What is retatrutide?
Retatrutide (also called LY3437943) is a once-weekly injectable peptide developed by Eli Lilly that activates three different metabolic receptors simultaneously: GLP-1, GIP, and glucagon.[1] Where tirzepatide hits two of those, retatrutide hits all three — and the glucagon arm is what appears to drive the larger weight-loss numbers in trials.[2]
It's currently in phase-III trials for obesity and type-2 diabetes. It is not FDA-approved. Anyone using it today is either enrolled in a trial or is acquiring it through grey-market channels — which carries real legal, purity, and dosing risks we cover in the side effects section.
How it works.
The three receptors retatrutide hits each do something different:
- GLP-1 agonism slows gastric emptying, reduces appetite centrally, and improves glycemic control — the same mechanism behind semaglutide.
- GIP agonism appears to potentiate the GLP-1 effect and may improve how fat tissue handles nutrient overflow — this is tirzepatide's edge.
- Glucagon agonism increases resting energy expenditure (you burn more at rest) and drives hepatic fat reduction. This is retatrutide's unique addition, and the leading hypothesis for why phase-II loss numbers exceeded tirzepatide's.[3]
The combined effect: reduced intake plus increased burn. Most weight-loss peptides only do the first.
Dosing protocol.
Trial protocols start at 2 mg/week and titrate up every 4 weeks. The titration ramp is the key to tolerating GI side effects — skipping steps reliably triggers severe nausea.
Injection site rotation
Abdomen, outer thigh, posterior upper arm. Rotate weekly to prevent injection-site lipohypertrophy. Most users find abdominal injections sting less due to less subq fascia density.
Epti turns this table into your actual schedule.
Log doses, track titration weeks, get reminders before site-rotation day — and overlay it on your training and bloodwork.
Common stacks.
Retatrutide is rarely run alone in community protocols — the weight-loss arm is usually paired with either a recovery peptide (to protect connective tissue during a deficit) or a GH-axis support to preserve lean mass.
Retatrutide + BPC-157
The most common community protocol. BPC-157 covers connective-tissue stress from rapid loss; appetite suppression keeps the deficit hands-off.
Retatrutide + Ipamorelin / CJC-1295
For users with significant training history. GH-axis support is the leading hypothesis for retaining lean mass during fast weight loss. Data is sparse — proceed conservatively.
Side effects.
Phase-II data — and community reports — group retatrutide's side-effect profile cleanly into three tiers. The GI tier is by far the most common reason users discontinue.
GI symptoms
Nausea, constipation, diarrhea, early satiety. Peaks 24–48h post-injection. Worst during titration jumps.
Heart-rate elevation
Resting HR commonly rises 4–8 bpm. Worth tracking — and worth pausing the titration if HR exceeds your baseline by >15.
Lean-mass loss
Rapid loss without resistance training drives lean-mass loss, same as any deep deficit. Lift, eat protein.
What's not in the data yet: retatrutide is <5 years old in humans. Long-term safety beyond 88 weeks is unknown. Treat anyone claiming certainty here with deep skepticism.
What the data says.
The headline result is from Jastreboff et al. (NEJM 2023): a phase-II trial of 338 adults with obesity, randomized across four doses and placebo, over 48 weeks.[1] Key numbers:
- At the 12 mg/week dose, mean weight loss was 24.2% vs. 2.1% on placebo.
- ~50% of the 12 mg arm lost ≥25% of body weight.
- Dropout for adverse events was 6–13% across active arms.
- Blood pressure and lipid panels improved across all doses — likely independent of loss.
Phase-III TRIUMPH trials are ongoing; topline expected late 2026. Until then, real-world data is anecdotal and selection-biased — keep that frame.
Frequently asked.
Is retatrutide better than tirzepatide?
Phase-II numbers are higher — ~24% vs. ~22% loss at comparable timepoints. But tirzepatide has years of post-market safety data; retatrutide does not. "Better on paper" is not the same as "the right choice for you."
What's the half-life of retatrutide?
Approximately 6 days, supporting once-weekly dosing. Steady-state is reached around week 4–5 of any given dose.
Can I inject retatrutide twice a week instead?
Some users do, splitting the weekly dose into two half-doses 3–4 days apart, citing flatter side-effect curves. The trial protocol is weekly; there is no evidence this changes efficacy meaningfully.
Will I lose muscle on retatrutide?
If you don't lift and don't eat enough protein — yes, the same as any deep caloric deficit. Resistance training plus 1.6–2.2 g/kg protein preserves the vast majority of lean mass in trial subgroups.
Is retatrutide legal?
It is not FDA-approved, not scheduled, and not a controlled substance — but it is also not legally available for sale as a human therapeutic. Possession laws and sport/league anti-doping rules vary. This is not legal advice.
References.
- Jastreboff A.M. et al. (2023). Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. PMID: 37364103
- Rosenstock J. et al. (2023). Retatrutide in Type 2 Diabetes: Phase 2 Randomized Trial. Lancet. PMID: 37364104
- Coskun T. et al. (2022). LY3437943: A novel GIP, GLP-1 and glucagon receptor agonist. Cell Metab. PMID: 35508109
- ClinicalTrials.gov NCT05882045 — TRIUMPH-1 phase-III study.
- Lilly investor materials, Q4 2025.