Retatrutide is the peptide readers ask us about the most that they also cannot legally buy. It is the drug behind the "24% weight loss" headlines, it is still in trials, and it is already being sold gray-market by people who are not Eli Lilly. That combination is worth an issue.
Peptide 101: Retatrutide (LY3437943)
Retatrutide is an experimental weekly injection from Eli Lilly that activates three metabolic hormone receptors — one more than tirzepatide. Phase 2 trials in obesity reported around 24% weight loss at one year, which would be the highest ever recorded for an injectable of this class. It is currently in Phase 3 trials (TRIUMPH) for obesity, diabetes, liver disease, sleep apnea, and obesity-related knee arthritis. Retatrutide is not FDA-approved and is only available through participation in a clinical trial.
How it works
If semaglutide turns down your appetite with one signal and tirzepatide adds a second metabolic lever, retatrutide pulls a third: it also tells your liver and fat tissue to burn through stored energy. Eating less and burning more — at the same time, from one molecule.
Your gut releases hormones after you eat that tell your brain you have had enough, and tell your pancreas to handle the incoming sugar. Retatrutide is a lab-made molecule shaped to press three of those hormone buttons at once — GLP-1 and GIP, which cut appetite and steady blood sugar, plus glucagon, which nudges the liver and fat tissue to burn stored energy instead of holding onto it. Semaglutide presses one of those buttons and tirzepatide presses two, so the third one is the whole idea here: eat less and burn a little more at the same time. That third button is also why retatrutide has its own side-effect profile, including a small rise in resting heart rate.
For the nerds
Retatrutide is a single synthetic peptide engineered to activate three incretin and glucoregulatory receptors simultaneously: GLP-1R, GIPR, and the glucagon receptor. GLP-1 activation reduces appetite, slows gastric emptying, and enhances glucose-dependent insulin secretion. GIP activation reinforces incretin signaling and modulates adipose tissue lipid handling. Glucagon receptor activation adds an energy-expenditure arm — increased hepatic fatty-acid oxidation and thermogenesis — that is absent from single or dual agonists. The combined signal drives both reduced caloric intake and increased caloric expenditure, explaining weight loss magnitudes that exceed GLP-1 or GLP-1/GIP agents.
What the evidence shows
Overall strength of the evidence, by our read: Emerging. Some of what that rests on:
Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial (2023) — In adults with obesity (BMI ≥30, or ≥27 with comorbidity), retatrutide 12 mg once weekly produced mean weight loss of approximately 24.2% at 48 weeks versus 2.1% with placebo. Roughly one-quarter of participants at the highest dose achieved ≥30% weight loss.
Retatrutide in patients with type 2 diabetes — Phase 2 trial (2023) — In adults with type 2 diabetes, retatrutide produced dose-dependent reductions in HbA1c and body weight at 36 weeks, with the highest dose achieving mean HbA1c reductions exceeding 2 percentage points.
Retatrutide for MASLD with fibrosis — Phase 2 sub-study (2024) — In participants with metabolic-dysfunction-associated steatotic liver disease, retatrutide reduced liver fat content by a median of more than 80% at 48 weeks in the highest-dose arm, with improvements in non-invasive fibrosis markers.
Preliminary or early findings are not the same as proof. Any use beyond a peptide's FDA-approved labeling (where one exists) is described here for educational purposes only and is not a recommendation.
Side effects and cautions
In Phase 2 trials, the most common adverse events mirrored the GLP-1 class: nausea, vomiting, diarrhea, and constipation, especially during dose escalation. Retatrutide's glucagon-receptor component is associated with a small increase in resting heart rate and modest, dose-dependent changes in hepatic and lipid markers that have been reported as transient. Rare events under investigation include pancreatitis and cholelithiasis (class effects). Long-term safety beyond trial windows is not yet characterized.
Reasons to avoid it, or to talk to a clinician first:
Personal or family history of medullary thyroid carcinoma (class caution inherited from GLP-1 agents)
Multiple endocrine neoplasia syndrome type 2 (MEN2) (class caution)
Known hypersensitivity to retatrutide or any component
History of pancreatitis
Pregnancy or breastfeeding
This is not a complete safety list.
Questions worth bringing to a clinician
Is retatrutide available to me through a clinical trial, and am I eligible to enroll?
Given that retatrutide is not FDA-approved, what approved options (semaglutide, tirzepatide) should I consider first?
How do the glucagon-receptor effects — including possible heart rate increase — apply to my cardiovascular history?
New research, translated
A weight-loss drug is being tested as part of cancer treatment
Mazdutide plus a hormonal IUD for early endometrial cancer · ClinicalTrials.gov · registered 2026 · mazdutide
This is a prospective, multicenter, randomized, double-blind, placebo-controlled, parallel-group clinical trial designed to evaluate the efficacy and safety of mazdutide combined with a levonorgestrel-releasing intrauterine system (LNG-IUS) for fertility-sparing treatment in overweight or obese patients with atypical endometrial hyperplasia (AEH) or early-stage endometrioid endometrial cancer. Eli…
Some women with early endometrial cancer or the precancer that comes before it want to keep their uterus so they can still have children, so instead of surgery they are treated with a hormone-releasing IUD. That treatment works less often in women who are overweight, because extra body fat keeps estrogen levels high and estrogen is what drives this particular cancer. This trial adds mazdutide — a GLP-1-type weight-loss injection — on top of the IUD, splits patients randomly so half get the real drug and half get a placebo, and then checks at 24 weeks whether more of the drug group had their lesions clear completely. Nothing has been found yet: this is a trial that is just starting, so treat it as a question being asked, not an answer.
Novo is testing two versions of the same weight-loss shot against each other
Cagrilintide, two presentations, placebo-controlled · ClinicalTrials.gov · registered August 2026 · cagrilintide
The purpose of this clinical study is to look at how well a study medicine called cagrilintide helps people living with excess body weight to lose weight. Participants will either get cagrilintide, the active study medicine being tested or placebo, a medicine that has no active medicine in it. Which treatment participants get is decided by chance. Cagrilintide is a new medicine under development t…
Cagrilintide is an experimental weight-loss injection that works on a different hormone than the GLP-1 drugs most people have heard of, and it is the other half of CagriSema. This nine-month study takes adults living with overweight or obesity, randomly assigns them to cagrilintide or to a placebo, and compares two different "presentations" of the drug — meaning two ways of packaging or delivering the same medicine, such as a different pen or concentration. The point is less about whether cagrilintide works and more about confirming that the version a pharmacy would eventually hand you behaves the same as the version used in earlier trials. It is a manufacturing and consistency question, and there are no results yet.
A peptide is being tried in COPD, which is new territory
Brenipatide vs. placebo, Phase 2, 52 weeks · ClinicalTrials.gov · registered August 2026 · brenipatide
The main purpose of this study is to assess if different dose levels of Brenipatide are safe and work the way they are intended to work in participants with moderate-to-severe Chronic Obstructive Pulmonary Disease (COPD), when compared with placebo. The study will last approximately one year.
COPD is long-term lung disease that makes breathing progressively harder, and the treatments we have manage symptoms rather than change the underlying damage. This one-year Phase 2 trial gives adults with moderate-to-severe COPD one of several doses of brenipatide or a placebo, with neither the patient nor the doctor knowing which, to see whether the drug is safe and whether it does what the lab work predicts. Phase 2 is the stage where a drug either shows a signal worth chasing or quietly does not — it is not the stage that proves anything works. Worth noting mostly because it puts a peptide in a disease area where peptides have not historically shown up.
What readers are asking
Three threads we are watching and will come back to:
Whether new-onset hair loss really is more common on tirzepatide than on semaglutide once you account for how fast people lose weight
Reports that people using unapproved retatrutide bought outside the trial system are losing less weight and reporting more cardiovascular symptoms than the trial data would predict — the most direct argument against the gray market we have seen
Where in the brain semaglutide's appetite and nausea effects actually originate, which may explain why some people get the appetite benefit without the nausea and others get both
About Retatrutide
Retatrutide has NOT been evaluated or approved by the U.S. Food and Drug Administration for any medical use. It is not an FDA-approved drug, and its sale for human use may violate federal law under FD&C Act § 505(a). Any research discussed is for educational purposes only.
Medical disclaimer
This letter is for educational and informational purposes only. It does not constitute medical advice, diagnosis, or treatment, and no provider-patient relationship is created by reading it. Peptides and medications discussed may not be FDA-approved for the uses described. Always consult your healthcare provider before making any health decision. Read our full medical disclaimer.