How to reconstitute retatrutide (5, 10 and 20 mg vials): bac water amounts and concentration math

To reconstitute a retatrutide vial, add a measured volume of bacteriostatic water (usually 1, 2 or 3 mL) slowly down the inside wall, swirl until clear, and label the vial with its concentration. A 10 mg vial with 2 mL gives 5 mg/mL, which is 50 mcg per unit on a U-100 insulin syringe. A 20 mg vial with 2 mL gives 10 mg/mL, or 100 mcg per unit. A 5 mg vial with 1 mL also gives 5 mg/mL. The volume you choose does not change how much peptide is in the vial. It only changes how concentrated the solution is, and so how much liquid carries a given mass.

What you need before you start

• The lyophilized vial. Lumera Labs supplies retatrutide as 5 mg, 10 mg and 20 mg vials. The 10 mg lot has a published HPLC-UV certificate at Lab Results; for the 5 mg and 20 mg vials a COA is available on request.
• Bacteriostatic water. Our bacteriostatic water is USP water with 0.9% benzyl alcohol in a 3 mL vial. Why bacteriostatic rather than sterile water is covered in bac water vs sterile water.
• A syringe to measure the solvent. A 1 mL or 3 mL syringe with a larger-bore needle is easier for the solvent step; a U-100 insulin syringe is what most labs use for drawing measured aliquots afterward. See insulin syringes for peptide research.
• Alcohol swabs and a label. The label matters more than people think. An unlabelled reconstituted vial is a vial of unknown concentration.

How to reconstitute retatrutide, step by step

1. Let the vial reach room temperature. Opening a cold vial pulls condensation onto the cake. Ten to fifteen minutes on the bench is enough.
2. Swab both stoppers (peptide vial and bac water vial) and let the alcohol dry.
3. Draw the exact solvent volume. Decide the target concentration first (the chart below), then draw exactly that volume of bacteriostatic water.
4. Add the water down the inside wall of the peptide vial, not directly onto the cake. A direct jet can foam the solution.
5. Swirl or roll gently until the solution is clear. Most lots dissolve within a minute or two. Do not shake or vortex.
6. Inspect it. A properly reconstituted solution is clear and colourless. If it is cloudy, has visible particles, or will not dissolve, read cloudy peptide troubleshooting before using it.
7. Label and refrigerate with peptide name, vial mass, volume added, resulting mg/mL, and the date.

The general technique (needle choice, venting, handling a stubborn cake) is the same for every lyophilized peptide and is covered in our peptide reconstitution reference.

How much bac water for retatrutide? Reconstitution chart

Every figure below was calculated, not estimated. Units refer to a U-100 insulin syringe, where 100 units equal 1 mL.

Two patterns are worth noticing. First, the 5 mg vial in 1 mL and the 10 mg vial in 2 mL produce the identical 5 mg/mL solution, so any working notes written for one apply to the other. Second, the 20 mg vial in 1 mL puts 200 mcg in every unit. At that concentration a one-unit reading error on the syringe barrel is a 200 mcg error, which is why most labs prefer 2 mL or 3 mL for the larger vial.

Retatrutide units to mg: the conversion

The syringe measures volume. The vial label tells you mass. Concentration connects them:

• mL = units x 0.01 (on a U-100 syringe)
• mg = mL x concentration (mg/mL)
• units = (target mg / concentration in mg/mL) x 100

Worked through for the three most common preparations, here is how many units carry a given mass of peptide:

The 120-unit cell is not a typo: 4 mg at 3.33 mg/mL is 1.2 mL, more than a 1 mL syringe holds. That is exactly the kind of result that tells you a more concentrated preparation suits the work better. For any combination not in the table, the peptide reconstitution calculator does the arithmetic, and the dedicated pages at /reconstitution/ (for example retatrutide 10 mg and retatrutide 20 mg) list the full chart for each vial. A longer explanation of units, mL, mg and mcg, including IU, is in peptide units to mg, explained.

Choosing a volume: the trade-off

There is no correct volume, only a trade-off between precision and convenience:

• More water, lower concentration means each unit carries less peptide, so the same measuring error on the barrel is a smaller mass error. The cost is more liquid per aliquot and more punctures to empty the vial.
• Less water, higher concentration keeps aliquot volumes small, but every unit carries more peptide and small reading errors grow.

Pick the concentration that puts your typical aliquot somewhere between roughly 10 and 50 units on the syringe, where graduations are easy to read. Then keep it constant across a study so results are comparable between vials.

How to store reconstituted retatrutide

Retatrutide is a 39-residue acylated peptide, and like other lipidated incretin analogues it is reasonably robust in solution, but no peer-reviewed stability study exists for research-grade retatrutide solutions specifically. Labs therefore use conservative general practice: 2 to 8 C, protected from light, accessed with a fresh needle each time, and discarded according to the conventional 28-day multi-dose window. If a vial must last longer, divide it into single-use aliquots and freeze them once. Detail on timing, room-temperature excursions and freezing is in how long reconstituted peptides last, and lyophilized storage is covered in our lyophilized storage reference.

Common reconstitution mistakes

• Not writing the concentration down. The single most common source of error. Two weeks later nobody remembers whether it was 2 mL or 3 mL.
• Mixing up vial sizes. A 20 mg vial treated as a 10 mg vial doubles every downstream mass. Check the label before drawing solvent.
• Reading the wrong syringe scale. Some 1 mL insulin syringes mark every 2 units. Know what each line means.
• Shaking. Foam is peptide stuck at the air-liquid interface. Swirl instead.

For background on the molecule itself, see retatrutide tri-agonist pharmacology and the summary of the published human studies in retatrutide clinical trials explained.

Frequently asked questions