Compound comparison
Retatrutide vs. Tirzepatide: What the Trials Actually Found
Retatrutide's phase 2 obesity trial reported a larger average weight loss than tirzepatide's phase 3 approval trial, which makes it tempting to read this as a contest retatrutide already won. It isn't one: retatrutide and tirzepatide have never been tested against each other, and the two numbers come from different trials, run on different populations, for different durations, at very different scale. Retatrutide is also not an approved medicine of any kind, while tirzepatide is FDA-approved and available by prescription as Zepbound and Mounjaro. This page lays out what each trial actually reported, not what stacking the two headline percentages next to each other would imply.
Retatrutide
Research use onlyTriple agonist: GIP, GLP-1, and glucagon receptors.
Evidence strength: Randomized trials. Randomized controlled trials in humans, at scale.
Sold for laboratory use. Not a legal medicine for people.
Tirzepatide
FDA-approvedDual agonist: GIP and GLP-1 receptors.
Evidence strength: Randomized trials. Randomized controlled trials in humans, at scale.
Approved for this use and available by prescription.
This is not a head-to-head trial
Retatrutide and tirzepatide have never been tested against each other in a head-to-head trial. Every result below comes from a separate trial, run on a different population, for a different duration, against its own placebo arm — read the table as three separate findings, not a scored comparison with a winner.
What each trial reported
Every row is its own trial. Duration, sample size, and phase are shown next to every result on purpose — without them, a result is not a fact you can compare to anything.
| Compound | Trial | Population | Result | Duration | n | Phase | Status |
|---|---|---|---|---|---|---|---|
| Retatrutide | — | Adults with obesity, no diabetes | −24.2% at 12 mg · placebo −2.1% | 48 wks | n=338 | phase 2 | Research use only |
| Retatrutide | TRANSCEND-T2D-1 | Adults with type 2 diabetes | −15.3% at 12 mg · placebo −2.6% | 40 wks | n=537 | phase 3 | Research use only |
| Tirzepatide | SURMOUNT-1 | Adults with obesity, no diabetes | −20.9% at 15 mg · placebo −3.1% | 72 wks | n=2539 | phase 3 | FDA-approved |
Why these numbers are not a comparison
A table with three rows next to each other invites a reader to do the subtraction anyway, so it is worth being explicit about what makes these three results incomparable. They come from three different trials, run by different research groups, in different phases of development, on different patient populations, for different durations, at three very different sample sizes — 338, 537, and 2,539 randomized participants. A trial testing a drug for 72 weeks in people without diabetes is not measuring the same thing as a trial testing a different drug for 40 weeks in people who have it. Stacking the topline percentages into a bar chart would flatten all of that away and leave only the one number that looks most dramatic, which is exactly the comparison this page declines to draw.
What retatrutide's trials found
Retatrutide's phase 2 obesity trial randomized 338 adults without diabetes to placebo or one of several retatrutide doses and followed them for 48 weeks. At the 12 mg dose, average body weight fell 24.2%, against 2.1% for placebo 1. That is the number most often quoted as retatrutide's headline result, and it is a real, randomized, placebo-controlled finding — but it is a phase 2 result, run in fewer than 350 people, in a population that excluded anyone with diabetes.
Retatrutide's phase 3 program has since produced a different picture. TRANSCEND-T2D-1 tested retatrutide in 537 adults who have type 2 diabetes — not the obesity-only population the phase 2 trial enrolled — over 40 weeks, and found average weight loss of 15.3% at the 12 mg dose against 2.6% for placebo 2. Type 2 diabetes tends to blunt the weight loss a GLP-1-based drug produces relative to a metabolically healthier population, which is one likely reason this phase 3 number reads lower than the phase 2 one — the two trials are not measuring the same patients, so the gap is not evidence the drug got weaker.
What tirzepatide's trial found
Tirzepatide's phase 3 approval trial, SURMOUNT-1, randomized 2,539 adults with obesity and no diabetes to placebo or tirzepatide and followed them for 72 weeks — closer to a year and a half, and roughly seven times the enrollment of retatrutide's phase 2 trial. At the 15 mg dose, average body weight fell 20.9%, against 3.1% for placebo 3. That is the trial regulators approved tirzepatide on, and it remains the largest, longest, and latest-phase result of the three in this table.
Availability is a separate question from evidence
Evidence and availability answer different questions, and this page keeps them separate on purpose. Tirzepatide is FDA-approved and sold by prescription under the brand names Zepbound and Mounjaro — a reader who wants it can get it through a licensed prescriber and pharmacy today. Retatrutide is not approved for any use. It carries no legitimate prescription supply chain, and what circulates outside of clinical trials is unregulated research-chemical stock with none of the manufacturing or purity oversight an approved drug carries. A larger phase 2 number does not change that: retatrutide's evidence base and its legal availability are two different facts, and only one of them favors retatrutide.
The bottom line
Read this table as three separate trial reports, not a race. Retatrutide's phase 2 result is the largest number here, from the smallest and shortest trial, in a population that excluded people with diabetes. Retatrutide's own phase 3 result, in a diabetes population, is smaller than tirzepatide's phase 3 result. Tirzepatide's number is the one backed by the largest, longest, latest-phase trial — and the only one of the three attached to a drug a reader can actually be prescribed.
Frequently asked questions
Has retatrutide been tested head-to-head against tirzepatide?
No. No published trial has randomized participants to receive one drug or the other in the same study. Every published result for each drug comes from its own separate trial, against its own placebo arm.
Why is retatrutide's weight-loss number bigger than tirzepatide's?
The two numbers come from different trials. Retatrutide's largest reported result is from a smaller, shorter, phase 2 trial in people without diabetes; tirzepatide's number is from a larger, longer, phase 3 trial that led to its approval. A bigger number from an earlier-phase, smaller trial is not the same claim as a bigger number from a head-to-head comparison.
Can I get a prescription for retatrutide?
No. Retatrutide is not FDA-approved for any use, so there is no legitimate prescription pathway for it. Tirzepatide is FDA-approved and available by prescription as Zepbound (for weight management) and Mounjaro (for type 2 diabetes).
References
- Jastreboff AM, Kaplan LM, Frías JP, et al. (2023). Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. New England Journal of Medicine. https://pubmed.ncbi.nlm.nih.gov/37366315/
- Bajaj HS, Welch M, Shah P, et al. (2026). Retatrutide in Adults with Type 2 Diabetes (TRANSCEND-T2D-1): A Randomised, Double-Blind, Placebo-Controlled, Phase 3 Trial. The Lancet. https://pubmed.ncbi.nlm.nih.gov/42250575/
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. (2022). Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine. https://pubmed.ncbi.nlm.nih.gov/35658024/
Medical disclaimer: This content is for general educational purposes only and is not medical advice, diagnosis, or treatment. Always consult a licensed healthcare professional before starting, stopping, or changing any treatment.