The Cold Open
Ozempic made GLP-1 drugs a household name. Retatrutide, Eli Lilly's newer entry, is posting numbers that outpace it: about a quarter of body weight lost on average, and liver fat nearly wiped out on the top dose. Phase 3 results arrive late this year. Below: the early data, and a muscle-loss myth that won't die.
Peptide of the Month: Retatrutide
What it is. Retatrutide is a once-weekly injectable from Eli Lilly, in the same broad family as Ozempic (semaglutide) and Mounjaro (tirzepatide) but with a twist. Think of it like a car. Two of the pathways it activates (GLP-1 and GIP) are the brakes on appetite: they slow digestion, boost insulin response, and signal fullness to the brain. The third, glucagon, is the accelerator on metabolism: it tells the liver to burn its own fat and nudges resting energy expenditure upward. Ozempic pumps one brake. Mounjaro pumps both. Retatrutide pumps both brakes and hits the gas.
What it's supposedly for. Weight loss and Type 2 diabetes are the headline uses, with serious interest building around fatty liver disease.
The evidence as of today. The early human data is unusually strong. In a 48-week obesity trial of about 340 adults, the top dose produced a 24% average body-weight reduction, outpacing what semaglutide or tirzepatide posted in theirs. In Type 2 diabetics, blood sugar control improved dramatically at higher doses. And in a 2024 fatty-liver trial, 86% of people on the top dose returned to fully normal liver fat.
One name to watch: TRIUMPH. That's the label on Lilly's pivotal Phase 3 program, the one that will decide whether retatrutide reaches pharmacy shelves. First results expected late 2026.
Retatrutide didn't just beat placebo. It outpaced the numbers Ozempic and Mounjaro posted in their own trials.
The catch. Retatrutide is not FDA-approved yet. That's expected to change late this year or in early 2027 once TRIUMPH reads out. For now, the only way to get it is through research-chemical vendors, a grey market where quality, sterility, and even the identity of what's in the vial are inconsistent. And that third receptor is new territory; long-term effects need Phase 3 scrutiny before anyone calls this settled.
Myth Check: "GLP-1 drugs destroy your muscle"
The loudest objection to Ozempic-style drugs, and now to retatrutide, is that the weight loss comes disproportionately from muscle. You'll hear it in gym locker rooms, lifting forums, and a few confident TikToks.
A 2025 retatrutide study measured this with body-composition scans. The muscle-to-fat ratio lost looked roughly the same as on any serious weight-loss approach. These drugs aren't specifically eating muscle. Rapid weight loss, by any method, reduces muscle along with fat. It always has.
So what: the reframe isn't "these drugs destroy muscle." It's "fast weight loss without strength training and adequate protein always costs you muscle." The medication isn't the villain. The setup around it is.
The rule: If you are losing weight rapidly, you need to be resistance training and eating a high-protein diet. Period.
Not medical advice. Most peptides discussed in this newsletter are investigational or research chemicals. Talk to a clinician before starting anything.