Retatrutide research spans Phase 1, Phase 2 and Phase 3
Looser clothes can raise hopes about your health. The studies measured less weight and improved blood readings. Those gains haven't settled the lasting risks.
The early studies leave lasting safety uncertain
A change on your scale can make you wonder about better health. Retatrutide hasn't passed all the checks for an approved medicine. Eli Lilly started testing in 2019 and reached larger studies by 2026. Phase 3 means larger studies before a drug can seek approval.
Phase 1 checked early safety; Phase 2 tested benefit and harm further. Retatrutide copies GLP-1, a hormone that helps curb hunger and lower high sugar. Other hormones it copies affect insulin, which moves sugar into cells for fuel, and the body's use of fat. Together, those effects seem to produce greater weight loss than medicines copying fewer hormones.
The main Phase 2 weight test ran for only 48 weeks. People in the group receiving the most drug lost an average 24.2% of starting weight [1]. Those with type 2 diabetes also had better sugar tests [2]. Your lasting benefits and possible risks need the ongoing larger Phase 3 studies.
Retatrutide stays in blood and helps the body use sugar
Retatrutide can work between shots because it briefly attaches to a common blood protein, a substance made by your body to carry other substances along in blood. That attachment slows removal by the kidneys. About half the retatrutide was still in blood after 6 days [4].
The drug's size helps explain how researchers designed it, rather than telling you how much to take. Each molecule, a tiny piece of the drug, weighs 4,731 daltons. Daltons are units for weighing something that small. The weight isn't an amount for a shot.
Cell Discovery reported lab studies of the drug in 2024 [3]. Those tests examined where hormones act on cells, without measuring anyone's weight loss. Retatrutide copied GIP most strongly, a hormone that adds insulin after meals and affects fat cells. The finding may explain part of the weight loss; your own response remains unknown.
Other effects reduce appetite and delay food passing from your stomach. By copying glucagon more gently, the drug helps the liver use fat for fuel. Full-strength glucagon could raise sugar too far; the gentler effect appears to avoid that rise. Your health still needs tests in people, beyond these lab findings.
Earlier lab tests, before testing in people, found more energy use without less food being eaten, suggesting that fat burning could add to the benefit of reduced hunger. That extra energy use doesn't yet establish a lasting benefit for you. Medicines copying only a pair of hormones lacked that extra fat-burning effect.
Phase 1b tested few people; Phase 2 tested more
Phase 1b tried repeated shots in a small type 2 diabetes study. Urva's 2022 Lancet report followed 72 adults with type 2 diabetes for only 12 weeks. Some received 0.5 mg weekly; another group received 3, then 6, then 9, then 12 mg. Those were study amounts, given with doctors watching for harm.
The highest group had 8.96 kilograms of extra weight loss compared with placebo, injections without study medicine. The authors' 90% confidence range was 6.75 to 11.16 kilograms of extra loss; this method covers the true average in about nine of ten repeats of a study like this. That range doesn't give you a personal chance of losing weight.
Half the drug was still in blood after 6 days. New health problems arose in 63% of participants, mostly affecting the stomach or bowels. The authors judged the risks acceptable for that early study [4].
Phase 2 checked weight loss over a longer period. Jastreboff's 2023 report followed 338 adults in a 48-week test. Their weight-for-height score was at least 30, or 27 to below 30 with a weight-related illness. The scores describe weight relative to height, rather than pounds alone.
The 1, 4, 8 and 12 mg groups lost more weight as amounts rose. At 12 mg, weight fell 24.2%, compared with 2.1% with placebo. Most stomach problems were rated mild or moderate, and pulse rises peaked around 24 weeks. At 12 mg, 18% stopped, chiefly because people couldn't tolerate sickness or bowel trouble [1].
Phase 2 separately followed people with type 2 diabetes. Rosenstock's 2023 Lancet report followed 281 adults with type 2 diabetes in a 36-week test. Doctors raised amounts from 0.5 to 12 mg in steps. At 24 weeks, average sugar fell at 12 mg, while placebo barely changed sugar.
At 36 weeks, weight fell 16.94%, compared with 3.00% with placebo. Problems with the stomach or bowels affected 35%; nobody died or developed dangerously low sugar [2]. See Retatrutide results for more findings and Retatrutide references on the references page for the reports.
The liver test involved a small, selected group. Sanyal's 2024 Nature Medicine study followed 98 people with high weight and fatty liver, without type 2 diabetes. On scans, at least 10% of each liver consisted of fat. At 12 mg over 24 weeks, 82.4% of the liver fat present before treatment was lost.
Normal meant fat made up below 5% of the liver; 86% of people reached that point [5]. The people and the amount of fat are different measures. At 48 weeks, the 12 mg group had lost 86.0% of its original liver fat, leaving a small share of that fat behind.
2024–2025 work found muscle loss and better kidney tests
Fat loss also came with less muscle. Coskun's smaller 2025 study used X-ray scans to measure fat and other tissue [12]. Higher amounts meant greater fat loss, but muscle and other tissue decreased too. Less weight doesn't tell you whether your strength has been protected.
Less protein in urine was encouraging for kidneys. A 2025 report checked kidney tests from Phase 2, the earlier benefit-and-harm studies [13]. Protein, a substance your body uses to build tissue and carry other substances, normally stays in blood; damaged kidneys can let some escape into urine. At 12 mg in type 2 diabetes, that leakage fell about 37% more than with placebo.
People with obesity had a 28-31% fall at 8-12 mg compared with placebo. Blood tests also suggested their kidneys filtered waste better. Both findings were encouraging, but didn't prove protection from kidney illness. Your kidneys' health over years still needs longer testing.
Tests suggested more fat burning helped explain weight loss. Pearson's 2026 work didn't test whether those changes prevent illness [14]. Higher amounts went with lower fats in blood and better use of insulin, the hormone that helps cells take in sugar. In people without type 2 diabetes, the authors' calculations suggested increased fat burning explained some weight loss. That doesn't tell you the weight you would lose from burning more fat.
Blood proteins that handle fat fell with harmful cholesterol. Wen's 2025 work involved people with type 2 diabetes and people with obesity [15]. The proteins studied help control how fats move through blood. Those proteins fell alongside harmful cholesterol, an encouraging change that may involve glucagon, a hormone affecting energy use. The study didn't prove that lower amounts of these proteins prevent illness for you.
Reviews compare existing findings rather than give treatment to more people. A 2025 Biomolecules review judged Phase 1/2 weight loss of about 24% a major advance [6]. Another 2025 review in World Journal of Cardiology ranked these drugs highest for weight loss [11]. Its authors expected heart and sugar benefits, but lasting health gains still need testing.
Phase 3 must check benefits and harm over time
Your health years later needs longer checks than an early weight test. TRIUMPH-2 studies adults with obesity and type 2 diabetes [7]. Another study checks serious heart problems and kidney health [8]. A further study compares retatrutide with tirzepatide [9], while the first Phase 1 test examined early safety and how long the drug remained in blood [10].
Phase 3 means larger studies before possible approval; results weren't published by mid-2026. Phase 2 found about 24% weight loss alongside improved sugar readings; those Phase 2 studies were carefully run, but Phase 2 tested too few people to settle every approval question. An encouraging average doesn't promise either benefit or safety for you.