mL solution.
- The volume of solution that contains a given mass is that mass divided by the concentration: 1 mg of a 5 mg/mL solution occupies 0.2 mL (200 microlitres).
- The trial doses (1, 4, 8, 9 and 12 mg once weekly) were delivered as a sponsor-prepared fixed-concentration solution; the published papers do not describe reconstitution because participants never performed it.
- A lyophilised peptide is a research reagent; reconstitution is a laboratory measurement step that a certificate of analysis makes checkable, not an instruction for use in people.
What does retatrutide reconstitution mean?
Research-grade retatrutide is supplied as a freeze-dried (lyophilised) powder in a sealed glass vial. In that state it is stable and light, but it cannot be measured out by volume. Reconstitution is the laboratory step of adding a known volume of sterile diluent so that the powder dissolves into a solution of known concentration. From that point on, any volume drawn from the vial contains a predictable mass of peptide, and the arithmetic below is all that connects the two.
It helps to be clear about what the clinical papers do and do not say. The NEJM, Lancet and Nature Medicine trials describe the doses (1 to 12 mg a week) and the route (subcutaneous), and the ClinicalTrials.gov entry for the obesity trial notes that dose escalation was achieved "by increasing the volume of administered study drug", which tells you the trial product was a fixed-concentration solution prepared by the sponsor. None of them describes reconstitution, because participants never did it. What follows is measurement arithmetic for a research sample, not a protocol for people.
The concentration math, in three lines
The whole subject fits in three formulas:
- Concentration (mg/mL) = peptide in vial (mg) ÷ diluent added (mL). A 10 mg vial plus 2 mL gives 5 mg/mL.
- Volume for a given mass (mL) = mass (mg) ÷ concentration (mg/mL). 1 mg of a 5 mg/mL solution is 0.2 mL.
- Microlitres = millilitres × 1,000. 0.2 mL is 200 µL, the unit a laboratory pipette is graduated in.
Everything else is a special case of those lines. Two worked examples using published trial doses as the reference masses: 4 mg, the lowest phase 3 dose, is 0.8 mL of a 5 mg/mL solution or 0.4 mL of a 10 mg/mL solution; and 12 mg, the top trial dose, is 1.2 mL at 10 mg/mL, which exceeds the content of a single 10 mg vial at any concentration. These are arithmetic illustrations of what the trial figures represent as quantities of solution, not amounts to prepare for anyone.
Divide the milligrams by the millilitres and you have the strength of the solution. Everything else is that one division.
Solution concentration calculator
Enter the vial content from the label, the diluent volume, and the mass of an aliquot. The calculator runs entirely in the browser; nothing is sent or stored. It is a laboratory aid for preparing solutions and aliquots of a research sample.
Solution concentration calculator (laboratory)
An aliquot of 1 mg corresponds to 0.200 mL (200 µL) of this solution.
The vial yields 10 aliquots of 1 mg.
Concentration arithmetic for a reconstituted research sample, computed in your browser; nothing is sent anywhere. This is a laboratory measurement aid for preparing solutions and aliquots. It is not a dosing tool and does not describe use in people.
Concentrations for the common vial sizes
Research suppliers list retatrutide in 5 mg, 10 mg and 20 mg vials. The table gives the concentration for the three most common diluent volumes and the volume of solution that then contains 1 mg, from which any other mass scales by multiplication.
| 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 laboratory rule falls out of the table: choose a diluent volume that keeps the aliquot volumes the experiment needs inside the accurate range of the pipette in use, typically 20 to 1,000 µL for a standard micropipette. Below that, small volumes carry large relative error; above it, the solution is needlessly dilute. How the vial sizes relate to the published dose ladder is covered in the vial-size guide.
Diluent and technique
Bacteriostatic water (sterile water with 0.9% benzyl alcohol as a preservative) is the standard diluent for multi-draw peptide vials in laboratory use, because the preservative limits bacterial growth once the seal has been punctured. Plain sterile water dissolves the peptide just as well but offers no protection after the first draw, so it suits single-use preparation. Whichever is used, the volume only sets the concentration: the mass in the vial does not change.
On technique, the points that matter for a research reagent are the ones that protect the molecule. Run the diluent slowly down the inside wall of the vial rather than onto the powder, swirl gently, and never shake: peptides are long chains that fold, and vigorous agitation can denature a fraction of the material, which lowers the effective concentration without any visible sign. Lyophilised retatrutide typically dissolves into a clear solution within a minute or two.
Storage of a reconstituted vial
Lyophilised peptide is the stable form; solution is the fragile one. Supplier guidance for research peptides is consistent on the essentials: keep the unopened powder refrigerated or frozen and away from light, and once reconstituted keep the vial refrigerated at 2 to 8 °C, upright, capped, and use it within the window the supplier states for the product.
The 6-day half-life reported in the phase 1b paper is a half-life in the body, not on the shelf; the two have nothing to do with each other. A certificate of analysis dated to the batch says what went into the vial; it cannot say what happens after the vial is opened, which is why the storage window matters.
Three classic reconstitution errors
Confusing vial content with a trial dose. A "10 mg vial" is 10 mg of peptide in total. The trials describe weekly doses of 1 to 12 mg. The vial label sits on one axis, the trial dose on another, and the concentration is the bridge between two different kinds of number.
Reading volume as mass. A volume of solution has no mass of peptide until it is multiplied by the concentration. At 5 mg/mL, 0.8 mL holds 4 mg; at 10 mg/mL the same 0.8 mL holds 8 mg. Writing the concentration on the vial at the moment of reconstitution removes the ambiguity.
Trusting the label without a certificate. Everything above assumes the vial contains what the label says. Only a batch-specific certificate of analysis (HPLC purity and mass) turns that assumption into a measurement. The dose ladders the trials used, which give the reference masses quoted above, are in the dosing schedule guide, and the whole dose picture in the retatrutide dosage guide. For research vials listed with a certificate of analysis per batch, see See it at OXpeptides (research use only).
Questions readers ask
How much diluent for a 10 mg retatrutide research vial?
Any volume works; it only changes the concentration. 1 mL gives 10 mg/mL, 2 mL gives 5 mg/mL, 3 mL gives 3.33 mg/mL. In a laboratory the volume is chosen so that the aliquots the experiment needs fall in a range the pipette measures accurately, and the concentration is written on the vial.
What concentration did the clinical trials use?
The published papers do not state the concentration of the trial product; participants received sponsor-prepared study drug at fixed doses of 1 to 12 mg per week. The registry notes that phase 2 escalation was achieved by increasing the injected volume, which implies a fixed-concentration product.
How many milligrams are in a 10 mg vial after reconstitution?
Still 10 mg. Reconstitution changes the volume and the concentration, not the mass of peptide. The mass in any aliquot is its volume multiplied by the concentration.
Is this page a dosing guide?
No. It describes the arithmetic of preparing a solution from a lyophilised research sample. The trial doses it mentions are cited as published figures, not as amounts for anyone to take.
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
- 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
- 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
- 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