How Much Bacteriostatic Water for a Retatrutide 10 mg or 20 mg Vial?
A 10 mg vial of retatrutide reconstituted with 2 mL of bacteriostatic water gives a concentration of 5 mg/mL; with 1 mL it gives 10 mg/mL. The 20 mg vial gives 10 mg/mL with 2 mL of water and 5 mg/mL with 4 mL. The calculation depends only on the quantity of peptide in the vial and the volume of water added, so the two vial sizes share every figure once the concentration matches.
View product →There is no single correct volume. Adding more water does not change how much peptide is in the vial. It spreads the same quantity through a larger volume, which changes how many syringe units correspond to a given mass. The tables on this page use 2 mL for the 10 mg vial because it produces round numbers on a U-100 syringe; 1 mL, 3 mL or 5 mL work identically as long as the resulting concentration is written on the vial.
Concentration by water volume
Concentration is peptide mass divided by water volume. A 10 mg vial holds 10 mg whether it receives 1 mL or 5 mL; only the mg/mL figure moves. The right-hand column converts each concentration into the number of U-100 units that contain 1 mg.
| Vial | Bacteriostatic water added | Concentration | Units containing 1 mg |
|---|---|---|---|
| 10 mg retatrutide | 1 mL | 10 mg/mL | 10 units |
| 10 mg | 2 mL | 5 mg/mL | 20 units |
| 10 mg | 3 mL | 3.33 mg/mL | 30 units |
| 10 mg | 5 mL | 2 mg/mL | 50 units |
| 20 mg retatrutide | 1 mL | 20 mg/mL | 5 units |
| 20 mg | 2 mL | 10 mg/mL | 10 units |
| 20 mg | 3 mL | 6.67 mg/mL | 15 units |
| 20 mg | 4 mL | 5 mg/mL | 20 units |
Before choosing a larger volume, check that the peptide vial has headspace for it. A 10 mL vial of bacteriostatic water reconstitutes five 10 mg vials at 2 mL each, or two 20 mg vials at 4 mL each with 2 mL to spare.
Reading the concentration on a U-100 syringe
A U-100 insulin syringe is graduated in units, and 100 units equal 1 mL. Units are therefore millilitres multiplied by 100: 0.1 mL is 10 units and 0.25 mL is 25 units. A 0.5 mL syringe carries 50 units and a 0.3 mL syringe carries 30 units on the same scale, with finer markings that make small volumes easier to read.
To find the volume that contains a given mass, divide the mass by the concentration. At 5 mg/mL (a 10 mg vial with 2 mL of water), the volume that contains 1 mg is 1 divided by 5, which is 0.2 mL, or 20 units. The table below runs the same calculation for four arbitrary amounts. They illustrate the arithmetic and nothing else.
| Mass (illustrative) | Volume at 5 mg/mL | U-100 units |
|---|---|---|
| 0.25 mg | 0.05 mL | 5 units |
| 0.5 mg | 0.1 mL | 10 units |
| 1 mg | 0.2 mL | 20 units |
| 2 mg | 0.4 mL | 40 units |
At 10 mg/mL, which is what the 20 mg vial gives with 2 mL of water, every volume in the table halves: a 1 mg draw measures 10 units and a 0.5 mg draw measures 5 units. Volumes below about 0.1 mL are hard to read accurately on a 1 mL syringe. Either move to a 0.3 mL or 0.5 mL syringe, or reconstitute with more water so that the same mass occupies a larger, more legible volume.
Retatrutide is supplied by Vistara Labs as a lyophilised material for research use. The quantities on this page illustrate the arithmetic of dilution and measurement only.
The 10 mg and 20 mg vials
Retatrutide is a 39-amino-acid synthetic peptide. Vistara Labs supplies it in 10 mg and 20 mg vials; the mass on the label is the lyophilised peptide in the vial, and the tables above treat it as one quantity. A 20 mg vial reconstituted with 4 mL of water is indistinguishable, in concentration, from a 10 mg vial reconstituted with 2 mL: both are 5 mg/mL, so a given mass occupies the same number of units from either. Certificates of analysis are published on the Lab Results page.
Retatrutide is a single peptide that activates three receptors, GIP, GLP-1 and glucagon, which is why it is described as a triple agonist; tirzepatide acts on two of the three. Like tirzepatide, it carries a fatty-acid side chain that binds albumin in the blood, which is what gives it a long half-life in the published trials. The side chain changes nothing in the arithmetic above; as with any peptide, add the water slowly down the vial wall and swirl rather than shake. Its trial record is covered in retatrutide in 2026: Phase 3 results and the FDA timeline, and the comparison in retatrutide vs tirzepatide.
Reconstitution technique
Let the vial reach room temperature, swab both stoppers, and run the measured water slowly down the inside wall of the vial rather than onto the powder. Swirl or tilt gently until the cake dissolves, never shake, and give any remaining powder a few minutes to settle into solution. Label the vial with the date and the concentration, then keep it refrigerated at 2 to 8 °C and protected from light. The full method, with the reason behind each step, is in how to reconstitute peptides with bacteriostatic water.
Why the solution should look clear
Properly reconstituted retatrutide forms a clear, colourless solution. Cloudiness, strands or a gel that will not disperse usually mean the water went in too fast or the vial was shaken; haze that appears later points to warm or bright storage. The causes are covered in why peptides gel or go cloudy.
Bacteriostatic water, not sterile water
Bacteriostatic water is sterile water with 0.9% benzyl alcohol, a preservative that allows a refrigerated vial to be accessed more than once; plain sterile water is intended for single access. The difference is explained in bacteriostatic water explained.
Frequently asked questions
How much bacteriostatic water do I add to a 10 mg vial of retatrutide?
There is no single correct volume. With 2 mL of bacteriostatic water a 10 mg vial of retatrutide gives 5 mg/mL; with 1 mL it gives 10 mg/mL; the 20 mg vial gives 10 mg/mL with 2 mL. Adding more water does not change the quantity in the vial, it spreads it through a larger volume, so write the concentration you chose on the vial.
How many 10 mg vials does a 10 mL vial of bacteriostatic water cover?
A 10 mL vial of bacteriostatic water reconstitutes five 10 mg vials at 2 mL each, or two 20 mg vials at 4 mL each with 2 mL to spare. Check that the peptide vial has headspace for the volume before choosing a larger one.
How many units on a U-100 syringe contain 1 mg at 5 mg/mL?
At 5 mg/mL (a 10 mg vial with 2 mL of water), the volume that contains 1 mg is 1 divided by 5, which is 0.2 mL, or 20 units. Units are millilitres multiplied by 100, so 0.1 mL is 10 units. The amounts on this page illustrate the arithmetic only.
What should reconstituted retatrutide look like?
Properly reconstituted retatrutide forms a clear, colourless solution. Cloudiness, strands or a gel that will not disperse usually mean the water went in too fast or the vial was shaken; haze that appears later points to warm or bright storage. More on the causes in why peptides gel or go cloudy.
Can I use sterile water instead of bacteriostatic water?
Only for a vial that will be accessed once. Bacteriostatic water is sterile water with 0.9% benzyl alcohol, a preservative that allows a refrigerated vial to be accessed more than once; plain sterile water is intended for single access. Details in bacteriostatic water explained.
How should a reconstituted retatrutide vial be stored?
Refrigerated at 2 to 8 °C, protected from light and labelled with the date and the concentration. Unopened lyophilised vials are kept cold and dark until they are needed. The full storage guide is how to store research peptides.
Work it out for a different vial or concentration
Our peptide calculator opens pre-set for a 10 mg vial with 2 mL of water and marks the 0.5 mg draw of 10 units on a syringe diagram. Change the vial size, water volume, mass or syringe and it recalculates. The 20 mg preset shows 1 mg at 20 units on 4 mL of water.
Products: Retatrutide 10 mg and 20 mg and bacteriostatic water 10 mL.
References
Retatrutide has been evaluated in randomised, placebo-controlled human trials through phase 3.
- Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 TrialRandomised, double-blind, placebo-controlled phase 2 obesity trial, 48 weeks.
- 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 USARandomised, double-blind, placebo- and active-controlled phase 2 trial; 281 participants with type 2 diabetes.
- Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trialRandomised phase 2a substudy; 98 participants with metabolic dysfunction-associated steatotic liver disease.
- Efficacy and safety of retatrutide, a GIP, GLP-1, and glucagon receptor agonist, in people with type 2 diabetes and inadequate glycaemic control with diet and exercise (TRANSCEND-T2D-1): a double-blind, randomised, phase 3 trialDouble-blind randomised phase 3 trial in type 2 diabetes (TRANSCEND-T2D-1).
Links open the study record on PubMed so the source can be checked directly. Listing a study is not a claim that its findings apply to any other use, product or route of administration. Research use only.
