These two are compared constantly, usually under their community shorthand — tirz for tirzepatide, reta for retatrutide. The most important difference is not a number: tirzepatide is an FDA-approved medicine a doctor can prescribe today, and retatrutide is investigational, not approved, and not legitimately available outside a clinical trial. This page is educational, not medical advice, and recommends neither.
| Tirzepatide (“tirz”) | Retatrutide (“reta”) | |
|---|---|---|
| Receptors | GIP + GLP-1 (dual agonist) | GIP + GLP-1 + glucagon (triple agonist) |
| Status | FDA approved | Investigational — not approved |
| Brand names | Mounjaro, Zepbound | None — no approved product exists |
| How you get it | Prescription | Clinical trials only |
| Development stage | Approved; new indications still being added | Phase 3 (TRIUMPH) reported |
| Next regulatory step | — | Lilly has said it plans to file with the FDA in Q1 2027 |
Tirzepatide activates two gut-hormone receptors, GIP and GLP-1. Both are involved in appetite and blood-sugar regulation, and acting on two at once is what set tirzepatide apart from single-receptor GLP-1 drugs such as semaglutide.
Retatrutide adds a third: the glucagon receptor. That reads as counter-intuitive, because glucagon raises blood sugar — but glucagon-receptor activity is also associated with increased energy expenditure, and the design pairs that with the appetite-reducing effects of the other two. That is the mechanistic reason retatrutide draws attention. It is also why it needs full characterisation rather than assumption: a third pathway is a third set of effects.
A Phase 2 trial published in the New England Journal of Medicine in 2023 first drew attention, reporting an average 24.2% body-weight reduction on the 12 mg dose at 48 weeks against 2.1% on placebo. The Phase 3 programme has since reported.
TRIUMPH-1 randomised 2,339 adults with obesity or overweight and at least one weight-related condition, without diabetes, to retatrutide 4 mg, 9 mg, 12 mg or placebo. Average weight reduction at 80 weeks:
| Group | Efficacy estimand | Treatment-regimen estimand |
|---|---|---|
| 4 mg | −19.0% | −17.6% |
| 9 mg | −25.9% | −23.7% |
| 12 mg | −28.3% | −25.0% |
| Placebo | −2.2% | −3.9% |
On 12 mg at 80 weeks, 62.5% of participants lost at least 25% of body weight, 45.3% lost at least 30%, and 27.2% lost at least 35%.
Three further Phase 3 trials have reported topline results: TRIUMPH-2 in adults with obesity and type 2 diabetes (up to 20.8% at 80 weeks, with A1C reduction up to 1.6 percentage points); TRIUMPH-3 in severe obesity with established cardiovascular disease (up to 22.6% at 80 weeks); and TRIUMPH-4 in obesity with knee osteoarthritis (up to 28.7% at 68 weeks, alongside reduced knee pain).
SURMOUNT-1 enrolled 2,539 adults with obesity or overweight and without diabetes, over 72 weeks. On the 15 mg dose, average weight reduction was 20.9% on the treatment-regimen estimand against 3.1% on placebo, and 22.5% on the efficacy estimand. Tirzepatide also has something retatrutide does not: an approved FDA label, and years of real-world use behind it.
This is the part most comparisons skip, and it matters more than any single figure. No head-to-head trial has reported. Every comparison you will read, this one included, places results from separate studies side by side — and those studies differ in ways that move the numbers:
Retatrutide's adverse events in TRIUMPH-1 were mostly the familiar gastrointestinal pattern for the class, rising with dose. On 12 mg against placebo: nausea 42.4% vs 14.8%, diarrhea 32.0% vs 13.5%, constipation 26.1% vs 10.9%, vomiting 25.3% vs 4.8%. Discontinuation because of adverse events was 11.3% on 12 mg against 4.9% on placebo — roughly double, and dose-dependent.
Two findings are distinctive rather than class-typical, and both are worth naming:
These are exactly the signals a full regulatory review exists to characterise, and they are one reason “the trial numbers are bigger” is an incomplete way to think about an unapproved drug. Tirzepatide's side effects, by contrast, are documented on an approved label that reflects a completed review, carries warnings and contraindications, and is updated as evidence accumulates.
Tirzepatide is approved under two brand names, and the list of approved uses has kept growing:
| Approved | Brand | Use |
|---|---|---|
| May 2022 | Mounjaro | Glycaemic control in adults with type 2 diabetes |
| November 2023 | Zepbound | Chronic weight management |
| December 2024 | Zepbound | Moderate-to-severe obstructive sleep apnea in adults with obesity |
| December 2025 | Mounjaro | Extension to paediatric patients aged 10 and over with type 2 diabetes |
| August 2026 | Mounjaro | Reducing the risk of major adverse cardiovascular events in adults with type 2 diabetes at high risk |
Two uses you may see claimed are not approved. Tirzepatide is not approved for heart failure — the SUMMIT trial was positive, but the application was withdrawn in 2025 after the FDA required a confirmatory trial. It is also not approved for MASH, despite occasional claims otherwise; the evidence there is Phase 2.
Retatrutide has completed pivotal Phase 3 trials with positive results, and Lilly has said it plans to file a Biologics License Application in the first quarter of 2027. Until a submission is reviewed and approved, it remains investigational: there is no approved retatrutide product, no approved dose, and no legitimate prescription route outside a trial.
Retatrutide is sold widely online as a research chemical. That market sits outside the regulatory system completely, and the specifics matter:
If you have been reading forums, the shorthand can be opaque. What the common abbreviations mean:
| Shorthand | Means | Status |
|---|---|---|
| tirz, tirze, tirzep | Tirzepatide | Approved |
| reta, retat | Retatrutide | Investigational |
| triple G, triple agonist, LY3437943 | Also retatrutide | Investigational |
| sema | Semaglutide | Approved |
| MJ | Mounjaro (tirzepatide, type 2 diabetes) | Approved |
| Zep, ZB | Zepbound (tirzepatide, weight and sleep apnea) | Approved |
| GLP-1 | Glucagon-like peptide-1, a receptor these drugs act on | — |
| GIP | Glucose-dependent insulinotropic polypeptide, the second receptor | — |
None of these names changes what either compound is. One is an approved medicine; the other is an investigational compound with promising trial data and an unfinished regulatory review.
They are the shorthand used in GLP-1 communities. Tirz (also tirze or tirzep) is tirzepatide, sold as Mounjaro and Zepbound. Reta is retatrutide, an investigational Eli Lilly compound, sometimes called the triple agonist or triple G, or by its development code LY3437943. It is not approved and is not sold as a prescription medicine.
No. Retatrutide has reported positive results from pivotal Phase 3 trials, but it remains investigational. Eli Lilly has said it plans to submit a Biologics License Application to the FDA in the first quarter of 2027. No approval date has been announced by the FDA or by Lilly.
No head-to-head trial has reported yet, so that question does not have a reliable answer. Retatrutide's separate trials produced larger average figures, but the studies differ in duration, population and statistical method, so putting the percentages side by side compares non-equivalent numbers. A direct comparison trial, TRIUMPH-5, is running and is expected to complete around the end of 2026.
Yes, and it has not reported. TRIUMPH-5 (NCT06662383) is a Phase 3 randomised double-blind trial of about 800 adults with obesity, comparing retatrutide against tirzepatide as an active comparator, with percent change in body weight at week 80 as the primary outcome. It is listed as active and no longer recruiting, with estimated completion in December 2026. It is the only trial in the programme with an active comparator.
In TRIUMPH-1, adults with obesity or overweight and no diabetes averaged 28.3% body-weight reduction on the 12 mg dose at 80 weeks on the efficacy estimand, against 2.2% on placebo. On the treatment-regimen estimand, which reflects what actually happened including people who stopped, the same group averaged 25.0%. A 104-week extension reported up to 30.3%. These are trial results for an unapproved drug, not an expected outcome for any individual.
In SURMOUNT-1, adults with obesity or overweight and without diabetes averaged 20.9% body-weight reduction on the 15 mg dose at 72 weeks on the treatment-regimen estimand, against 3.1% on placebo. The efficacy estimand for the same group was 22.5%. Individual results vary widely.
Tirzepatide acts on two receptors, GIP and GLP-1. Retatrutide acts on those two plus the glucagon receptor, which is why it is called a triple agonist. Glucagon-receptor activity is associated with increased energy expenditure, but a third pathway also means a third set of effects to characterise.
The gastrointestinal pattern is familiar for the class and rises with dose. One finding is distinctive: dysesthesia, an abnormal skin sensation such as tingling or burning, was reported by up to 12.5% of retatrutide participants in TRIUMPH-1 against 0.9% on placebo. Discontinuation because of adverse events was 11.3% on the 12 mg dose against 4.9% on placebo. The earlier Phase 2 trial also reported a dose-dependent rise in heart rate.
No. There is no legitimate route outside a clinical trial. Retatrutide has no USP monograph, is not a component of any approved drug, and is not on the FDA's 503A bulks list, so there is no lawful compounding pathway either. The FDA has issued warning letters to multiple vendors selling it as an unapproved new drug.
Largely no. The FDA declared the tirzepatide shortage resolved in October 2024, and enforcement discretion ended for 503A pharmacies in February 2025 and for 503B outsourcing facilities in March 2025. A court upheld the FDA's decision in May 2025. A narrow route survives where a prescriber documents a specific clinical need the approved product cannot meet for an individual patient, but that is not the same as routine compounded programmes.
That is a decision for you and your physician, and this page does not make it. One is an approved medicine a doctor can prescribe; the other is investigational and unavailable outside a trial, which in practice answers the question for now.