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Proper retatrutide reconstitution technique is critical for research integrity — see the retatrutide phase 2 trial data (NEJM 2023) for context on why reconstitution stability affects experimental outcomes.
Step-by-Step Retatrutide Reconstitution Protocol
Retatrutide is a triple GLP-1/GIP/glucagon receptor agonist currently in Phase III clinical trials. Phase II data (N=338, 48 weeks) showed a mean 24.2% reduction in body weight at the highest dose tested (12 mg weekly) — the largest weight-loss result reported in a randomized controlled trial at the time of publication.[1] This page covers reconstitution volumes, dosage calculations, and handling considerations for research use.
Retatrutide Dosage: What Phase II Trials Used
The Phase II trial (Jastreboff et al., 2023) tested five weekly subcutaneous doses over 48 weeks in adults with obesity (BMI ≥30 or ≥27 with a comorbidity). Dose-escalation schedules were used; the final maintenance doses and observed weight loss were:
| Maintenance Dose | Mean Weight Loss at 24 Weeks | Placebo-Adjusted |
|---|---|---|
| 1 mg weekly | 8.7% | 7.9% |
| 4 mg weekly | 17.3% | 16.5% |
| 8 mg weekly | 22.8% | 22.1% |
| 12 mg weekly | 24.2% | 23.5% |
The trial used a dose-escalation protocol to the maintenance level rather than starting at the full dose, consistent with GLP-1 class peptide tolerability practices. The Phase III trial (TRIUMPH) is using a similar escalation approach. As of 2026, retatrutide does not have an approved dosage; the Phase II figures are the only clinical reference available for research protocols.
Reconstitution by Vial Size
Retatrutide is sold in lyophilized form in various vial sizes. The standard approach is to reconstitute with bacteriostatic water (BAC water, 0.9% benzyl alcohol) and calculate the concentration based on the volume added. See the retatrutide research peptide Canada page for current vial sizes and batch COAs.
Use this formula: Concentration (mg/mL) = Vial mass (mg) ÷ BAC water added (mL)
| Vial Size | BAC Water Added | Final Concentration | Volume per 1 mg dose | Volume per 4 mg dose | Volume per 8 mg dose |
|---|---|---|---|---|---|
| 2 mg | 1.0 mL | 2 mg/mL | 0.5 mL (50 IU) | — | — |
| 5 mg | 1.0 mL | 5 mg/mL | 0.2 mL (20 IU) | 0.8 mL (80 IU) | — |
| 5 mg | 2.5 mL | 2 mg/mL | 0.5 mL (50 IU) | 2.0 mL (200 IU) | — |
| 10 mg | 2.0 mL | 5 mg/mL | 0.2 mL (20 IU) | 0.8 mL (80 IU) | 1.6 mL (160 IU) |
| 10 mg | 5.0 mL | 2 mg/mL | 0.5 mL (50 IU) | 2.0 mL (200 IU) | 4.0 mL (400 IU) |
IU values are based on a standard U-100 insulin syringe (100 IU = 1.0 mL). For a U-100 syringe: 10 IU = 0.1 mL.
Step-by-Step Reconstitution Protocol
- Allow the vial to reach room temperature (15–20 min from refrigerator). Cold lyophilized powder reconstitutes unevenly.
- Swab the vial septum with an alcohol swab and allow to dry (30 seconds).
- Draw the calculated volume of BAC water into a clean syringe.
- Inject BAC water slowly against the inner glass wall of the vial — not directly onto the lyophilized cake. This minimizes mechanical denaturation of the peptide.
- Do not shake the vial. Gently swirl or roll between palms until the powder is fully dissolved. A clear, colourless solution is expected. Any persistent particulate matter is a quality concern.
- Inspect the solution before each use. Discard if particulate, discoloured, or showing signs of degradation.
- Label the vial with the reconstitution date.
Stability and Storage of Reconstituted Retatrutide
No published stability data specific to research-grade retatrutide solutions exists in the peer-reviewed literature as of 2026. However, general peptide stability principles and manufacturer data for the related compound tirzepatide (Mounjaro) provide reference points:
- Lyophilized powder: Stable at room temperature for shipping; long-term storage at −20°C to −80°C is recommended. Avoid repeated freeze-thaw cycles.
- Reconstituted solution with BAC water: 2–8°C refrigeration; conservative guidance is 28 days. Benzyl alcohol provides bacteriostatic (not indefinite) protection. Some research protocols extend to 42 days at 2–8°C for similar peptide solutions.
- Do not freeze reconstituted solutions. Freeze-thaw cycles can cause peptide aggregation and degradation of solution integrity.
- Light exposure: Store in original vial, away from light. UV exposure degrades GLP-1 class peptides.
For the Mounjaro (tirzepatide) pen device specifically, Eli Lilly’s published data shows stability at 2–8°C up to 36 months and at room temperature (≤30°C) for 21 days after first use. Retatrutide’s structural similarity (same amino acid chain length, similar fatty acid modification) suggests analogous stability behaviour, but this should not be treated as established data for retatrutide specifically.
Injection Site and Administration Notes
In the Phase II trial, retatrutide was administered as a weekly subcutaneous injection. Injection site rotation is standard practice for subcutaneous peptides to prevent lipohypertrophy — common rotation sites are the abdomen (at least 2 inches from navel), outer thigh, and outer upper arm.
The Phase II trial reported the most common adverse events at higher doses were nausea, vomiting, and diarrhea — consistent with other GLP-1 class peptides and more pronounced during dose escalation. The dose-escalation schedule used in the trial:
- Weeks 1–4: Starting dose (0.5 mg or 2 mg depending on arm)
- Weeks 4–8: First escalation
- Continue to maintenance dose over 16–20 weeks total escalation period
Specific escalation schedules for the Phase III TRIUMPH trial have not been published in detail as of this writing.
Retatrutide vs Semaglutide vs Tirzepatide: Dosage Comparison
| Compound | Receptor Targets | Highest Trial Dose | Peak Weight Loss (RCT) | Trial Duration |
|---|---|---|---|---|
| Semaglutide (Wegovy) | GLP-1 | 2.4 mg weekly | ~15.2% (STEP-1, 68 wk) | 68 weeks |
| Tirzepatide (Zepbound) | GLP-1 + GIP | 15 mg weekly | ~20.9% (SURMOUNT-1, 72 wk) | 72 weeks |
| Retatrutide | GLP-1 + GIP + GCGR | 12 mg weekly | ~24.2% (Phase II, 48 wk) | 48 weeks (Phase II) |
Comparing these figures requires caution: the trials used different populations, timeframes, and dose-escalation schedules. Retatrutide’s Phase II 48-week data cannot be directly compared to 68–72 week data from approved drugs. Head-to-head trial data does not exist as of 2026.
Frequently Asked Questions
What is the retatrutide dosage used in clinical trials?
The Phase II trial tested weekly subcutaneous doses of 1 mg, 4 mg, 8 mg, and 12 mg as maintenance doses, with dose escalation over the first 16–20 weeks. The highest dose (12 mg) showed the greatest weight loss (~24.2% at 48 weeks). In May 2026, Lilly reported Phase 3 TRIUMPH-1 results using 4 mg, 9 mg and 12 mg maintenance doses over 80 weeks, with a mean 28.3% weight reduction at 12 mg.
How do I reconstitute a 5 mg retatrutide vial?
Add 1.0 mL of bacteriostatic water to a 5 mg vial for a 5 mg/mL concentration (20 IU per 1 mg), or add 2.5 mL for a 2 mg/mL concentration (50 IU per 1 mg). Inject the water slowly against the inner glass wall, then swirl gently until dissolved. Do not shake.
How long does reconstituted retatrutide last?
Reconstituted retatrutide in bacteriostatic water should be stored at 2–8°C and used within 28 days as a conservative guideline, consistent with similar GLP-1 class peptide solutions. Benzyl alcohol provides bacteriostatic protection but not indefinite stability. Do not freeze reconstituted solutions.
What is the difference between retatrutide and tirzepatide?
Tirzepatide is a dual GLP-1/GIP receptor agonist (two receptor targets). Retatrutide adds a third receptor target — the glucagon receptor (GCGR). Glucagon receptor activation increases energy expenditure, which may contribute to retatrutide’s greater weight loss signal. Tirzepatide (Mounjaro/Zepbound) is approved in Canada and many other countries; retatrutide is in Phase III trials as of 2026 and is not approved.
Where can I buy retatrutide in Canada?
Retatrutide is available as a research peptide from Canadian suppliers including Anglo Peptides. It is not an approved drug in Canada. Research-grade retatrutide should be ≥98% HPLC purity with mass spectrometry confirmation; request a batch-specific certificate of analysis before purchasing.
References
- Jastreboff AM, et al. Triple-hormone-receptor agonist retatrutide for obesity — a Phase 2 trial. N Engl J Med. 2023;389(6):514–526. PMID 37385013
- Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med. 2021;384(11):989–1002. PMID 33567185
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387(3):205–216. PMID 35658024
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