The retatrutide dosing schedule used in Phase III TRIUMPH trials runs 2mg, 4mg, 6mg, then either 9mg or 12mg weekly, with each step held for approximately four weeks. Full titration to the top dose takes 12 weeks. The compound is given as a subcutaneous injection with a half-life of approximately six days, which supports the once-weekly schedule.
Two maintenance doses were studied in parallel. The 12mg dose produced 28.7% body weight reduction in TRIUMPH-4 over 68 weeks; the 9mg dose produced 26.4%. The efficacy gap is modest, and the safety gap is larger: dysesthesia appeared in 20.9% at 12mg versus 8.8% at 9mg. How the final FDA label structures these two doses will shape real-world prescribing.
Retatrutide is investigational as of April 2026 and not available by prescription or legitimate compounding. The schedule described here is the trial protocol, not a usage recommendation. This article covers the rationale behind the titration design, how it compares with semaglutide and tirzepatide, and the practical considerations that will apply once retatrutide reaches market.
What retatrutide dosing looks like
Retatrutide (LY3437943) is Eli Lilly's triple agonist peptide, activating GLP-1, GIP, and glucagon receptors. The dosing format follows the template established by semaglutide and tirzepatide: once-weekly subcutaneous injection from a pre-filled pen, stepwise titration from a low starting dose to maintenance, and indefinite continuation for weight loss maintenance.
The pharmacokinetic profile drives the weekly schedule. A half-life of approximately six days means steady-state plasma concentrations are reached after four to five weeks of consistent dosing at any given step. That is also why each titration step is held for four weeks: it gives the body time to reach equilibrium before the next dose increase, reducing the gastrointestinal effects that dominate the early side effect profile.
Injection sites are the abdomen, thigh, or upper arm, rotated weekly to minimise local skin reactions. The pen device is functionally equivalent to what Lilly uses for Mounjaro and Zepbound, so patients familiar with those products will find retatrutide administration familiar when it launches.
What the research says
The TRIUMPH Phase III protocol uses a fixed five-step titration: 2mg (weeks 1 to 4), 4mg (weeks 5 to 8), 6mg (weeks 9 to 12), and then maintenance at 9mg or 12mg from week 13 onward. This schedule was carried over from the Phase II trial (Jastreboff et al., New England Journal of Medicine, 2023, PMID: 37366315), where it was established as effective and tolerable.
| Week | Weekly Dose | Step Purpose |
|---|---|---|
| 1 to 4 | 2mg | Initial tolerance, starting dose |
| 5 to 8 | 4mg | First escalation, GI acclimation |
| 9 to 12 | 6mg | Mid-titration, monitoring continues |
| 13 to 16 | 9mg | Pre-maintenance (9mg arm) or step to 12mg |
| 17 onward | 9mg or 12mg | Maintenance dose, continued indefinitely |
The design rationale is straightforward: GLP-1 receptor agonists produce the strongest gastrointestinal side effects at first exposure and on dose escalation. A gradual titration reduces nausea, vomiting, and diarrhoea sufficiently to keep most participants on treatment through the escalation period. Trial-reported GI effects in TRIUMPH-4 were predominantly mild-to-moderate and resolved by the maintenance phase for most participants.
The choice between 9mg and 12mg as maintenance was tested head to head in TRIUMPH-4. The efficacy difference (26.4% vs 28.7%) was statistically meaningful but clinically modest. The safety difference was larger: dysesthesia appeared in 8.8% at 9mg and 20.9% at 12mg, and trial discontinuation rates ran higher at 12mg as well. See the full safety breakdown in Retatrutide Side Effects.
Practical breakdown
Assume retatrutide launches with a label that mirrors the TRIUMPH protocol: 2mg starting dose, 4-week step intervals, 9mg and 12mg both available as maintenance options. The practical questions for clinicians and patients then become: how strictly should the four-week step be followed, when should titration be slowed, and which maintenance dose is the default starting point.
On slower titration: extending each step to six or eight weeks when participants experience persistent nausea is already standard with semaglutide and tirzepatide. The trade-off is delayed time to maintenance dose (and therefore delayed peak efficacy) against better tolerability and adherence. For most patients, the adherence benefit outweighs the efficacy delay: participants who discontinue during titration get zero long-term benefit.
On maintenance dose selection, the case for defaulting to 9mg is strong. The efficacy advantage over tirzepatide (26.4% vs 22.5% in SURMOUNT-1) is preserved at 9mg; the dysesthesia rate is approximately one third of the 12mg rate; the discontinuation rate is lower. Participants who tolerate 9mg well and have not reached their weight loss goal can escalate to 12mg, but the marginal 2.3 percentage points of additional loss may not justify the safety cost for most users. The TRIUMPH-4 clinical trial results article has the full efficacy versus dose data.
On administration: injection site rotation matters more with retatrutide than with semaglutide because of the dysesthesia signal, which some early case discussions have suggested may present at or near injection sites in a subset of participants. The prevailing evidence points to dysesthesia as a systemic rather than local effect, but site rotation is standard practice and worth maintaining regardless.
Comparison: retatrutide vs semaglutide vs tirzepatide dosing
All three compounds use weekly subcutaneous injection with stepwise titration. The starting and maintenance doses differ, but the structural pattern is the same.
| Compound | Starting | Maintenance | Titration |
|---|---|---|---|
| Retatrutide | 2mg | 9mg or 12mg | ~12 weeks |
| Tirzepatide (Zepbound) | 2.5mg | 5mg to 15mg | ~20 weeks |
| Semaglutide (Wegovy) | 0.25mg | 2.4mg | ~16 weeks |
| Liraglutide (Saxenda) | 0.6mg | 3.0mg | ~5 weeks (daily) |
Retatrutide reaches maintenance faster than semaglutide or tirzepatide (12 weeks vs 16 to 20). The trade-off is fewer intermediate dose options for fine-tuning tolerability. Where semaglutide offers six titration steps and tirzepatide offers six as well, retatrutide offers five, with larger mg gaps between them. This is a design choice rather than a flaw: the Phase II data showed the titration worked, and Lilly carried it into Phase III without modification.
For the broader class context see Retatrutide vs Tirzepatide and Retatrutide vs Semaglutide, which cover the full head-to-head picture across efficacy, safety, and cost.
Frequently asked questions
What is the starting dose of retatrutide in Phase III trials?
Retatrutide trials start at 2mg weekly, given as a subcutaneous injection. This is the dose used for the first four weeks of the TRIUMPH protocol. The starting dose is low enough to minimise gastrointestinal side effects during the first exposure and allows tolerance to develop before escalation.
How is retatrutide titrated up to the maintenance dose?
The TRIUMPH titration schedule runs 2mg, 4mg, 6mg, then either 9mg or 12mg weekly. Each step is held for approximately four weeks before escalation. Full titration to the 12mg maintenance dose takes 12 weeks. This is similar to the titration used for semaglutide and tirzepatide, with longer step durations than typical off-label compounding protocols.
Is 9mg or 12mg the better maintenance dose?
The efficacy difference is modest (26.4% at 9mg versus 28.7% at 12mg over 68 weeks in TRIUMPH-4), but the dysesthesia rate more than doubles (8.8% at 9mg versus 20.9% at 12mg). Many clinicians will probably use 9mg as the default maintenance dose post-approval, reserving 12mg for participants who tolerate 9mg well and need further weight loss. This is a personal clinical judgement call that the label will inform.
How is retatrutide injected?
Retatrutide is administered by subcutaneous injection, typically into the abdomen, thigh, or upper arm. Rotate sites each week to reduce local skin reactions. The Phase III trials use pre-filled pen devices similar to Lilly's existing incretin products (Mounjaro, Zepbound). Self-administration at home is standard practice, with training typically provided at the first prescription.
What is the half-life of retatrutide?
Approximately six days, which is what supports the once-weekly dosing schedule. Steady-state plasma concentrations are reached after four to five weeks of consistent dosing at a given step. Missing a single dose is not catastrophic given the long half-life, but chronic inconsistency compromises efficacy and can increase side effects on re-initiation.
Can the titration be slowed for patients who do not tolerate the schedule?
Yes. The TRIUMPH trials allowed slower titration for participants with significant gastrointestinal side effects. Extending each step from four weeks to six or eight weeks is a common practice with semaglutide and tirzepatide and will likely carry over to retatrutide. The trade-off is slower time-to-maintenance-dose against improved tolerability.
What happens if a dose is missed?
Standard guidance for weekly GLP-1 agonists is: if the missed dose is noticed within 72 hours, take it and resume the regular schedule. If more than 72 hours have passed, skip that dose and take the next one at the usual time. Do not double-dose to compensate. Retatrutide will likely follow the same guidance once approved.
Is retatrutide available at any compounding pharmacy?
No. Retatrutide is investigational and not available through legitimate compounding pharmacies. It has not been approved by the FDA and is not on any approved compounding list. Grey-market sources selling retatrutide cannot verify purity, sterility, or dose accuracy, and carry substantial safety risks independent of the trial-reported side effect profile.
This article is for informational and educational purposes only and does not constitute medical advice. Retatrutide is an investigational drug. It has not been approved by the FDA, EMA, MHRA, or any other regulatory agency. Dosing information presented here describes clinical trial protocols, not recommendations for personal use. The compound is not available through legitimate prescription or compounding channels as of April 2026. Consult a licensed healthcare provider before starting, stopping, or changing any medication. Peptide File reports on research and does not sell, prescribe, or recommend sources for any compound discussed.