Understanding Peptides and the Research Behind Retatrutide

What are peptides?

Peptides are small chains of amino acids — the same building blocks that make up proteins, just in shorter sequences. Your body already produces thousands of them naturally, and they act as messengers: telling cells when to release insulin, when you've eaten enough, when to grow tissue, when to sleep. Because peptides speak the body's own chemical language, they've become one of the most active areas in modern medicine.

Peptide therapeutics aren't new. Insulin, first used to treat diabetes in 1922, is a peptide — and over a century later, it's still saving lives. Since then, scientists have developed peptide treatments for a wide range of conditions including diabetes, osteoporosis, multiple sclerosis, chronic pain, certain cancers, and HIV. What's changed recently is the sophistication: researchers can now engineer peptides to be more stable, longer-lasting, and far more targeted than anything nature produces on its own.

The GLP-1 revolution

The current wave of attention around peptides started with a class of drugs called GLP-1 receptor agonists. GLP-1 (glucagon-like peptide-1) is a hormone your gut releases after you eat. It tells your pancreas to release insulin, slows down digestion, and signals to your brain that you're full.

The story took an unusual turn in 1992, when researcher John Eng was studying the venom of the Gila monster — a lizard native to the southwestern US. He discovered a peptide called exendin-4 that mimicked human GLP-1 but was far more resistant to being broken down in the body. That serendipitous discovery launched an entire therapeutic class, eventually leading to medications like exenatide, liraglutide, semaglutide (Ozempic, Wegovy), and tirzepatide (Mounjaro, Zepbound). ScienceDirect

These drugs have transformed treatment for type 2 diabetes and obesity. But the science hasn't stopped there.

Enter retatrutide

Retatrutide (code name LY3437943) is the next step in this evolution. Developed by Eli Lilly, it's a single engineered peptide that activates three different hormone receptors at once: GLP-1, GIP (glucose-dependent insulinotropic polypeptide), and glucagon. Earlier drugs targeted one or two of these. Retatrutide hits all three — which is why it's sometimes called a "triple agonist."

The idea is that each receptor contributes something different. GLP-1 reduces appetite and improves insulin response. GIP supports insulin secretion and influences fat metabolism. Glucagon, perhaps counter-intuitively, helps increase energy expenditure when activated in this controlled way. Together, the three pathways appear to produce results that go beyond what single- or dual-target drugs achieve.

What the research shows

The clinical data so far has been striking.

In a Phase 2 trial published in the New England Journal of Medicine, participants with obesity who took the highest dose of retatrutide for 48 weeks lost an average of around 24% of their body weight.

In TRIUMPH-1, the Phase 3 trial announced by Eli Lilly in May 2026, participants taking 12mg of retatrutide weekly for 80 weeks lost an average of 28.3% of their body weight — roughly 70 pounds. That level of weight loss is comparable to bariatric surgery, which has historically been the most effective intervention available. Scientific AmericanScientific American

For people with type 2 diabetes, a separate Phase 3 trial showed an average weight loss of around 36.6 pounds on the highest dose, alongside a 1.7 to 2 percent reduction in HbA1c (a long-term blood sugar marker). Scientific American

Research has also explored retatrutide's effects on liver health. In a Phase 2a trial focused on metabolic dysfunction-associated steatotic liver disease (formerly called fatty liver disease), retatrutide produced up to an 82% reduction in liver fat, with the majority of participants on higher doses returning to normal liver fat levels. Nature

Safety and what's still unknown

No drug is without trade-offs. The most common side effects reported across retatrutide trials have been gastrointestinal — nausea, diarrhea, and vomiting — which is consistent with other drugs in the GLP-1 family. A smaller number of participants (between 2.3% and 4.5%) reported dysesthesia, a burning or painful skin sensation that researchers say needs further study. Scientific American

It's also important to be clear about where things stand: retatrutide is not yet approved by any regulator. Phase 3 results are positive, but the drug still needs to complete the full regulatory review process before becoming available as a prescription medication. Long-term safety data — beyond the 80-week trial window — is still being gathered.

The bigger picture

Retatrutide is one drug, but it represents something broader: a shift in how medicine approaches complex, chronic conditions. Rather than targeting a single pathway, researchers are increasingly designing peptides that engage multiple biological systems at once — a closer match to how the body actually regulates itself.

For anyone following the science of metabolic health, peptide therapeutics are likely to keep producing significant developments over the next decade.

This page is for educational purposes only and does not constitute medical advice. Retatrutide is an investigational medication and is not currently approved for use. Always consult a qualified healthcare professional before considering any treatment.