⚠️ Research use disclaimer

Retatrutide (LY3437943) is an investigational compound and is not approved by the FDA, MHRA, or any other regulatory body for clinical use as of July 2026. All dosage information on this page is drawn from published clinical trial protocols (Phase 2 NEJM 2023; TRIUMPH Phase 3 program) and is presented for research and educational purposes only. This is not medical advice. Do not use this information to self-administer or prescribe any compound.

What is retatrutide and why does dosage matter?

Retatrutide (LY3437943) is a first-in-class triple receptor agonist developed by Eli Lilly that simultaneously activates three metabolic hormone receptors: GLP-1 (glucagon-like peptide-1), GIP (glucose-dependent insulinotropic polypeptide), and glucagon. This triple mechanism distinguishes retatrutide from every other compound in its class — tirzepatide activates GLP-1 and GIP (dual), while semaglutide activates GLP-1 alone (single).

The addition of the glucagon receptor — absent from all other approved or late-stage metabolic peptides — activates energy expenditure pathways that add a thermogenic component unique to retatrutide. This is widely believed to explain the unprecedented magnitude of weight reduction observed in both Phase 2 and Phase 3 clinical trials.

Understanding retatrutide dosage is critical precisely because of this triple mechanism. Each receptor pathway activates at different plasma concentrations and produces different physiological effects. The dose escalation protocol used in the TRIUMPH trials is not arbitrary — it is designed to allow each receptor layer to be introduced progressively, minimising adverse effects while maximising efficacy over time.

TRIUMPH Phase 3 data — what we know as of July 2026

28.7%
Weight reduction at 12mg — TRIUMPH-4 (68 weeks)
28.3%
Weight reduction at 12mg — TRIUMPH-1 (80 weeks, 2,339 patients)
58.6%
Participants achieving ≥25% loss — TRIUMPH-4
16.8%
Weight reduction in T2D cohort — TRANSCEND-T2D-1

TRIUMPH-4 (December 2025) was the first successful Phase 3 readout for retatrutide. The trial enrolled adults with obesity or overweight and knee osteoarthritis. Participants on the 12mg weekly dose achieved a mean body weight reduction of 28.7% at 68 weeks — equivalent to an average of 32.3kg (71.2 lbs) lost. This is the largest weight-loss signal ever reported in a Phase 3 randomised controlled trial of any GLP-1-class compound.

TRIUMPH-1 (May 2026) confirmed these results at larger scale — 2,339 participants in a general obesity population achieved 28.3% weight reduction at 80 weeks. In participants with BMI ≥35, a 104-week extension cohort achieved 30.3% — entering bariatric surgery territory without the surgical risk.

Key Phase 3 secondary findings

Beyond weight loss, TRIUMPH-4 reported a 75.8% reduction in knee osteoarthritis pain (WOMAC pain subscale), a ~20% reduction in LDL cholesterol, a 14mmHg reduction in systolic blood pressure at 12mg, and a 72% reversal rate of prediabetes to normoglycemia. A 60.6% reduction in apnoea-hypopnea index (AHI) was also confirmed in the TRIUMPH-1 sleep apnoea substudy at the ADA Scientific Sessions, June 2026.

As of July 2026, Eli Lilly has confirmed a Q4 2026 NDA filing target with the FDA. Market launch is projected for Q1–Q2 2028 based on standard FDA review timelines. Five additional TRIUMPH Phase 3 trials covering type 2 diabetes, cardiovascular outcomes, sleep apnoea, liver disease, and chronic low back pain remain ongoing.

Retatrutide dosage tiers — Phase 2 and Phase 3 reference

The table below summarises all dosage levels studied in the Phase 2 NEJM 2023 trial and the TRIUMPH Phase 3 programme. Note that 9mg and 12mg are the Phase 3 validated maintenance doses, while 1mg, 4mg, and 8mg remain Phase 2 / titration reference points.

Dose Phase Role in protocol Weight reduction signal Notes
1mg / week Phase 2 Research reference only ~7.9% at 48 weeks Meaningful signal present but not used in Phase 3. Not a standalone maintenance dose.
2mg / week Titration Starting dose (weeks 1–4) Low — tolerability phase The TRIUMPH Phase 3 starting point. GI adaptation period. Not therapeutic for weight loss at this level.
4mg / week Titration Escalation step (weeks 5–8) ~17.5% at 48 weeks (Phase 2) Largest relative dose jump (4× from 1mg). GI side effects most commonly peak here. Monitor closely.
8mg / week Phase 2 Escalation step (weeks 9–12) ~22.8% at 48 weeks (Phase 2) Phase 2 maintenance dose. Not evaluated as standalone Phase 3 maintenance. Bridge to 9mg or 12mg.
9mg / week Phase 3 Phase 3 maintenance option 26.4% at 68 weeks (TRIUMPH-4) Recommended for participants with dysesthesia sensitivity. Lower side effect burden than 12mg with strong efficacy.
12mg / week Phase 3 max Primary Phase 3 maintenance dose 28.7% at 68 weeks (TRIUMPH-4) · 28.3% at 80 weeks (TRIUMPH-1) The primary Phase 3 efficacy dose. Highest weight reduction signal ever recorded in a Phase 3 GLP-1 trial. Highest dysesthesia incidence (20.9% vs 8.8% at 9mg).

Week-by-week titration schedule — TRIUMPH Phase 3 protocol

The following titration schedule is based on the TRIUMPH Phase 3 clinical trial protocol, consistent with the approach used in both TRIUMPH-4 and TRIUMPH-1. Retatrutide reaches its 12mg maintenance dose at week 13 — notably faster than tirzepatide (16–20 weeks) and semaglutide (16 weeks).

2
Weeks 1–4 · Starting dose
2mg once weekly
The GI adaptation phase. The 2mg starting dose is not therapeutic in terms of weight loss — its purpose is to allow GI and receptor systems to adapt before higher plasma concentrations arrive. Most researchers experience minimal side effects at this level. Subcutaneous injection into abdomen, thigh, or upper arm on a rotating basis.
4
Weeks 5–8 · First escalation
4mg once weekly
The largest relative jump in the protocol (4× increase from the starting dose). This is when GI side effects — nausea, decreased appetite, vomiting — most commonly peak. Phase 2 data suggests approximately 17.5% weight reduction at maintenance across 48 weeks if a research subject were held at this level. The GIP receptor pathway begins to produce meaningful signal here.
8
Weeks 9–12 · Second escalation
8mg once weekly
The bridge dose. Used in Phase 2 as a maintenance level, now functioning as a transitional step toward 9mg or 12mg in Phase 3. Phase 2 data showed approximately 22.8% weight reduction at this level. The glucagon receptor thermogenic pathway is now active at meaningful concentrations — energy expenditure increases become measurable at 8mg and above.
12
Week 13 onwards · Maintenance
9mg or 12mg once weekly
9mg: Phase 3 validated maintenance option with 26.4% weight reduction at 68 weeks (TRIUMPH-4). Dysesthesia incidence 8.8% — significantly lower than 12mg. Preferred for participants with heightened skin sensitivity. 12mg: Primary Phase 3 efficacy dose. 28.7% weight reduction at 68 weeks, 28.3% at 80 weeks. Highest efficacy, highest side effect burden — 20.9% dysesthesia incidence. The data supports 9mg as a strong alternative when tolerability is prioritised over maximum efficacy.
Important — dose restart guidance

If research use is interrupted for more than 2 weeks, clinical protocols recommend restarting at a lower dose level rather than resuming at the previous maintenance dose. Re-exposure at high concentrations after a gap carries a higher risk of GI adverse effects as receptor sensitivity partially resets during the off period.

Retatrutide vs tirzepatide vs semaglutide — efficacy comparison

The following comparison uses Phase 3 data across all three compounds at their primary maintenance doses. Retatrutide’s triple-agonist mechanism produces the largest weight reduction signal of any GLP-1-class compound studied in a randomised Phase 3 trial to date.

Retatrutide 12mg (Phase 3 — TRIUMPH-1, 80 weeks)
28.3%
Triple agonist: GLP-1 + GIP + Glucagon · NDA target Q4 2026
Tirzepatide 15mg (Phase 3 — SURMOUNT-1, 72 weeks)
22.5%
Dual agonist: GLP-1 + GIP · FDA approved (Zepbound)
Semaglutide 2.4mg (Phase 3 — STEP-1, 68 weeks)
14.9%
Single agonist: GLP-1 only · FDA approved (Wegovy)
Bariatric surgery (Roux-en-Y gastric bypass — historical benchmark)
~30%
Surgical benchmark — retatrutide at 12mg approaches this without surgical risk

Side effect profile by dose level

Retatrutide’s side effect profile follows a dose-dependent pattern consistent with other GLP-1 class compounds, with one novel finding specific to its triple mechanism: dysesthesia (abnormal skin sensations including tingling, burning, or crawling sensations), which appears to be linked to glucagon receptor activation and is not observed with semaglutide or tirzepatide.

2mg
Starting dose · Weeks 1–4

  • Mild nausea (low incidence)

  • Reduced appetite

  • Occasional fatigue

  • Dysesthesia: rare

4mg
Escalation · Weeks 5–8

  • Nausea (peak incidence)

  • Decreased appetite (strong)

  • Vomiting (some subjects)

  • Diarrhoea / constipation

  • Dysesthesia: low

8mg
Escalation · Weeks 9–12

  • GI effects moderate

  • Injection site reactions

  • Heart rate increase (mild)

  • Dysesthesia: emerging

9mg
Phase 3 maintenance

  • GI effects diminishing

  • Dysesthesia: 8.8%

  • Headache (some subjects)

  • Hair thinning (rare)

12mg
Phase 3 maximum dose

  • Dysesthesia: 20.9%

  • Nausea (residual)

  • Heart rate elevation

  • Hair thinning (some)

  • Cholecystitis risk (elevated)

Dysesthesia — the novel retatrutide side effect

Dysesthesia (abnormal skin sensations including tingling, burning, or crawling feelings) is a novel finding specific to retatrutide and is believed to be linked to glucagon receptor activation — a pathway absent in tirzepatide and semaglutide. In TRIUMPH-4, dysesthesia occurred in 8.8% of participants at 9mg and 20.9% at 12mg, compared to ~1–2% in the placebo group. This is a primary reason the 9mg dose is considered an efficacy-versus-tolerability trade-off worth evaluating in research contexts.

Sourcing retatrutide for UK research — what to look for

Retatrutide is not yet approved for prescription use and has no legal commercial manufacturing pathway as of July 2026. Research peptide suppliers in the UK supply retatrutide pen kits for scientific research use only. When evaluating a UK supplier, the following criteria are the minimum acceptable standard:

Authentication checklist

Certificate of Analysis (COA) — Every authentic batch must be accompanied by a COA from an independent third-party laboratory. The COA should display the lab’s header, batch number, compound identity confirmation, and purity percentage. A COA that only shows the supplier’s own branding with no independent lab reference is not a valid COA.

Batch traceability — The batch number on the COA must match the batch number on the pen kit itself. No match means no traceability — a significant quality concern.

UK dispatch — Legitimate UK research peptide suppliers dispatch from UK addresses. Packages arriving from China with 10–14 day delivery windows are not UK-sourced, regardless of what the website claims. UK dispatch means 2–3 day delivery as standard.

Registered business — Verify the supplier has a Companies House registration and a UK VAT number. Telegram-only ordering with no traceable business registration is a major red flag for counterfeit product.

Prefilled pen format — Synedica UK’s Retatrutide 40mg is supplied as a prefilled injection pen kit, which offers precise, repeatable dosing — critical for research consistency. Raw lyophilised powder requires reconstitution and carries higher contamination risk.

⚠️ Counterfeit warning

Counterfeit retatrutide pen kits manufactured in China have entered the UK market in 2025–2026. These products are frequently sold via Telegram channels, priced significantly below market rate, and ship with fraudulent COA documentation. Counterfeits have been found to contain incorrect compounds, incorrect concentrations, or no active ingredient at all. Purchase only from traceable UK-registered suppliers with independently verifiable batch documentation.

Frequently asked questions — retatrutide dosage

What is the starting dose of retatrutide in the TRIUMPH protocol?+
Based on the TRIUMPH Phase 3 clinical trial protocol, retatrutide titration begins at 2mg once weekly for the first 4 weeks. This low starting dose allows GI adaptation before higher plasma concentrations of the triple-agonist compound are introduced. The 2mg level is not therapeutic for weight loss — its purpose is tolerability.
What is the maximum retatrutide dose studied in Phase 3?+
The maximum dose evaluated in the TRIUMPH Phase 3 programme is 12mg once weekly. Both TRIUMPH-4 (December 2025) and TRIUMPH-1 (May 2026) used 9mg and 12mg as the two primary maintenance dose arms. The 12mg dose produced 28.7% mean body weight reduction at 68 weeks in TRIUMPH-4 — the largest Phase 3 GLP-1-class weight-loss signal ever recorded.
How does retatrutide dosage compare to tirzepatide and semaglutide?+
Retatrutide at 12mg weekly produced 28.3% weight reduction at 80 weeks (TRIUMPH-1), compared to 22.5% for tirzepatide at 15mg weekly (SURMOUNT-1, 72 weeks) and 14.9% for semaglutide 2.4mg weekly (STEP-1, 68 weeks). Retatrutide also reaches its maintenance dose faster — 13 weeks vs 16–20 weeks for tirzepatide and 16 weeks for semaglutide. The trade-off is a novel side effect (dysesthesia) not seen with the other two compounds, linked to retatrutide’s unique glucagon receptor activity.
How long does it take to reach the 12mg maintenance dose?+
Following the TRIUMPH Phase 3 titration protocol: 2mg (weeks 1–4) → 4mg (weeks 5–8) → 8mg (weeks 9–12) → 12mg (week 13 onwards). The 12mg maintenance dose is reached at week 13 — notably faster than tirzepatide, which takes 16–20 weeks, and semaglutide, which takes 16 weeks.
What is dysesthesia and why is it specific to retatrutide?+
Dysesthesia refers to abnormal skin sensations including tingling, burning, or crawling feelings. It is a novel side effect specific to retatrutide and is believed to be linked to glucagon receptor activation — a pathway not present in tirzepatide or semaglutide. In TRIUMPH-4, dysesthesia occurred in 8.8% of participants at 9mg and 20.9% at 12mg. This is why 9mg is considered a valid lower-side-effect alternative for research subjects who show sensitivity. Dysesthesia is generally mild-to-moderate and does not require discontinuation in most cases.
Where can I buy retatrutide pen kits for research in the UK?+
Synedica UK supplies pharmaceutical-grade Retatrutide 40mg prefilled pen kits for research use. All kits are accompanied by a Certificate of Analysis (COA) from an independent laboratory, dispatched from the UK (2–3 day delivery), and come with free returns. Visit synedica-uk.shop to view current stock and documentation. When comparing UK suppliers, always verify independent COA documentation, UK dispatch, and registered business details before purchase.

Synedica UK — Retatrutide 40mg research pen kit

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