Retatrutide Dosing Protocol: Phase 2 Trial Reference for Researchers

Retatrutide (LY3437943) is the first triple receptor agonist — simultaneously targeting GLP-1, GIP, and Glucagon receptors — to reach human clinical trials. The Phase 2 dose-finding study (NCT04999256), published in the New England Journal of Medicine in July 2023 (doi:10.1056/NEJMoa2301972), enrolled 338 adults across seven cohorts over 48 weeks. The primary endpoint was percentage change in body weight from baseline at week 24, with 48-week data reported as a key secondary outcome.

Phase 2 cohorts included Retatrutide at 1mg, 4mg, 8mg, and 12mg administered weekly (QW), and 4mg, 8mg, and 12mg administered every two weeks (Q2W), compared against placebo. All active cohorts used a step-up escalation from a 2mg initiation dose, with the rate of escalation proportional to target maintenance dose. The 12mg QW cohort achieved the highest reported mean weight loss of any compound in a completed trial to date: −24.2% from baseline at 48 weeks.

This article summarises Phase 2 dosing protocols as published, for use as a research reference. Retatrutide supplied by RetaLABS is for laboratory research use only — not for human consumption.

Escalation intervals were 4 weeks at each step. The following summarises the weekly (QW) cohort escalation schedules as reported in the published trial:

Weeks	1mg Cohort	4mg Cohort	8mg Cohort	12mg Cohort
1–4	1 mg	2 mg	2 mg	2 mg
5–8	1 mg (maint.)	4 mg (maint.)	4 mg	4 mg
9–12	—	—	6 mg	8 mg
13–16	—	—	8 mg (maint.)	10 mg
17–48	—	—	—	12 mg (maint.)

The 1mg cohort was a single-step initiation — dosed at 1mg throughout without escalation. This cohort served as the low-dose reference arm and still produced a mean −7.9% weight loss at 48 weeks, demonstrating clinically meaningful activity even at sub-therapeutic doses relative to the maximum studied.

Researchers should note that Phase 2 trial protocols are designed with safety margins appropriate for a first-in-human dose-finding context. Phase 3 TRIUMPH protocols may use different escalation schedules — published Phase 3 data will supersede Phase 2 reference schedules once available.

RetaLABS Retatrutide is available in 20mg and 30mg lyophilised vials. Reconstitution with bacteriostatic water establishes the working concentration from which individual doses are drawn.

20mg vial — reconstituted with 2.0mL bacteriostatic water → 10mg/mL

Dose	Volume at 10mg/mL	U100 Units
1 mg	0.10 mL	10 units
2 mg	0.20 mL	20 units
4 mg	0.40 mL	40 units
6 mg	0.60 mL	60 units
8 mg	0.80 mL	80 units

30mg vial — reconstituted with 3.0mL bacteriostatic water → 10mg/mL

Same concentration as above; the 30mg vial provides sufficient quantity for the full 12mg escalation schedule (2 + 2 + 2 + 2 + 4 + 4 + 4 + ... mg per week). At 10mg/mL, 10mg weekly doses require 1.0mL per injection — which may necessitate splitting across two injections at the same site visit for researchers preferring smaller injection volumes.

For reconstitution technique, see the Peptide Reconstitution & Storage Guide. Reconstituted Retatrutide should be stored refrigerated at 2–8°C and used within 4 weeks.

Retatrutide's triple agonist mechanism produces a pharmacodynamic profile that differs meaningfully from dual or single-receptor agents, with implications for research protocol design:

• GLP-1 receptor agonism: suppresses appetite via hypothalamic and brainstem pathways, slows gastric emptying, stimulates glucose-dependent insulin secretion
• GIP receptor agonism: augments insulin secretion, improves adipose tissue lipid handling, contributes to weight loss through mechanisms distinct from GLP-1 (consistent with Tirzepatide dual agonism data)
• Glucagon receptor agonism: increases hepatic glucose output (requiring GLP-1 co-agonism to prevent net hyperglycaemia), elevates energy expenditure via thermogenic mechanisms, promotes lipolysis — this third pathway is the primary mechanistic distinction from Tirzepatide

Pharmacokinetic parameters from Phase 2 data:

• Half-life: approximately 6 days (enables once-weekly subcutaneous dosing)
• Tmax: approximately 24–72 hours post-injection
• Steady state: reached at approximately 4–5 weeks of weekly dosing

The glucagon receptor component is implicated in both the elevated energy expenditure signal and the nausea profile observed at higher doses in Phase 2. Researchers designing protocols that compare Retatrutide to Tirzepatide or Semaglutide should account for the additional glucagon pathway as a confounding variable in multi-compound designs.

The Phase 2 trial reported weight loss at 24 and 48 weeks across all cohorts. Key 48-week findings from Jastreboff et al. (NEJM 2023):

Cohort	Mean Weight Change at 48 Weeks
Placebo	−2.1%
1 mg QW	−7.9%
4 mg QW	−16.3%
8 mg QW	−22.8%
12 mg QW	−24.2%

A notable feature of the Phase 2 data is that weight loss curves at week 48 had not yet plateaued in the higher-dose cohorts — suggesting that a longer maintenance period (as used in SURMOUNT and STEP programmes, which ran to 72–88 weeks) may produce further reductions. Phase 3 TRIUMPH trial results will clarify the long-term plateau trajectory.

For mechanistic comparison with Tirzepatide, see the Retatrutide vs Tirzepatide Research Comparison. For full Phase 2 data context, see the Retatrutide Research Guide.

Retatrutide Phase 2 injections were administered subcutaneously in the abdomen, with the same site rotation principles applicable to all once-weekly peptide research protocols. Higher-dose cohorts (8mg, 12mg) produced injection site reactions (redness, swelling, itching) at a higher frequency than lower-dose cohorts — consistent with the expected local tissue response to larger volumes of GLP-1 class peptides at concentrated doses.

Key protocol considerations specific to Retatrutide research:

• GI adverse event rate: higher than Tirzepatide at comparable dose ranges in Phase 2 — nausea, vomiting, and decreased appetite were the most common adverse events, predominantly occurring during dose escalation; the glucagon receptor component may contribute to the elevated upper GI signal
• Energy expenditure observation: researchers should note that glucagon agonism increases resting metabolic rate — studies measuring energy expenditure endpoints should baseline-measure before the first dose and at each escalation step
• Liver lipid effects: glucagon receptor agonism drives hepatic lipid export; protocols examining hepatic steatosis endpoints may observe more rapid change compared to GLP-1-only designs
• Phase 3 protocols: TRIUMPH trial designs may use different escalation schedules and will be published in their clinical trial registration — researchers should consult the current NCT registration for the most up-to-date protocol specifications

All content is for laboratory research reference only. RetaLABS Retatrutide is not for human consumption.