The low-and-slow schedule
The schedule used across this library is deliberately conservative: 2 mg weekly for the first four weeks, 3 mg for weeks 5–6, then 4 mg for weeks 7–8. Every step is a single 1 mg increase, and it is only taken when two things are true — no side effects on the current dose, and appetite suppression has clearly worn off. The aim is to hold the lowest dose that is still working for as long as it keeps working, rather than overloading the GLP-1, GIP and glucagon receptors and burning through your response early.
| Phase | Weekly dose | Notes |
|---|---|---|
| Weeks 1–4 | 2 mg / week | Starting dose. Hold the full four weeks — no early increases. |
| Weeks 5–6 | 3 mg / week | Step up 1 mg only if weeks 1–4 were side-effect free and appetite suppression has faded. |
| Weeks 7–8 | 4 mg / week | Second 1 mg step, same two conditions. If 3 mg still works, stay on 3 mg. |
| Beyond week 8 | Hold lowest working dose | No routine climb. Stay as low as possible for as long as possible. |
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 | 3 mg | 4 mg |
|---|---|---|---|---|
| 1 mL | 10 mg/mL | 20 u | 30 u | 40 u |
| 2 mL | 5 mg/mL | 40 u | 60 u | 80 u |
| 3 mL | 3.33 mg/mL | 60 u | 90 u | 120 u (two draws) |
3 mL BAC water is the default used across this library — 60, 90 and 120 units for 2, 3 and 4 mg. The wider marks make each dose easier to read accurately; 4 mg exceeds one syringe, so it is drawn as two injections. See the ultimate guide to peptide reconstitution for the full protocol.
Side effects and when to hold
Side effects track dose closely. In the Phase 2 obesity trial nausea was reported by around 40% of participants on the highest arms versus roughly 15% on the lowest, with decreased appetite, transient diarrhoea and constipation following the same pattern. At 2–4 mg those effects are far less common — which is exactly why this schedule stays there.
The practical rule: if anything is off — nausea, loose stools, poor sleep, a flat mood, an unusually high resting heart rate — do not step up. Hold the current dose for another two weeks and reassess. Only move up 1 mg when the week has been completely clean and food suppression has faded to the point the dose is no longer doing much. Two conditions, one milligram, never both steps at once.
Because the glucagon arm raises hepatic energy expenditure, small transient rises in resting heart rate (~3–6 bpm) were seen on the high trial arms. Staying low keeps that load off the system as well.
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. Solvent specs are in the bacteriostatic water guide and full storage 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 recommended retatrutide schedule?+
The conservative low-and-slow schedule used across this library is 2 mg weekly for weeks 1–4, 3 mg weekly for weeks 5–6, then 4 mg weekly for weeks 7–8. Each step is a single 1 mg increase, and it is only taken if the previous dose was tolerated with no side effects and appetite suppression has clearly faded. If the current dose is still working, hold it rather than climbing.
Why keep the dose as low as possible for as long as possible?+
Receptor responsiveness is finite. Pushing the dose up while a lower one is still producing appetite suppression overloads the GLP-1, GIP and glucagon receptors, blunts the response and wastes headroom you may need later. Holding the lowest dose that is still doing something keeps sensitivity intact, keeps side effects low and stretches each vial further.
When should the dose be increased by 1 mg?+
Only when two conditions are both true: no side effects at the current dose, and food suppression has dropped off so the dose is no longer doing much. If either is untrue, stay where you are for another 2 weeks and reassess. Never jump 2 mg or more in a single step.
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. This library uses 3 mL as the default: a 10 mg vial with 3 mL BAC water gives 3.33 mg/mL, so 2 mg = 60 units on a U-100 insulin syringe, 3 mg = 90 units and 4 mg = 120 units (drawn as two injections). Always confirm with the reconstitution calculator before drawing.
What are the most common retatrutide side effects?+
Trial data (Jastreboff et al., NEJM 2023) reported nausea, decreased appetite, transient diarrhoea and constipation, all clearly dose-dependent and most frequent at the high trial arms. Any of these at a low dose is a signal to hold, not to step up.
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.