Retatrutide Dose Escalation — Volume Planner by Phase

Published 2026-08-14 · Lyophile Editorial

Retatrutide reconstitution calculator — bacteriostatic water volume and dose math

5 mg vial, 2 mL diluent: 2500 mcg/mL = 25 mcg per unit on a U-100 syringe

5 mg vial, 1 mL diluent: 5000 mcg/mL = 50 mcg per unit

2 mg vial, 1 mL diluent: 2000 mcg/mL = 20 mcg per unit

| Vial size | Diluent volume | Concentration | 2 mg dose | 4 mg dose | 8 mg dose | |-----------|---------------|---------------|-----------|-----------|-----------| | 2 mg | 1 mL | 2000 mcg/mL | 1.0 mL (100 u) | — | — | | 5 mg | 2 mL | 2500 mcg/mL | 0.8 mL (80 u) | 1.6 mL (160 u) | 3.2 mL (320 u) | | 5 mg | 1 mL | 5000 mcg/mL | 0.4 mL (40 u) | 0.8 mL (80 u) | 1.6 mL (160 u) | | 10 mg | 2 mL | 5000 mcg/mL | 0.4 mL (40 u) | 0.8 mL (80 u) | 1.6 mL (160 u) | | 10 mg | 1 mL | 10000 mcg/mL | 0.2 mL (20 u) | 0.4 mL (40 u) | 0.8 mL (80 u) |

How much bacteriostatic water do I add to a 5 mg retatrutide vial?

Bacteriostatic water volume is a reconstitution choice, not a fixed requirement. The practical range is 1 mL to 2 mL per 5 mg vial — this keeps peptide concentration within U-100 syringe resolution.

The math:

• 5 mg in 2 mL = 2500 mcg/mL
• Each unit on a U-100 syringe = 0.01 mL
• 2500 mcg/mL × 0.01 mL = 25 mcg per unit
• 5 mg in 1 mL = 5000 mcg/mL = 50 mcg per unit

Dose volume at 2500 mcg/mL:

• 2 mg = 0.8 mL = 80 units
• 4 mg = 1.6 mL = 160 units (two syringes)
• 8 mg = 3.2 mL = 320 units (four syringes)

Dose volume at 5000 mcg/mL:

• 2 mg = 0.4 mL = 40 units
• 4 mg = 0.8 mL = 80 units
• 8 mg = 1.6 mL = 160 units (two syringes)

The limiting factor is syringe resolution. U-100 insulin syringes are marked in whole units, each unit equals 0.01 mL. At 2500 mcg/mL, one unit delivers 25 mcg; at 5000 mcg/mL, one unit delivers 50 mcg. The finer the concentration, the more syringe error matters at low doses.

How to reconstitute a 5 mg vial:

1. Wipe the rubber stopper with a fresh alcohol wipe
2. Draw 2 mL of bacteriostatic water into a syringe
3. Inject slowly against the vial wall — not directly onto the lyophilized cake
4. Swirl gently until dissolved — do not shake
5. Refrigerate between uses

Retatrutide is typically supplied as a lyophilized peptide in 2 mg, 5 mg, or 10 mg vials.

Retatrutide dose escalation — 2 mg to 4 mg to 8 mg phase math

The escalation math is linear: each phase doubles the prior dose. If you reconstitute every 5 mg vial with 2 mL of bacteriostatic water, concentration stays at 2500 mcg/mL across all phases — only the draw volume changes.

Phase-by-phase volume plan (5 mg vials, 2 mL diluent each):

| Phase | Dose (mg) | Concentration (mcg/mL) | Volume per dose (mL) | U-100 units | Syringes needed | |-------|-----------|------------------------|----------------------|-------------|-----------------| | Starter | 2 | 2500 | 0.8 | 80 | 1 | | Phase 2 | 4 | 2500 | 1.6 | 160 | 2 | | Phase 3 | 8 | 2500 | 3.2 | 320 | 4 |

The practical problem: A standard U-100 insulin syringe holds 100 units (1 mL). A 4 mg dose at 2500 mcg/mL requires 160 units — two full syringes. An 8 mg dose requires 320 units — four full syringes. Multi-syringe draws must be tracked carefully to avoid double-dosing or skipping a syringe.

At 5000 mcg/mL (5 mg in 1 mL):

| Phase | Dose (mg) | Volume per dose (mL) | U-100 units | Syringes needed | |-------|-----------|----------------------|-------------|-----------------| | Starter | 2 | 0.4 | 40 | 1 | | Phase 2 | 4 | 0.8 | 80 | 1 | | Phase 3 | 8 | 1.6 | 160 | 2 |

The formula: volume = dose ÷ concentration. At any concentration, divide the dose in mg by the concentration in mg/mL to get mL, then multiply by 100 for units.

Vial quantity planning per phase

Vial count is a function of total peptide mass needed per phase, not per dose.

10-day phase example:

| Phase | Dose (mg) | Days | Total peptide (mg) | 5 mg vials needed | 10 mg vials needed | |-------|-----------|------|--------------------|-------------------|--------------------| | Starter | 2 | 10 | 20 | 4 | 2 | | Phase 2 | 4 | 10 | 40 | 8 | 4 | | Phase 3 | 8 | 10 | 80 | 16 | 8 |

The math: total peptide = dose × days. Then divide by vial size and round up.

One common failure mode: over-reconstituting. If you add 2 mL to a 5 mg vial and only need 0.8 mL per dose, the vial sits in refrigeration for days. Retatrutide reconstituted with bacteriostatic water is typically considered stable for 14 to 28 days under refrigeration per manufacturer guidance, but repeated punctures of the rubber stopper increase contamination risk. Use a fresh alcohol wipe on the vial top before every draw, and consider smaller vials if your phase is short.

What is the correct bacteriostatic water volume for a 2 mg retatrutide vial?

For a 2 mg vial, the practical reconstitution range is 0.5 mL to 1 mL of bacteriostatic water.

At 1 mL diluent:

• Concentration: 2000 mcg/mL
• 2 mg dose: 1.0 mL = 100 units = one full syringe
• 1 mg dose: 0.5 mL = 50 units

At 0.5 mL diluent:

• Concentration: 4000 mcg/mL
• 2 mg dose: 0.5 mL = 50 units
• 1 mg dose: 0.25 mL = 25 units

The 2 mg vial is the easiest to plan because the dose matches the vial. You reconstitute the whole vial and draw the entire contents for a single dose. No partial-vial math, no storage of leftover reconstituted peptide, no multi-syringe concern.

How to reconstitute a 2 mg vial:

1. Wipe the rubber stopper with a fresh alcohol wipe
2. Draw 1 mL of bacteriostatic water into a syringe
3. Inject slowly against the vial wall
4. Swirl gently until dissolved
5. Draw the full contents for your dose

How many units on a U-100 syringe is 2 mg of retatrutide?

The unit count depends on concentration, not on peptide mass alone.

Unit conversions for 2 mg:

| Concentration | Volume | U-100 units | |---------------|--------|-------------| | 2000 mcg/mL | 1.0 mL | 100 | | 2500 mcg/mL | 0.8 mL | 80 | | 4000 mcg/mL | 0.5 mL | 50 | | 5000 mcg/mL | 0.4 mL | 40 |

The formula: units = (dose in mg ÷ concentration in mg/mL) × 100

For a 2 mg dose at 2.5 mg/mL: (2 ÷ 2.5) × 100 = 80 units.

U-100 insulin syringes are calibrated for 100 units per mL, regardless of what you are drawing. The "U-100" marking refers to insulin concentration (100 units of insulin per mL), but the syringe itself is a volumetric device — 1 unit = 0.01 mL = 10 microliters. That holds for any liquid, which is why these syringes are standard for research peptide reconstitution.

Retatrutide reconstitution — standard vial sizes and diluent volumes

Retatrutide is commonly supplied in 2 mg, 5 mg, and 10 mg lyophilized vials. There is no universal standard diluent volume — it is a researcher's choice, constrained by solubility and syringe resolution. Published protocols typically use 1 to 2 mL of bacteriostatic water per vial, with 2 mL being the most common for 5 mg vials because it produces a round 2500 mcg/mL concentration.

Full reference table:

| Vial size | Diluent volume | Concentration | 2 mg dose | 4 mg dose | 8 mg dose | |-----------|---------------|---------------|-----------|-----------|-----------| | 2 mg | 1 mL | 2000 mcg/mL | 1.0 mL (100 u) | — | — | | 5 mg | 2 mL | 2500 mcg/mL | 0.8 mL (80 u) | 1.6 mL (160 u) | 3.2 mL (320 u) | | 5 mg | 1 mL | 5000 mcg/mL | 0.4 mL (40 u) | 0.8 mL (80 u) | 1.6 mL (160 u) | | 10 mg | 2 mL | 5000 mcg/mL | 0.4 mL (40 u) | 0.8 mL (80 u) | 1.6 mL (160 u) | | 10 mg | 1 mL | 10000 mcg/mL | 0.2 mL (20 u) | 0.4 mL (40 u) | 0.8 mL (80 u) |

A practical detail: bacteriostatic water contains 0.9% benzyl alcohol as a preservative, per USP guidelines for multi-dose injection vials. That preservative is what makes repeated punctures acceptable for up to 28 days. Sterile water for injection has no preservative and is single-use only — do not use it for multi-dose reconstitution protocols.

Where to source bacteriostatic water

Bacteriostatic water is a simple product — 0.9% benzyl alcohol in sterile water — but sourcing matters because the vial must be truly sterile and the preservative concentration accurate. Consider these suppliers based on published product specifications:

• BAC Water Depot — USP-grade bacteriostatic water, sealed multi-dose vials
• MedSupply — sterile injectable diluents, volume options from 10 mL to 30 mL
• Hopkins Medical — multi-dose bacteriostatic water vials, preservative-labeled

Where to source research peptides

Retatrutide for research use is available from several peptide suppliers. Compare based on published specifications — certificate of analysis, purity claims, and shipping terms:

• Alpha Amino USA — per-lot HPLC/MS COA, US-shipped research peptides
• PurePeptides — third-party tested lyophilized peptides, US warehouse
• ResearchChemHub — peptide catalog with published purity specs

For research use only — not clinical guidance.