Reader’s guide · First edition · September 2026 · Revised October 1, 2026
Retatrutide
A guide to the triple GIP / GLP-1 / glucagon agonist
Retatrutide adds a third hormone, glucagon, to the GLP-1 and GIP pair behind tirzepatide. In Phase 3 trials it has produced the largest weight loss ever recorded for a drug, approaching what bariatric surgery achieves. It is not yet approved, and no pharmacy can lawfully compound it. This guide covers what the trials show, what the drug does to the body, and why the vials being sold as retatrutide today are not what they claim.
Who this is for
Written for adults who are following retatrutide's path to approval, or who have been offered a product called retatrutide by a clinic, pharmacy, or website. It summarizes the published record and FDA's position. It is not medical advice.
Retatrutide (Lilly's LY3437943) is a synthetic peptide engineered to activate three receptors at once: GLP-1 and GIP, the incretin pair that tirzepatide targets, plus the glucagon receptor. A fatty-acid side chain binds it to albumin in the blood, giving it a half-life of about six days, so it is injected once a week.
- Class
- Triple GIP, GLP-1, and glucagon receptor agonist
- Route
- Subcutaneous injection, once weekly
- Half-life
- About 6 days
- Status
- Investigational. Phase 3 (TRIUMPH) program under way; Lilly has said it plans to file for FDA approval in the first quarter of 2027. Not approved anywhere as of this edition.
- Maker
- Eli Lilly and Company
Not approved, and not compoundable
There is no approved retatrutide product and no lawful compounded one. In April 2025 the FDA told every state board of pharmacy that retatrutide is not eligible for compounding under either section 503A or 503B. Anything sold as retatrutide today is an unapproved drug from an unregulated source. Section 05 explains.
GLP-1 and GIP are gut hormones that prepare the body for a meal: more insulin, less appetite, slower digestion. Glucagon is the opposite signal in one respect, telling the liver to release sugar, but it also raises energy expenditure and burns liver fat. Retatrutide pairs the appetite-cutting incretins with glucagon's calorie-burning effect, while the incretins keep glucagon's sugar-raising side in check.
| GLP-1 | GIP | Glucagon | |
|---|---|---|---|
| Pancreas | Insulin up, glucagon down, when sugar is high | Insulin up after meals | Would raise sugar; offset by the incretins |
| Stomach | Slows emptying; meals feel larger | Little direct effect | Little direct effect |
| Brain | Lowers appetite and food reward | Separate appetite circuits; may blunt nausea | Modest appetite effect |
| Liver and energy | Indirect, via weight loss | Better fat handling and insulin sensitivity | Burns liver fat; raises energy expenditure |
The glucagon lever is what separates retatrutide from tirzepatide: more weight loss and a striking drop in liver fat, but also a larger rise in resting heart rate.
Phase 3 figures are topline results
The TRIUMPH numbers below come from Lilly's press releases and have not yet appeared in peer-reviewed journals as of this edition. They describe Lilly's manufactured drug at verified doses under trial monitoring. None of it applies to a vial bought outside a trial.
Body weight at 80 weeks on 12 mg, adults with obesity without diabetes (TRIUMPH-1); placebo −2.2%
Share of 12 mg participants who lost 30% or more of their body weight at 80 weeks (TRIUMPH-1)
Body weight at 68 weeks on 12 mg, obesity with knee osteoarthritis (TRIUMPH-4); placebo −2.1%
Relative reduction in liver fat at the higher doses in the Phase 2 liver substudy
The Phase 2 obesity trial reported 24.2% weight loss at 48 weeks on 12 mg, and weight was still falling when it ended. In TRIUMPH-1, participants with a starting BMI of 35 or more who continued to 104 weeks reached an average loss of 30.3%. In type 2 diabetes, Phase 2 cut HbA1c by about 2 points at 36 weeks. Lower doses did less: 19.0% at 4 mg and 25.9% at 9 mg over 80 weeks in TRIUMPH-1.
There is no labeled dose, because there is no label. What exists is the trial protocol: start low, step up roughly every four weeks, and settle at a maintenance dose chosen by tolerability.
| Dose | Role | What it produced (TRIUMPH-1, 80 weeks) |
|---|---|---|
| 2 mg | Starting dose in the escalation schedules | Not a maintenance dose |
| 4 mg | Lowest maintenance dose | −19.0% body weight |
| 9 mg | Middle maintenance dose | −25.9% body weight |
| 12 mg | Highest maintenance dose | −28.3% body weight |
Escalation steps and timing differ between trials. Whatever dose is eventually approved may not match these.
What the next table is, and is not
It expresses the trial escalation for the vial size sold under this name, so that a reader can recognize it and check the arithmetic. It is not an instruction. There is no lawful compounded retatrutide, and the vials sold today have no verified concentration, so the units column assumes a label nobody has checked.
- Vial
- 30 mg, as sold; identity and quantity unverified
- Water added
- 1.5 mL bacteriostatic water
- Concentration
- 20 mg/mL if the label is accurate. One concentration for the whole course.
- Syringe
- 0.3 mL insulin syringe with half-unit marks for draws under 30 units; 1 mL syringe above that
| Weeks | Weekly dose | Draw | Units | Used per 4 weeks |
|---|---|---|---|---|
| 1–4 | 2 mg | 0.1 mL | 10 | 8 mg (1 vial; most discarded at 28 days) |
| 5–8 | 4 mg | 0.2 mL | 20 | 16 mg (1 vial) |
| 9–12 | 8 mg | 0.4 mL | 40 | 32 mg (2 vials) |
| 13+ | 12 mg | 0.6 mL | 60 | 48 mg (2 vials) |
| Middle arm | 9 mg | 0.45 mL | 45 | 36 mg (2 vials) |
The 4, 9, and 12 mg maintenance doses are the Phase 3 arms; 2 mg is a starting dose only. Trials monitored heart rate at every step; a self-directed user does not.
What each draw looks like · 1 mL (100-unit) U-100 insulin syringe
The plunger tip sits on the line for the dose. Units are marks on a U-100 syringe: 100 units is 1 mL. The picture assumes the concentration in the table above; a different amount of water changes every line.
- Side effects track the dose. In TRIUMPH-1, adverse events led 4.1% of the 4 mg group, 6.9% of the 9 mg group, and 11.3% of the 12 mg group to stop, against 4.9% on placebo.
- Heart rate rose in a dose-dependent way, by roughly 5 to 10 beats per minute, peaking around week 24 and easing afterward. Trials monitored it; a self-directed user would not.
- Weight loss continued past a year at every dose, so the trials ran 68 to 104 weeks. Short-term results understate the full effect.
Compounding pharmacies may only work from three kinds of ingredient: one with a United States Pharmacopeia monograph, one that is a component of an FDA-approved drug, or one on FDA's approved bulk-substances list. Retatrutide is none of these. The tirzepatide shortage that once let pharmacies compound copies never applied here, because there is no product to be short of.
| Date | Event |
|---|---|
| 2023 | Phase 2 results published in the New England Journal of Medicine and The Lancet |
| Apr 2025 | FDA writes to every state board of pharmacy and the medical boards: retatrutide is not eligible for compounding under 503A or 503B |
| Sep 2025 | FDA warning letters to sellers of unapproved retatrutide products labeled "for research purposes" but sold with dosing instructions |
| Dec 2025 | First Phase 3 readout (TRIUMPH-4) |
| May 2026 | Pivotal obesity trial readout (TRIUMPH-1) |
| Q1 2027 | Lilly's stated target for filing with the FDA |
What is actually in the vial
Products sold as retatrutide come from chemical suppliers, not pharmacies, with no identity, potency, or sterility testing anyone is accountable for. Independent testing of such vials has found under-dosed, over-dosed, and mislabeled product. There is no concentration you can trust, so there is no dose you can calculate.
- A licensed pharmacy that offers compounded retatrutide is acting outside the law. That is a reason to doubt everything else it tells you.
- "Research use only" and "not for human consumption" on the label are legal cover for the seller, not a description of quality.
- The only lawful ways to receive retatrutide today are a registered clinical trial or Lilly's pre-approval expanded-access program, which is listed on ClinicalTrials.gov and requires a physician.
- Adverse events from unregulated product are not captured by any safety system, so no one learns from them.
People are using unregulated retatrutide now. The risks are not hypothetical, and they are different in kind from the side effects of the drug itself.
- Identity
- No one has verified that the vial contains retatrutide rather than another peptide, a fragment, or nothing.
- Potency
- Label strength is unverified. Dosing errors of several-fold are possible without any mistake on your part.
- Sterility
- Injectables must be sterile and endotoxin-free. Chemical suppliers do not certify either for human use.
- Monitoring
- Trials tracked heart rate, rhythm, and liver enzymes. Self-use has none of that.
- Interactions
- Nothing is known about combining it with insulin, other GLP-1 drugs, or common medicines outside a trial.
- Recourse
- No pharmacy, prescriber, or manufacturer is accountable if something goes wrong.
Most of retatrutide's side effects are the familiar incretin ones, at higher rates than tirzepatide at comparable weight loss. Two signals are specific to the glucagon component and worth knowing before approval.
| Event | 12 mg | 9 mg | Placebo |
|---|---|---|---|
| Nausea | 43% | — | — |
| Dysesthesia (skin tingling, burning, sensitivity) | 21% | 9% | — |
| Stopped for adverse events | 18% | 12% | 4% |
Dashes: not reported in the topline release. Diarrhea, constipation, and vomiting were also common. Full tables await publication.
Heart rate
Resting heart rate rose by roughly 5 to 10 beats per minute, more than with semaglutide or tirzepatide, peaking around six months and easing after. Whether that matters for people with heart disease is what the TRIUMPH cardiovascular outcomes trial is designed to answer.
- Skin sensations (dysesthesia) were mostly mild and rarely caused people to stop, but they were common at 12 mg and were not seen with earlier incretin drugs.
- Expect the thyroid C-cell boxed warning, pancreatitis, gallbladder, and aspiration-under-anesthesia cautions that apply to the whole class to carry over to any approved label.
- Long-term data beyond two years do not exist yet. Neither does experience in pregnancy, adolescents, or people with significant kidney or liver disease outside the dedicated trials.
There are honest answers to these questions and there are evasive ones. The honest answers today are short.
- Is this FDA-approved? (It is not, as of this edition.)
- Is this a registered clinical trial? What is the NCT number, and who is the principal investigator?
- Is this Lilly's expanded-access program? Which physician applied on my behalf?
- If it is neither, where does the vial come from, and what does the seller's label say about human use?
- Which pharmacy compounded it, and can they explain how that squares with FDA's April 2025 letter?
- Who will check my heart rate and rhythm, and how often?
- What is the concentration, who tested it, and can I see the certificate of analysis?
- What happens, and who is responsible, if I have a serious side effect?
The patient's option today
Tirzepatide is approved, available in a manufacturer's pen, and covered by the same trial-grade evidence. For most people asking about retatrutide, it is the drug their prescriber can actually give them while the Phase 3 program finishes.
Latest
Developments since this edition, newest first. Items here are dated and sourced; they update the guide without re-editing it.
Full TRIUMPH-2 results published in The Lancet: 20.8% weight loss on 12 mg at 80 weeks in type 2 diabetes
In 1,152 adults with obesity and type 2 diabetes, weight fell 12.7%, 19.1%, and 20.8% on 4, 9, and 12 mg against 4.0% on placebo, and A1C fell 1.4 to 1.6 points. Side effects led 3.8% to 11.6% to stop (4.9% on placebo), skin sensations (dysesthesia) affected 4.5% to 7.3%, and Lilly still plans to file in the first quarter of 2027.
Full TRIUMPH-2 results to be presented September 30
Lilly will show the complete Phase 3 data in adults with obesity and type 2 diabetes at a European diabetes meeting symposium, the first peer-presented look at numbers so far known only from press releases.
Lilly and FDA dispute whether retatrutide is a drug or a biologic
Lilly argues the 41-residue peptide crosses the 40-amino-acid line for a biologic and should get 12 years of exclusivity; FDA counts only alpha amino acids, which would leave five. The answer sets how long generics are kept out.
TRIUMPH-2 and TRIUMPH-3 succeed; FDA filing set for early 2027
Top doses produced 20.8 percent weight loss in adults with obesity and type 2 diabetes and 22.6 percent in those with established cardiovascular disease at 80 weeks. Lilly will file in the first quarter of 2027 after finishing manufacturing data, so approval before late 2027 is unlikely.
References
- 1.Jastreboff AM, et al. Triple-hormone-receptor agonist retatrutide for obesity: a phase 2 trial. N Engl J Med. 2023;389(6):514-526.
- 2.Rosenstock J, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, parallel-group, phase 2 trial. Lancet. 2023;402(10401):529-544.
- 3.Sanyal AJ, et al. Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial. Nat Med. 2024;30:2037-2048.
- 4.Eli Lilly and Company. Lilly's triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs along with substantial relief from osteoarthritis pain in first successful Phase 3 trial (TRIUMPH-4). Press release, December 2025. https://investor.lilly.com/news-releases/news-release-details/lillys-triple-agonist-retatrutide-delivered-weight-loss-average
- 5.Eli Lilly and Company. Lilly's triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial (TRIUMPH-1). Press release, May 21, 2026. https://investor.lilly.com/news-releases/news-release-details/lillys-triple-agonist-retatrutide-delivered-powerful-weight-loss
- 6.ClinicalTrials.gov. A study of retatrutide (LY3437943) in participants who have obesity or overweight (TRIUMPH-1). NCT05929066. https://clinicaltrials.gov/study/NCT05929066
- 7.ClinicalTrials.gov. Pre-approval expanded access of retatrutide (LY3437943). NCT07629401. https://clinicaltrials.gov/study/NCT07629401
- 8.U.S. Food and Drug Administration. Letter to executive officers, state boards of pharmacy: retatrutide in compounded drug products. April 2025. https://www.pharmacy.ohio.gov/documents/pubs/special/compounding/fda%20letter%20-%20retatrutide%20in%20compounded%20drug%20products.pdf
- 9.U.S. Food and Drug Administration. Warning letters to sellers of unapproved retatrutide products (GLP-1 Solution, GenLabMeds, ASN-Labs). September 9, 2025. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/warning-letters/glp-1-solution-715883-09092025
- 10.U.S. Food and Drug Administration. Human drug compounding: sections 503A and 503B of the Federal Food, Drug, and Cosmetic Act. https://www.fda.gov/drugs/guidance-compliance-regulatory-information/human-drug-compounding
- 11.Coskun T, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: from discovery to clinical proof of concept. Cell Metab. 2022;34(9):1234-1247.