What is retatrutide? The research so far

What retatrutide is

Retatrutide (development code LY3437943) is an investigational peptide developed by Eli Lilly. It is engineered to activate three receptors at once: the GLP-1 receptor, the GIP receptor, and the glucagon receptor. That is why it is usually described as a triple agonist, or a GGG agonist.

It is not an approved medicine anywhere. It has been studied in phase 1 and phase 2 trials published in peer-reviewed journals, and phase 3 programs are underway. Everything currently known about it in humans comes from those trials.

The nickname "GLP-3" that circulates online is not a scientific term. There is no hormone called GLP-3; the name appears to have been invented to imply a successor to GLP-1 drugs.

How triple agonism works

Each of the three receptors contributes something different, and the rationale for combining them is that their effects stack.

• GLP-1 receptor. Enhances glucose-dependent insulin release, suppresses glucagon, slows gastric emptying and reduces appetite through brain signalling. This is the same pathway used by semaglutide. Our GLP-1 guide covers it in detail.
• GIP receptor. The other major incretin pathway. Adding GIP activity to GLP-1 activity — as tirzepatide does — produced larger effects in trials than GLP-1 alone, though the mechanism is still debated.
• Glucagon receptor. The novel addition. Glucagon raises blood glucose, which sounds counterproductive, but it also increases energy expenditure and promotes hepatic fat mobilization. The theory is that GLP-1 and GIP activity offsets the glucose effect while the energy-expenditure benefit remains.

In short, the first two arms mainly reduce energy intake and the third mainly increases energy output. That combination is the reason researchers expected larger effects than from single or dual agonists.

What the trials found

The headline results come from a phase 2 randomized, double-blind, placebo-controlled trial in adults with obesity, published in 2023, plus a parallel phase 2 trial in adults with type 2 diabetes.

• Body weight. In the obesity trial, participants on the highest studied doses showed mean weight reductions in the region of 24 percent at 48 weeks, compared with roughly 2 percent on placebo. Those are the largest reductions reported in a trial of a pharmacological agent to date.
• No clear plateau. Weight was still declining at the end of the study period in the higher-dose groups, which is unusual and is part of why the results drew so much attention.
• Glucose control. In the type 2 diabetes trial, HbA1c reductions were substantial across dose groups, with weight loss also observed.
• Liver fat. A sub-study in participants with metabolic dysfunction-associated steatotic liver disease reported large reductions in liver fat content, with a high proportion reaching normal levels.

Two caveats matter. These are phase 2 trials — moderate in size and length, designed to identify dose and signal rather than to prove long-term benefit. And every result above comes from the pharmaceutical-grade compound administered under trial supervision.

How it compares

Cross-trial comparisons are imperfect — different populations, durations and designs — but the rough picture from published trials is a ladder. Semaglutide (GLP-1 alone) produced mean weight reductions around 15 percent in its obesity trial; tirzepatide (GLP-1 plus GIP) around 20 to 22 percent at the highest dose; retatrutide's phase 2 result sits above those.

The important asymmetry is evidence maturity. Semaglutide and tirzepatide have completed phase 3 programs, regulatory review and, in several cases, cardiovascular outcome trials. Retatrutide has none of that yet. A larger number in a shorter trial is not the same as a better-established medicine.

Reported side effects

• Gastrointestinal. Nausea, vomiting, diarrhea and constipation were the most frequent adverse events, generally dose-related and most common during dose escalation.
• Heart rate. Trials reported dose-dependent increases in heart rate that appeared to peak partway through the study and then decline. The long-term significance is unknown.
• Glucose handling. Because glucagon receptor activation can raise glucose, glycemic effects were monitored carefully; in the published trials glucose control improved overall, but this remains a mechanism worth watching.
• Lean mass. As with other agents producing large weight loss, a portion of the loss is lean tissue. How best to mitigate that is unresolved.

What is still unknown

• Whether the phase 2 weight and metabolic results hold up in larger phase 3 trials.
• Whether it reduces hard outcomes such as cardiovascular events or mortality.
• What happens after stopping — regain has been substantial with related agents.
• Long-term safety beyond the roughly one-year trial windows published so far.
• Whether the heart rate increase, hepatic effects or lean mass loss have clinical consequences over years of use.

Regulatory status

Retatrutide is not approved by the FDA or any other major regulator. It is an investigational drug, which means the only lawful route to receiving it is participation in a clinical trial.

Material sold online as "research retatrutide" is outside that system entirely: not manufactured to pharmaceutical standards, not identity- or purity-verified for human use, and not subject to any regulatory oversight. The FDA has repeatedly warned about compounded and counterfeit products in this drug class. We do not publish sourcing, reconstitution or dosing information, and nothing on this page should be read as suggesting any use outside a trial.

Common questions

Is retatrutide FDA approved?

No. It is investigational and in clinical development. Approval would require successful phase 3 results and regulatory review.

What does retatrutide do?

In published trials it activates the GLP-1, GIP and glucagon receptors, reducing appetite and food intake while increasing energy expenditure, producing large reductions in body weight, HbA1c and liver fat.

Is retatrutide safe?

Its safety profile in phase 2 trials was broadly consistent with other incretin agents, with mostly gastrointestinal effects plus a dose-dependent heart rate increase. Safety beyond about a year, and in larger populations, has not been established.

Is retatrutide the same as "GLP-3"?

No. "GLP-3" is not a real hormone or drug class. It is informal shorthand some sellers and forums use for triple agonists like retatrutide.

How is it different from tirzepatide?

Tirzepatide activates two receptors, GLP-1 and GIP. Retatrutide adds glucagon receptor activation. Tirzepatide is approved; retatrutide is not.

Where to go from here

We track new metabolic and longevity research as it publishes and write it up with full citations so you can read the underlying studies yourself.