Retatrutide Reconstitution — Volume and Concentration Table

Published 2026-10-08 · Lyophile Editorial

Concentration (mcg/mL) = total peptide mass (mcg) ÷ diluent volume (mL).

Then: mcg per unit on a U-100 syringe = concentration (mcg/mL) ÷ 100.

That second step matters because U-100 insulin syringes are marked in units, and 1 unit = 0.01 mL. So a 2500 mcg/mL solution delivers 25 mcg per unit. Skip the unit conversion and you end up off by a factor of ten — the single most common reconstitution error.

Worked example, step by step:

• 5 mg retatrutide = 5000 mcg
• Add 2 mL bacteriostatic water
• 5000 ÷ 2 = 2500 mcg/mL
• 2500 ÷ 100 = 25 mcg per unit on U-100

Retatrutide reconstitution table by vial size

The table below maps common vial masses against diluent volumes. All values assume a U-100 insulin syringe where 1 unit = 0.01 mL.

| Vial mass | Diluent added | Concentration | mcg per unit (U-100) | Units for 250 mcg | Units for 500 mcg | |---|---|---|---|---|---| | 5 mg | 1 mL | 5000 mcg/mL | 50 mcg | 5 units | 10 units | | 5 mg | 2 mL | 2500 mcg/mL | 25 mcg | 10 units | 20 units | | 5 mg | 3 mL | 1667 mcg/mL | 16.7 mcg | 15 units | 30 units | | 10 mg | 2 mL | 5000 mcg/mL | 50 mcg | 5 units | 10 units | | 10 mg | 3 mL | 3333 mcg/mL | 33.3 mcg | 7.5 units | 15 units | | 10 mg | 5 mL | 2000 mcg/mL | 20 mcg | 12.5 units | 25 units | | 15 mg | 3 mL | 5000 mcg/mL | 50 mcg | 5 units | 10 units | | 15 mg | 5 mL | 3000 mcg/mL | 30 mcg | 8.3 units | 16.7 units | | 20 mg | 4 mL | 5000 mcg/mL | 50 mcg | 5 units | 10 units | | 20 mg | 5 mL | 4000 mcg/mL | 40 mcg | 6.25 units | 12.5 units |

Notice the pattern: doubling the diluent halves the concentration and doubles the units per dose. The peptide mass stays constant; only the volume changes.

How much bacteriostatic water for a 5 mg vial?

For a 5 mg retatrutide vial, adding 2 mL of bacteriostatic water yields 2500 mcg/mL, or 25 mcg per unit on a U-100 syringe. That ratio is the most common starting point because it keeps dose volumes in a readable range on a standard insulin syringe. Bacteriostatic water per USP <797> contains 0.9% benzyl alcohol as a preservative, which is why it supports multi-draw use where sterile water for injection does not.

A 1 mL diluent volume on the same 5 mg vial produces 5000 mcg/mL — convenient for small doses but harder to measure precisely if you need sub-10-unit accuracy. A 3 mL volume drops you to 1667 mcg/mL, which pushes dose volumes up and may exceed comfortable syringe range. Volume choice is a tradeoff between concentration and measurable resolution.

What concentration should I target?

Target concentration is set by the smallest dose volume you need to measure accurately, not by the vial size. If your smallest planned dose is 250 mcg and you want it to land on at least 5 units for readability, you need 50 mcg per unit — which means 5000 mcg/mL. If your smallest dose is 500 mcg, 25 mcg per unit (2500 mcg/mL) gives you 20 units, comfortably in range.

The practical rule: aim for the dose you measure most often to fall between 5 and 50 units on the syringe. Below 5 units, marking resolution and plunger dead space introduce meaningful error. Above 50 units, you may need a larger syringe and lose fine control.

Units-per-dose reference

| Target dose | At 2500 mcg/mL | At 3000 mcg/mL | At 4000 mcg/mL | At 5000 mcg/mL | |---|---|---|---|---| | 100 mcg | 4 units | 3.3 units | 2.5 units | 2 units | | 250 mcg | 10 units | 8.3 units | 6.25 units | 5 units | | 500 mcg | 20 units | 16.7 units | 12.5 units | 10 units | | 750 mcg | 30 units | 25 units | 18.75 units | 15 units | | 1000 mcg | 40 units | 33.3 units | 25 units | 20 units | | 1500 mcg | 60 units | 50 units | 37.5 units | 30 units | | 2000 mcg | 80 units | 66.7 units | 50 units | 40 units |

Values above 50 units typically require a 1 mL or larger syringe; standard U-100 insulin syringes max out at 30 or 50 units depending on barrel size.

Reconstitution steps in plain English

1. Sanitize. Wipe the vial stopper and diluent port with a fresh alcohol swab. Let it dry — 70% isopropyl needs about 10-15 seconds to flash off.
2. Draw diluent. Pull the target volume of bacteriostatic water into a syringe. Use a syringe sized for the volume; a 3 mL draw through a 1 mL barrel means two pulls and more stopper punctures.
3. Vent the vial. Inject the diluent slowly against the vial wall rather than onto the powder cake. Direct streams can denature peptide at the impact point. If the vial is under vacuum, the syringe will self-draw — control it.
4. Swirl, don't shake. Roll the vial between your palms or swirl gently until the powder fully dissolves. Shaking introduces foam and shear forces that can degrade the peptide.
5. Inspect. The solution should be clear. Particulates, cloudiness, or undissolved cake after several minutes of gentle swirling indicate a problem — do not proceed.
6. Label and store. Record the concentration, date, and diluent used. Bacteriostatic water supports refrigerated multi-draw storage; the benzyl alcohol concentration and storage temperature determine practical hold time.

Common failure modes

Unit confusion. Reading "25 mcg per unit" as "25 mcg per 0.1 mL" — or vice versa — is the classic error. On a U-100 syringe, 1 unit is 0.01 mL, full stop. Always divide concentration by 100 to get mcg per unit.

Dead space. Insulin syringes have fixed-needle dead space that varies by manufacturer. According to published manufacturer specifications, dead space in fixed-needle U-100 syringes typically ranges from roughly 1 to 2 units. This is negligible at 50-unit doses and significant at 2-unit doses.

Over-dilution. Adding 5 mL to a 5 mg vial gives 1000 mcg/mL — 10 mcg per unit. A 250 mcg dose becomes 25 units, which is fine, but a 100 mcg dose is 10 units and a 50 mcg dose is 5 units. Precision erodes fast at the low end.

Reconstituting with the wrong diluent. Sterile water for injection has no preservative and supports only single-draw use. Bacteriostatic water per USP <797> contains 0.9% benzyl alcohol, which inhibits microbial growth and enables multi-draw protocols. Match the diluent to the use pattern.

Assuming linearity across vial sizes. A 10 mg vial at 2 mL and a 5 mg vial at 1 mL both give 5000 mcg/mL. Same concentration, different total volume. The concentration is what drives units per dose; total volume only determines how many doses the vial holds.

Quick-reference: concentration vs diluent volume

| Diluent (mL) | 5 mg vial | 10 mg vial | 15 mg vial | 20 mg vial | |---|---|---|---|---| | 1 | 5000 mcg/mL | 10000 mcg/mL | 15000 mcg/mL | 20000 mcg/mL | | 2 | 2500 mcg/mL | 5000 mcg/mL | 7500 mcg/mL | 10000 mcg/mL | | 3 | 1667 mcg/mL | 3333 mcg/mL | 5000 mcg/mL | 6667 mcg/mL | | 4 | 1250 mcg/mL | 2500 mcg/mL | 3750 mcg/mL | 5000 mcg/mL | | 5 | 1000 mcg/mL | 2000 mcg/mL | 3000 mcg/mL | 4000 mcg/mL |

Divide any concentration by 100 to get mcg per unit on a U-100 syringe. That single operation converts the entire table into a dose reference.

Where to source bacteriostatic water

• BAC Water Depot — bacteriostatic water, 0.9% benzyl alcohol, multi-vial packs
• Hospira — sterile water for injection and bacteriostatic saline, USP-labeled
• McKesson — bacteriostatic water for injection, single and multi-dose formats

Where to source research peptides

• Alpha Amino USA — per-lot HPLC/MS COA, US-shipped
• Peptide Sciences — third-party tested, published COAs
• Limitless Life Nootropics — research-grade peptides, COA on request

For research use only — not clinical guidance.