GLP-3 (Retatrutide): What the Name Means and What the Trials Show

GLP-3 is a nickname for retatrutide, an experimental once-weekly injection from Eli Lilly that switches on three hormone receptors at once: GIP, GLP-1 and glucagon. There's no natural hormone called GLP-3, and Lilly calls the label "scientifically inaccurate". GLP 3, GLP3 and GLP-3 RT all point to the same drug. In Lilly's TRIUMPH-1 trial, people on the top 12 mg dose lost an average of 28.3% of their body weight over 80 weeks. As of September 2026, no approved version exists anywhere.

Most of the confusion around GLP-3 starts with the name. The 3 suggests a third hormone in a series, and there isn't one.

Is there a real GLP-3 hormone?

No. Your body makes GLP-1 and GLP-2, and nothing numbered 3.

Both come from one gene, GCG, which builds a precursor protein called proglucagon. According to the gene record at the U.S. National Center for Biotechnology Information, proglucagon is cut into four finished peptides: glucagon, glucagon-like peptide-1 (GLP-1), glucagon-like peptide-2 (GLP-2) and a glicentin-like peptide. That's the whole family.

The two real ones do very different jobs. GLP-1 tells the pancreas to release insulin after a meal, slows how fast your stomach empties and helps you feel full, which is why drugs that copy it are used for diabetes and weight loss. GLP-2 mostly acts on the gut lining. The GLP-2 drug teduglutide (Gattex) is approved in the US for short bowel syndrome, according to its FDA label, and has nothing to do with weight loss.

Lilly's own FAQ explains where the name came from. It says "GLP-3" is a label "used informally in the media" for triple hormone receptor agonists, and that it stuck because retatrutide hits three receptors. Lilly's preferred term is "triple agonist". The numbering only works if you count receptors: semaglutide works on one, tirzepatide on two and retatrutide on three. Headline writers ran with it anyway.

What GLP-3 RT, GLP3 and "reta" mean

Retatrutide has picked up more names than most approved drugs, and only two of them are official.

GLP-3 RT is the name that confuses people most. It shows up almost only on seller listings, often with "retatrutide" in brackets next to it, and no regulator, journal or Lilly document defines it. We couldn't find any official source that says what the letters RT stand for. The label is a seller's choice, and it doesn't tell you what's in the vial or how much of it there is.

The numbers are seller shorthand too. A US seller the FDA warned in August 2026 listed "GLP-1 S (Semaglutide)", "GLP-2 T (Tirzepatide)" and "GLP-3 Reta (Retatrutide)" side by side. On that list, "GLP-2" meant tirzepatide, a drug with no link to the real GLP-2 hormone.

Lilly discovered and developed retatrutide itself, and the Eli Lilly retatrutide page covers what the company has said about it. There's no brand name yet either. A brand name usually arrives with approval, the way tirzepatide became Mounjaro and Zepbound. Our retatrutide brand name page tracks that, and if you've been saying it wrong, the pronunciation guide sorts it out.

Is retatrutide a GLP-1, and what does it do?

Retatrutide is one molecule that activates three receptors: the GLP-1 receptor, the GIP receptor and the glucagon receptor. So it's a GLP-1 drug in the sense that it works on GLP-1 receptors, but it does more than semaglutide (GLP-1 only) or tirzepatide (GLP-1 and GIP). Our retatrutide vs GLP-1 comparison covers where that line falls.

Lilly's 2022 paper in Cell Metabolism, which described the molecule before it reached big trials, laid out the idea. GLP-1 and GIP mainly cut how much you eat. Glucagon was added to raise the energy the body burns, on top of eating less. The same paper reported that in lab tests, retatrutide acts more strongly at the GIP receptor than at the other two, which are roughly balanced.

A caution on that energy-burning part: it was shown in mice. Trials in people have measured weight, blood sugar and side effects, and those results are strong, but no published retatrutide trial has shown how much of the weight loss comes from burning more versus eating less. The how retatrutide works page goes through each receptor.

Retatrutide lasts a long time in the blood. In the Phase 1b trial published in The Lancet in 2022, its half-life was about 6 days, which is why it's given once a week. The half-life page explains what that means for missed doses and stopping.

What do the GLP-3 trials show?

Weight loss is the headline. As of September 2026, Lilly has reported weight results from one Phase 2 obesity trial and five Phase 3 trials, four of them in obesity and one in type 2 diabetes.

The TRIUMPH figures come from Lilly's press releases and use what Lilly calls the efficacy estimand, which assumes everyone kept taking the drug as planned. Counting everyone who was randomized, dropouts included, TRIUMPH-1's 12 mg group lost 25.0% on average. The TRANSCEND-T2D-1 row uses that second, stricter method, from the published Lancet paper. Both kinds of number are real; they answer different questions.

TRIUMPH-1 also reported how many people hit big milestones on 12 mg. Nearly half (45.3%) lost 30% or more of their body weight, and 65.3% ended the trial with a BMI under 30. In an extension for people who started with a BMI of 35 or higher, those first assigned to 12 mg averaged 30.3% at 104 weeks.

The lowest dose held up too. On 4 mg, reached after a single dose increase, TRIUMPH-1 participants lost 19.0% at 80 weeks, and fewer stopped over side effects (4.1%) than on placebo (4.9%).

For scale, the landmark trials of the approved drugs came in lower. Semaglutide 2.4 mg averaged 14.9% at 68 weeks in STEP 1, and tirzepatide 15 mg averaged 20.9% at 72 weeks in SURMOUNT-1, both counting everyone randomized. Those are different trials with different people and lengths, so they don't make a head-to-head comparison. TRIUMPH-5, which tests retatrutide against tirzepatide directly, hadn't reported as of September 2026. The comparisons hub lays out each matchup with its caveats.

Our read: if you want the fairest rough comparison, set TRIUMPH-1's 25.0% against SURMOUNT-1's 20.9%, since both count dropouts. On that basis the gap is about 4 points, not 7.

The results hub breaks down weight loss by month and by dose, the benefits page covers what improved besides weight, and the clinical trials hub lists every trial, including the ones still running.

Side effects and safety

Retatrutide's side effects look like those of other GLP-1 drugs, with a couple of additions. In TRIUMPH-1, the most common were stomach-related and mostly rose with the dose.

About 4 in 10 people on 12 mg (42.4%) had nausea at some point, against about 1 in 7 on placebo. The skin symptom, dysesthesia, is the less familiar one: burning, tingling or oversensitive skin. In TRIUMPH-4 it hit 20.9% of people on 12 mg, and Lilly says most cases in TRIUMPH-1 were mild to moderate and resolved during treatment.

Heart rate goes up too. In the Phase 2 trial, the rise depended on dose, peaked at 24 weeks and then eased. The same trial found that starting at 2 mg rather than 4 mg partly reduced stomach side effects, and TRIUMPH-1 started everyone at 2 mg.

What nobody knows yet is how retatrutide behaves over many years. The longest placebo-controlled data run 80 weeks, and Lilly's trials on heart and kidney outcomes are still going. The side effects hub covers each effect in depth, and is retatrutide safe? weighs the whole picture.

Retatrutide dosage in the trials

TRIUMPH-1, the biggest Phase 3 trial reported so far, used a simple pattern: start at 2 mg once a week, then step up every 4 weeks until reaching the assigned dose. Gradual step-ups like this are the only kind of schedule with published results behind them.

The earliest-week column is our arithmetic from the 4-week steps, not a figure Lilly published. Every dose in the trials was a shot under the skin, once a week.

That table describes what the trials did. It isn't a plan to follow, and dose decisions belong with a doctor. The dosage hub covers doses, units and vial math, the how to take retatrutide section covers mixing, injecting and storage, and the calculators handle reconstitution and projected weight loss.

Is GLP-3 approved, and how do people get it?

Nothing is approved as of September 2026. Lilly says retatrutide "has not been approved by any regulatory agency" and is legally available only through its clinical trials. Its July 2026 release adds that retatrutide "cannot be legally sold or marketed for human use".

The next step has a date. Lilly plans to submit a Biologics License Application to the FDA in Q1 2027, once it finishes the manufacturing data the application needs. The FDA review comes after that, and Lilly hadn't given a launch date as of September 2026. Our FDA approval page tracks each milestone.

Until then, there are two legal routes:

• Clinical trials. Lilly is still running retatrutide trials in obesity, type 2 diabetes, chronic low back pain, heart disease, kidney disease and fatty liver disease, but most have finished enrolling. As of September 2026, ClinicalTrials.gov lists 33 retatrutide studies and only 2 are recruiting: Lilly's SYNERGY-Outcomes trial in fatty liver disease, and a small study in China that isn't Lilly's. The clinical trials hub shows how to search for one near you.
• Expanded access. Lilly opened a single-patient program in 2026 (NCT07629401) for adults whose BMI is still 35 or more on the highest available dose of an approved weight drug, who have at least two serious obesity-related complications and who can't join a trial. If all of that fits, ask your doctor: the request has to come from them, through the Lilly Answers Center.

For most people, that leaves approved drugs like tirzepatide or semaglutide, which a doctor can prescribe now. The how to get retatrutide page goes through each route, and the price page covers what's known about cost.

What the "GLP-3" sold online actually is

Search "GLP-3 peptide" and you'll find vials of powder labeled GLP-3 RT, GLP3 or retatrutide, often with small print saying they aren't meant for people. None of them come from Lilly, since Lilly doesn't sell retatrutide at all. Its FAQ warns that these products "may contain unknown ingredients, harmful contaminants and impurities", or too much, too little or the wrong ingredient entirely.

Regulators have acted on this. In December 2024 the FDA sent a warning letter to a US peptide seller, calling its "Retatrutide" product an unapproved new drug even though the label said it wasn't for people, because the seller's website showed it was meant for human use. The August 2026 letter to the "GLP-3 Reta" seller also called its products unapproved new drugs.

Two Australian reports show what can go wrong. In June 2026 Victoria's Department of Health reported 6 cases of acute liver injury since January in people using a product labeled retatrutide, and said a contaminant was a possible cause. In September 2026 the TGA, Australia's drug regulator, tested a vial sold as retatrutide after a serious reaction: it held no retatrutide at all, only undeclared semaglutide at about 8 times the level in approved semaglutide products.

People on r/Retatrutide and similar forums describe buying these vials anyway, mixing the powder with bacteriostatic water and dosing by syringe units. Those are reports, not evidence, and they show where the extra risks come in: a wrong amount per vial, a math error when mixing, or a batch that was never tested.

If you're reading about retatrutide for sale online, the one thing that separates a checkable seller from a guess is a batch-specific test report from an independent lab: identity, purity, the actual amount per vial and, ideally, endotoxin or sterility results, with a batch number that matches the vial in your hand. No batch report, a generic or reused report, Lilly branding, pens, or claims of approval are red flags.

Our read: the trial numbers on this page come from Lilly's own drug, made for its trials. A vial labeled GLP-3 RT has none of that behind it, so the 28.3% and the side effect rates above describe what retatrutide can do, not what any particular vial will do.

Who retatrutide is being studied for

Obesity is the main target, but Lilly's Phase 3 program is wider. Its trials cover type 2 diabetes, knee osteoarthritis pain, obstructive sleep apnea, chronic low back pain, heart and kidney outcomes, and fatty liver disease (MASLD). In TRIUMPH-1's sleep apnea group, Lilly reported breathing interruptions during sleep dropped by up to 36.1 events an hour, a 60.6% fall from an average starting point of 58.6.

The conditions hub goes through each one: diabetes, blood pressure, fatty liver, kidney health, inflammation and cravings. The women's hub covers pregnancy and fertility, menopause, dosing and side effects in women.

Online, people also combine retatrutide with other peptides and drugs. No trial has tested retatrutide together with tirzepatide, semaglutide or any other weight-loss drug, and several of those work on the same receptors, so their side effects can add up. The stacks hub says what's known and what isn't for each pairing.

The trial that will say the most about long-term safety is TRIUMPH-Outcomes, which is following about 10,000 people with heart or kidney disease. Its ClinicalTrials.gov record estimates that main data collection ends in February 2029. The news page is updated as Lilly and regulators publish new results.