How Fast Does Retatrutide Work? A Week-by-Week Timeline From the Phase 2 and TRIUMPH Trials

Written by Olha Orlyk. Updated October 26, 2026.

Retatrutide starts working within the first weeks, but the number everyone quotes, 28% body weight lost, takes 80 weeks to reach. In the phase 2 trial, people on the highest doses had lost about 17% of their body weight by week 24 and 24% by week 48, and the curve was still heading down when the study ended. In the phase 3 TRIUMPH-1 trial reported in May 2026, the 12 mg group reached 28.3% at 80 weeks, and a subgroup followed to 104 weeks hit 30.3%. So the honest answer to how fast retatrutide works has two parts: fast enough that most people see the scale move inside a month, and slow enough that the headline result is a two-year project.

Below is the week-by-week picture pieced together from the published trials, a comparison with semaglutide and tirzepatide at matching timepoints, the reasons the first twelve weeks are deliberately slower than they could be, and what none of this tells you about a vial bought online.

Retatrutide hits three receptors, GLP-1, GIP and glucagon, and the GLP-1 part works the same way semaglutide does: it slows stomach emptying and tells the brain's appetite centres that you have eaten. That effect is present from the first injection. People in the trials described smaller portions, loss of interest in snacks and feeling full sooner within the first week, before any weight had changed. The glucagon component, which raises energy expenditure and pushes the liver to burn fat, is the part that is unique to retatrutide and the reason its curve keeps falling after the other drugs flatten. It is also the part that needs the most careful escalation, because glucagon on its own raises heart rate.

The first month's weight loss in the trials was modest in absolute terms, a few percent of body weight, and part of it was water as glycogen stores dropped. That is the same with every GLP-1 drug. What you should not expect in month one is anything close to the trial's monthly average, because the trial's 12 mg group was not on 12 mg in month one. The mechanism in full is in what is retatrutide.

No single study reports every timepoint, so this table combines the 48-week phase 2 trial published in the New England Journal of Medicine in 2023 with the 80-week and 104-week TRIUMPH-1 results Lilly released in May 2026. Doses differ slightly between the two programmes (phase 2 tested 1, 4, 8 and 12 mg; phase 3 tested 4, 9 and 12 mg), which is why the columns are grouped as low, middle and high.

Two things stand out. The 4 mg arm, which looks underwhelming at week 24, catches up almost entirely by week 104 once participants in the extension were allowed to escalate to their maximum tolerated dose. And the placebo arm's 19% at week 104 is not a misprint: those participants were switched to retatrutide for the extension, which is why their loss jumps. Read the 104-week column as what happens when everyone eventually gets the drug.

In TRIUMPH-1, nobody started on 12 mg. The protocol began at a low dose and stepped up every four weeks, so a participant assigned to 12 mg typically did not reach it until month three or four. That design exists because the gut effects of all three receptor pathways stack: nausea was reported by 42% of the 12 mg group, diarrhea by 32%, vomiting by 25%, and 11.3% of that group stopped the drug because of side effects, compared with 4.9% on placebo. Faster escalation would have pushed those numbers higher and lost more people. The slow start is the price of the fast finish. The escalation schedules used in each trial are set out in retatrutide dosage in clinical trials, which is a description of study protocols, not a guide to dosing anything you might have obtained outside a trial.

Cross-trial comparisons are imperfect because the populations and durations differ, but the broad shape is consistent across the three drugs: similar first-month losses, then separation. Semaglutide 2.4 mg (STEP 1) reached about 15% at 68 weeks and had largely plateaued. Tirzepatide 15 mg (SURMOUNT-1) reached about 21% at 72 weeks and was flattening. Retatrutide 12 mg reached 28% at 80 weeks and was still falling in the 104-week extension. In other words, retatrutide is not dramatically faster at the start; it is slower to stop.

The detailed matchup with tirzepatide, including side-effect rates, is in retatrutide vs tirzepatide, and the four-drug comparison is in retatrutide vs Ozempic, Wegovy and Zepbound.

The trials give three signals. People with type 2 diabetes lost less and more slowly: TRIUMPH-2 reported 20.8% at the end of the study versus 28.3% in TRIUMPH-1, which mirrors what happens with semaglutide and tirzepatide, because diabetes blunts weight loss on every GLP-1 drug. People with established cardiovascular disease in TRIUMPH-3 lost 22.6%. People with heavier starting weights lost more in absolute kilograms but a similar percentage, which is why the trials report percentages. Sex and age had small effects in the phase 2 analysis. None of this predicts an individual's curve; the spread within each dose group was wide, and roughly a quarter of the 12 mg group in TRIUMPH-1 lost less than 25%, which is still a large amount.

Retatrutide trials have not yet published a formal withdrawal phase the way STEP 1 and SURMOUNT-4 did for semaglutide and tirzepatide, but there is no reason to expect it to be different: when those drugs are stopped, people regain roughly two thirds of the lost weight within a year. The speed of weight loss on the way down does not protect against regain on the way up. This matters for anyone thinking of retatrutide as a short, fast course; the trial evidence is for continuous use.

Retatrutide is not approved anywhere. Lilly has said it expects to file for approval after the TRIUMPH programme completes, with a decision unlikely before 2027. Every number in this article comes from pharmaceutical-grade drug, dosed by protocol, with monthly monitoring of heart rate, blood pressure, liver enzymes and side effects. Vials sold online as retatrutide, GLP-3 or RT are not covered by that evidence. Independent testing of such products has repeatedly found wrong doses, wrong peptides, and contamination, and in March 2026 the FDA issued a warning letter to a prominent online seller for marketing unapproved peptides for human use. If a product like that produces a different timeline, faster, slower or nothing, that tells you about the product, not about retatrutide. We do not provide dosing or sourcing guidance for unapproved products; the question of legal access is covered in how to get retatrutide and the naming confusion in what is GLP-3 RT.

Gastrointestinal effects follow the same pattern as weight loss, just earlier: they peak in the week or two after each dose increase and fade as the body adapts. The retatrutide-specific effects, a rise in resting heart rate of several beats per minute and tingling or altered skin sensation (dysesthesia, reported by 12.5% of the 12 mg group versus 0.9% on placebo), appeared in the first months and were mostly mild. Heart rate increases in phase 2 peaked around week 24 and then drifted back toward baseline. The full list, with trial percentages, is on our retatrutide side effects page.

Lilly released TRIUMPH-1 in May 2026, with TRIUMPH-2 and TRIUMPH-3 following over the summer; the company has indicated a regulatory filing once the programme is complete, which points to a possible approval decision in 2027. Pricing has not been announced. Tirzepatide's current self-pay prices through LillyDirect ($299 to $449 a month for the Zepbound KwikPen) are the closest guide to what a Lilly obesity drug costs out of pocket in 2026. We track the regulatory timeline in approval watch and the cost question in how much will retatrutide cost.

Retatrutide works quickly enough to notice within a month and keeps working for longer than any approved weight-loss drug tested so far: about 17% at six months, 24% at a year, 28% at 80 weeks and 30% at two years in the highest-dose groups, with the placebo-corrected effect still growing at the last measurement. The early phase is deliberately slow because the dose is escalated over three to four months to keep side effects manageable, so the first twelve weeks understate what the drug eventually does. All of those numbers describe the trial drug under trial conditions. Until retatrutide is approved, the only way to get that timeline is to be in one of the studies that produced it.

Retatrutide is an investigational drug. This article summarizes published trial results and does not provide guidance on using unapproved products.