Retatrutide (LY3437943) and semaglutide sit at opposite ends of the injectable metabolic peptide timeline. Semaglutide is an approved, well-characterized GLP-1 mono-agonist with years of post-marketing data. Retatrutide is an investigational triple receptor agonist still in Phase 3, with the largest weight loss numbers ever published for a pharmaceutical at the time of its Phase 2 readout. They are injected the same way, once weekly, subcutaneously, but almost everything else differs.
Mechanism: One Receptor vs Three
Semaglutide activates a single target, the GLP-1 receptor. That drives glucose-dependent insulin secretion, suppresses glucagon release, slows gastric emptying, and acts on hypothalamic appetite centers. The result is reduced caloric intake with improved glycemic control.
Retatrutide activates three: GLP-1, GIP (glucose-dependent insulinotropic polypeptide), and the glucagon receptor. The GLP-1 and GIP arms produce appetite suppression and insulin sensitization similar to tirzepatide. The glucagon arm is the genuine departure. Glucagon receptor agonism raises hepatic glucose output and increases resting energy expenditure, adding a thermogenic component that neither semaglutide nor tirzepatide has.
Practically, that means semaglutide works almost entirely on the intake side of the energy balance equation, while retatrutide works on both intake and expenditure. It is the most plausible explanation for the gap in trial results.
Dose Ranges: A 5-Fold Difference
This is the difference that matters most when you sit down to reconstitute a vial. Semaglutide is dosed in fractions of a milligram. Retatrutide is dosed in whole milligrams, topping out five times higher than semaglutide ever goes.
- Semaglutide starting dose: 0.25 mg (250 mcg) once weekly, held for 4 weeks
- Semaglutide maximum labeled dose: 2.4 mg once weekly (Wegovy)
- Retatrutide starting dose: 1 mg once weekly, held for 4 weeks
- Retatrutide maximum dose studied in Phase 2: 12 mg once weekly
- Half-life: roughly 7 days for semaglutide, roughly 6 days for retatrutide (both support weekly cadence)
Reconstitution: Concentration Strategy Diverges
Because semaglutide doses are small, a low concentration keeps the draw volume readable on a U-100 syringe. A 2 mg vial with 2 mL of bacteriostatic water gives 1 mg/mL, so a 0.25 mg starting dose is 0.25 mL, or 25 units. Clean and unambiguous.
Retatrutide at that same 1 mg/mL concentration would require 12 mL for a 12 mg dose, which is absurd. A 10 mg vial with 2 mL of BAC water gives 5 mg/mL, so a 1 mg starter dose is 0.2 mL (20 units) and a 4 mg dose is 0.8 mL (80 units). At 12 mg, even 5 mg/mL leaves you drawing 2.4 mL, which exceeds a 1 mL syringe. Researchers running the top of the titration ladder typically reconstitute at 10 mg/mL (10 mg vial with 1 mL BAC water) so a 12 mg dose lands at 1.2 mL, or split the dose across two injections.
- Semaglutide 2 mg vial + 2 mL BAC water = 1 mg/mL (0.25 mg = 25 units)
- Semaglutide 10 mg vial + 2 mL BAC water = 5 mg/mL (2.4 mg = 48 units)
- Retatrutide 10 mg vial + 2 mL BAC water = 5 mg/mL (1 mg = 20 units, 4 mg = 80 units)
- Retatrutide 10 mg vial + 1 mL BAC water = 10 mg/mL (8 mg = 80 units, 12 mg = 120 units, over one syringe)
Run both compounds through the PeptiTools reconstitution calculator before your first injection of a new vial. Enter the vial size in mg, the BAC water volume in mL, and your target dose, and it returns the exact unit mark with a live syringe diagram.
Titration Schedules Side by Side
Both compounds require slow escalation, and for the same reason: gastrointestinal side effects are dose-dependent and front-loaded. Both protocols hold each step for 4 weeks minimum before advancing.
- Semaglutide: 0.25 mg for weeks 1-4, 0.5 mg for weeks 5-8, 1.0 mg for weeks 9-12, 1.7 mg for weeks 13-16, then 2.4 mg from week 17 onward.
- Retatrutide: 1 mg for weeks 1-4, 2 mg for weeks 5-8, 4 mg for weeks 9-12, 8 mg for weeks 13-16, then 12 mg from week 17 onward.
- Either protocol: if nausea, vomiting, or diarrhea is intolerable at a given step, hold at the current dose an extra 4 weeks rather than pushing forward.
- Retatrutide only: the glucagon arm can transiently raise fasting glucose and heart rate early in treatment, so monitor both if you have diabetes, prediabetes, or a cardiac history.
What the Trial Data Actually Shows
The Phase 2 retatrutide obesity trial (NEJM, 2023) reported mean body weight reductions of 17.5% at 4 mg, 22.8% at 8 mg, and 24.2% at 12 mg over 48 weeks. Roughly a quarter of participants at the top dose lost 30% or more of body weight, a magnitude previously seen only after bariatric surgery.
The STEP 1 semaglutide trial reported 14.9% mean body weight reduction at 2.4 mg over 68 weeks. Semaglutide also carries something retatrutide does not yet have: hard cardiovascular outcome data. The SELECT trial demonstrated a 20% reduction in major adverse cardiovascular events in people with overweight or obesity and established cardiovascular disease.
The critical caveat is that these were separate trials with different durations, populations, and enrollment criteria. There has never been a head-to-head randomized comparison of retatrutide and semaglutide. The 24.2% vs 14.9% figure is a comparison of two independent readouts, not a measured difference.
Regulatory Status and Practical Availability
Semaglutide is FDA-approved as Ozempic (type 2 diabetes) and Wegovy (chronic weight management), with published prescribing information, established manufacturing, and a known safety profile from millions of patient-years. Retatrutide has no approved indication anywhere. It exists as an investigational compound in Phase 3 trials and as a research chemical sold for laboratory use, meaning purity, actual mg content, and sterility are not verified by any regulator.
That asymmetry matters more than any efficacy number. With retatrutide you are accepting uncertainty about what is actually in the vial on top of uncertainty about long-term safety of chronic triple agonism, which no one has more than a few years of data on.
Which Comparison Is More Useful
If your real question is about the incremental step rather than the extremes, the closer comparison is retatrutide against tirzepatide, since both cover GLP-1 and GIP and the only variable is the glucagon arm. Retatrutide vs semaglutide compares two generations at once, so the gap conflates the GIP addition with the glucagon addition.
Storage: Identical Requirements
Both are lyophilized powders stable at room temperature before reconstitution and refrigerated at 2-8°C for up to 28 days after. Neither should be frozen once mixed, and neither should be shaken. Discard either if the solution turns cloudy or develops particulates. See the peptide storage guide for handling detail.