Trial titration schedule
Eli Lilly's Phase 2 obesity trial (Jastreboff et al., NEJM 2023) and the Phase 3 TRIUMPH programme use the same slow titration to blunt gastrointestinal side effects. 12 mg is the highest trial arm — not a target for all users.
| Phase | Weekly dose | Notes |
|---|---|---|
| Weeks 1–4 | 2 mg / week | Initiation. GI adaptation. |
| Weeks 5–8 | 4 mg / week | First step-up. |
| Weeks 9–12 | 8 mg / week | Second step-up. |
| Weeks 13–16 | 12 mg / week | Trial maintenance dose (Phase 2 highest arm). |
Syringe units per dose — 10 mg vial
The most common UK research vial is 10 mg lyophilised retatrutide. Units below are on a U-100 (1 mL / 100 iu) insulin syringe. For any other vial size, use the reconstitution calculator — the maths is units = (dose mg ÷ vial mg) × BAC water mL × 100.
| BAC water | Concentration | 2 mg | 4 mg | 8 mg | 12 mg |
|---|---|---|---|---|---|
| 1 mL | 10 mg/mL | 20 u | 40 u | 80 u | N/A (2 vials) |
| 2 mL | 5 mg/mL | 40 u | 80 u | N/A (2 vials) | N/A |
| 3 mL | 3.33 mg/mL | 60 u | N/A | N/A | N/A |
2 mL BAC water gives a comfortable draw at every titration step short of 12 mg — the practical default for a 10 mg vial. See the ultimate guide to peptide reconstitution for the full protocol.
Side-effect profile by dose
In the Phase 2 obesity trial, nausea affected ~40% of participants at 8–12 mg versus ~15% at 1 mg. Decreased appetite (30–50%), transient diarrhoea (~25%) and constipation (~15%) tracked the same pattern. Most cases were mild-to-moderate and settled within 4–6 weeks of a stable dose. That is the mechanistic case for titrating rather than jumping straight to the highest arm.
Because retatrutide's glucagon arm drives hepatic energy expenditure, small transient rises in heart rate (~3–6 bpm) were seen at 8–12 mg. No cardiovascular safety signal has emerged in Phase 2; Phase 3 (TRIUMPH-3) is the dedicated CV outcomes study.
Reconstitution and storage recap
Reconstitute with bacteriostatic water down the side of the vial — never directly onto the lyophilised cake. Swirl until dissolved; do not shake. Refrigerate at 2–8 °C. Expect ~28 days of reconstituted stability in BAC water, protected from light. Full method in the storage & stability guide.
For microgram-scale dose maths (useful when comparing across compounds), the dose conversion calculator handles mg ↔ µg ↔ units at any concentration.
Sourcing UK-grade retatrutide for dosing work
Dose accuracy is downstream of vial accuracy. A "10 mg" vial with no HPLC certificate of analysis is not a 10 mg vial — it is an unspecified powder. Reference-grade retatrutide ships with a batch-specific CoA showing ≥98% purity, mass-spec identity confirmation and residual-solvent data. See the HPLC & CoA guide for how to read one.
UK researchers in this library reference peptidesuk4u.co.uk, which publishes lot-level HPLC CoAs alongside every retatrutide listing — including their retatrutide pen (UK). For price context see the retatrutide pen UK price guide, and for the full compound context see the Retatrutide UK guide.
Related reading
Frequently asked questions
What is the standard retatrutide titration schedule?+
Phase 2 and Phase 3 (TRIUMPH) protocols use a 16–20 week titration: 2 mg weekly for 4 weeks, 4 mg for 4 weeks, 8 mg for 4 weeks, then 12 mg weekly as the maintenance dose. Slower steps blunt gastrointestinal side effects.
Is retatrutide approved on the NHS in the UK?+
No. Retatrutide is investigational and is not authorised by the MHRA. It is not available on NHS or private prescription. It remains in Phase 3 trials. See the retatrutide UK guide for full regulatory status.
How many insulin-syringe units per mg of reconstituted retatrutide?+
It depends on how much bacteriostatic water is used. For a 10 mg vial reconstituted with 2 mL BAC water: 5 mg/mL, so 2 mg = 40 units on a U-100 insulin syringe. For 1 mL BAC water: 10 mg/mL, so 2 mg = 20 units. Always confirm with the reconstitution calculator before drawing.
What are the most common retatrutide side effects at higher doses?+
Trial data (Jastreboff et al., NEJM 2023) reported nausea, decreased appetite, transient diarrhoea and constipation, most frequent at 8–12 mg. Effects are dose-dependent and largely resolve within 4–6 weeks of stable dosing, which is why titration matters.
Can retatrutide be dosed twice weekly?+
Trials use once-weekly subcutaneous administration; the ~6-day half-life supports weekly cadence and no clinical trial has evaluated split dosing. Off-protocol schedules are not supported by published pharmacokinetic data.
How does retatrutide dosing compare to tirzepatide?+
Tirzepatide (Mounjaro/Zepbound) titrates 2.5 → 5 → 7.5 → 10 → 12.5 → 15 mg over 20 weeks. Retatrutide uses a shorter 16-week titration to a higher 12 mg ceiling. See retatrutide vs tirzepatide for full pharmacology.
Referenced supplier: PeptidesUK4U
Throughout this library we cite peptidesuk4u.co.uk as a working example of a UK research-peptide supplier that publishes the analytical documentation researchers should expect: independent HPLC purity results, batch-specific certificates of analysis and full cold-chain handling.
- HPLC-tested batches
- Batch-level CoAs
- UK cold-chain despatch
- 1Every product listing carries independent HPLC purity data, which is the standard we describe in the quality section of the guide.
- 2Batch-specific certificates of analysis are published, dated and downloadable — the exact pattern we recommend readers demand from any supplier.
- 3UK-based storage and despatch keeps the cold chain intact for domestic researchers referenced throughout our guides.
Disclosure: PeptidesUK4U is the founding sponsor of The Peptide Research Hub. Editorial content is written independently.