Retatrutide Weight Loss: Trial Results, Doses, and Timeline

Summary: Retatrutide produced 17.5%, 22.8%, and 24.2% weight loss at 4, 8, and 12 mg in Phase 2, and around 28.7% in the Phase 3 TRIUMPH-4 readout, making it the largest reduction reported for any obesity drug in late-stage trials.

This content is for informational purposes only and is not medical advice. Always consult a qualified healthcare provider before starting, changing, or stopping any medication.

Retatrutide produced 17.5%, 22.8%, and 24.2% mean body weight loss at the 4 mg, 8 mg, and 12 mg doses over 48 weeks in its Phase 2 obesity trial, and approximately 28.7% at 68 weeks in the Phase 3 TRIUMPH-4 readout reported by Eli Lilly in late 2025 to 2026 ^[1][2]. Those numbers make retatrutide the most effective injectable obesity drug ever tested in a randomized trial. It is not FDA approved. As of May 2026 the only legal way to receive it is through an active clinical trial.

The rest of this page is the data, the mechanism, and the access reality.

What retatrutide is

Retatrutide is a once-weekly subcutaneous injection developed by Eli Lilly. It is a single peptide that activates three hormone receptors at once: GLP-1, GIP, and glucagon. Semaglutide hits one of those targets. Tirzepatide hits two. Retatrutide hits all three ^[5]. The drug is sometimes called a triple agonist or a GLP-3, though "GLP-3" is marketing shorthand, not a real receptor.

The glucagon arm is the new lever. GLP-1 and GIP signaling reduce appetite and slow gastric emptying. Glucagon receptor activation raises basal energy expenditure and pushes the liver toward fat oxidation. The combination produces both lower calorie intake and higher calorie burn, which is the mechanistic argument for why retatrutide outperforms drugs that only suppress appetite.

Phase 2 weight loss results, dose by dose

The Phase 2 trial that put retatrutide on the map was published by Ania Jastreboff and colleagues in the New England Journal of Medicine in 2023 ^[1]. The study enrolled 338 adults with BMI of 30 or greater (or 27 or greater with a weight-related condition) and randomized them to placebo or retatrutide at one of four maximum dose levels. After 48 weeks of titration and treatment, the mean percent change in body weight from baseline was:

At the 12 mg dose, more than a quarter of participants lost at least 30% of their starting body weight. That had never been reported before for an injectable obesity drug. The weight loss curve at 48 weeks was still declining at the 8 mg and 12 mg doses, meaning participants had not plateaued. That open-ended trajectory is what made Lilly and outside investigators expect even larger numbers at the longer Phase 3 endpoint.

Phase 3 TRIUMPH-4 readout

TRIUMPH-4 is the obesity arm of Lilly's Phase 3 retatrutide program. Lilly reported topline results in late 2025 and 2026, with the headline figure landing around 28.7% mean weight loss at 68 weeks at the maximum tolerated dose ^[2]. The trial used a longer treatment window than Phase 2 (68 versus 48 weeks), which captured more of the late-trajectory decline that Phase 2 had only hinted at.

For context, the comparable Phase 3 numbers for the two approved competitors are roughly 21% (tirzepatide at 15 mg in SURMOUNT-1) and 15% (semaglutide at 2.4 mg in STEP-1) at 68 to 72 weeks ^[3][4]. The retatrutide number is meaningfully higher than either, and is closer to the weight loss seen with sleeve gastrectomy at one year, which typically runs 25 to 30% depending on the cohort.

Why retatrutide beats tirzepatide and semaglutide

The simplest answer is the glucagon receptor. The harder answer is that adding glucagon to a GLP-1 plus GIP backbone is not free. Glucagon raises blood glucose if unopposed, which is the opposite of what you want in a diabetes drug. Lilly's balancing act was to combine glucagon with strong enough GLP-1 and GIP activity to neutralize the hyperglycemic effect while preserving the energy-expenditure benefit. The Phase 2 data showed they pulled it off. HbA1c dropped in participants with prediabetes despite the glucagon component, suggesting the GLP-1 and GIP contributions dominate glycemic control while glucagon contributes the metabolic-rate piece.

The practical consequence is dose-dependent. At low retatrutide doses the appetite effects look similar to high-dose tirzepatide. At high retatrutide doses the cumulative effect of all three pathways pushes weight loss into a range that single and dual agonists cannot reach. The 12 mg arm in Phase 2 averaged 24.2% loss; the placebo arm averaged 2.1%. Almost no participants on 8 mg or 12 mg failed to lose weight.

Approximate timeline of weight loss

The Phase 2 trajectory data shows a fairly consistent month-by-month decline once the dose is escalated past the initiation level. The titration schedule used in TRIUMPH-4 starts at 2 mg weekly for the first four weeks, doubles to 4 mg for the next four weeks, then steps up at 4-week intervals to the assigned maximum dose. Most participants do not reach their target dose until somewhere between weeks 16 and 24.

The numbers in that table are eyeballed off the Phase 2 and Phase 3 curves and represent the higher-dose arms. People on the 4 mg or 8 mg arms run lower. People with a higher starting BMI tend to lose a larger percentage of body weight than people closer to the BMI 27 enrollment floor, which matches what semaglutide and tirzepatide trials also showed.

How long does retatrutide take to work

Appetite suppression usually starts within the first 1 to 2 weeks. Measurable weight change on the scale typically lags by a few weeks because the early effect is dominated by water shifts and slowed gastric emptying. By week 4, most participants in Phase 2 had lost 2 to 5 pounds. By week 12, the high-dose arms had crossed the 8 to 10% loss mark. If you have not seen any movement on the scale by 8 to 12 weeks of consistent dosing past the initiation phase, the most common explanations are under-dosing, calorie compensation through eating more between doses, or inaccurate scale measurement.

Who can access retatrutide

There are three paths, and only one of them is legal in the United States.

1. Clinical trial enrollment. Lilly's TRIUMPH program is the only sanctioned route. Active sites can be found on clinicaltrials.gov by searching for "retatrutide." Most enrollment for TRIUMPH-4 closed before the Phase 3 readout, but follow-on trials in osteoarthritis, sleep apnea, and cardiovascular outcomes have been adding participants. Inclusion criteria typically require BMI of 30 or higher (or 27 with a weight-related comorbidity) and exclude people with type 1 diabetes, recent cardiovascular events, or active malignancy.

2. FDA approval. Lilly has signaled an obesity submission once the full TRIUMPH program reads out. Best-case timeline puts FDA action somewhere in 2027 or 2028. Until then, no licensed US pharmacy can dispense retatrutide.

3. The research-peptide gray market. Online vendors sell vials labeled "retatrutide for research use only, not for human consumption." This market exists because the drug is not yet a Schedule or DEA-controlled substance, so the supply side faces fewer barriers than a controlled drug would. People who self-dose from these vials are taking on risks that are not present with brand-name tirzepatide or semaglutide. There is no manufacturer accountability, no chain-of-custody validation, no FDA inspection of the source facility, and no guarantee that the powder in the vial is the molecule on the label.

Side effects and safety signals

The Phase 2 and Phase 3 safety profile looks similar to tirzepatide and semaglutide: nausea, vomiting, diarrhea, and constipation are the dominant adverse events, almost all mild to moderate, and concentrated in the dose-escalation phase. About 6 to 16% of Phase 2 participants on the active arms discontinued for adverse events, a rate that scales with dose. There was no signal for pancreatitis, medullary thyroid carcinoma, or major adverse cardiovascular events in the trials reported so far, but the populations and follow-up windows are not yet large enough to rule those out at population scale.

Two signals to watch as the Phase 3 program completes: a small increase in resting heart rate (a few beats per minute, similar to other GLP-1 drugs), and reports flagged in independent reviews of possible increases in bone fractures and kidney function changes at high doses. These have not been confirmed as causal in the regulatory record, and they will be the focus of label discussions if and when Lilly files.

Does retatrutide work for everyone

In the Phase 2 12 mg arm, more than 90% of participants who completed the trial lost at least 5% of baseline weight, and more than 60% lost at least 20%. That is the highest response rate reported for any obesity drug at this duration. Non-response at this magnitude is rare. When non-response does happen, the usual culprits are real-world adherence (skipped doses, dose interruptions for travel or supply gaps), calorie compensation, or undiagnosed conditions that suppress weight loss independent of appetite (hypothyroidism, Cushing's, certain psychiatric medications).

Weight loss by BMI category followed the same pattern other GLP-1 trials showed. Higher starting BMI produced larger absolute weight loss in pounds but similar percentage loss. People in the BMI 30 to 35 range and people in the BMI 40 and above range both lost roughly comparable percentages of their starting weight, though the BMI 40+ group lost more pounds.

Weight regain after stopping

The retatrutide trials have not yet published a structured withdrawal study, but tirzepatide and semaglutide both show substantial regain when treatment stops. The SURMOUNT-4 withdrawal study showed about 14% regain after one year off tirzepatide in people who had previously lost 21%. Retatrutide is mechanistically similar, and the same physiology that drives regain (appetite normalization, possibly a downshift in basal energy expenditure as glucagon signaling fades) will apply. Plan for retatrutide as a chronic medication, not a finite-course treatment, if and when it reaches the market.

Common questions about retatrutide weight loss

How much weight loss does retatrutide cause?

Phase 2 produced 17.5%, 22.8%, and 24.2% mean loss at 4 mg, 8 mg, and 12 mg over 48 weeks. Phase 3 TRIUMPH-4 reported about 28.7% at 68 weeks at the top dose.

How does retatrutide compare to tirzepatide and semaglutide?

Tirzepatide tops out around 21% in SURMOUNT-1 and semaglutide around 15% in STEP-1. Retatrutide outperforms both because it adds glucagon receptor activation to GLP-1 and GIP.

Is retatrutide FDA approved?

No. As of May 2026 it is not approved by any regulator. Lilly is expected to file with the FDA after the full Phase 3 program reads out, with possible action in 2027 or 2028.

How can I get retatrutide?

Legally, only through an active clinical trial. Search clinicaltrials.gov for current TRIUMPH and related trials. Some research-peptide vendors sell unapproved retatrutide online, which is not legal for human use.

How long does retatrutide take to work?

Appetite suppression usually begins within 1 to 2 weeks. Scale movement typically appears by week 4. The 8 to 10% loss mark is usually crossed around week 12 on the higher dose arms.

What is the retatrutide dose for weight loss?

Phase 3 used a titration starting at 2 mg weekly, doubling to 4 mg after 4 weeks, then stepping up every 4 weeks to a maximum of 8 mg or 12 mg depending on tolerance and study arm.

What does retatrutide do that tirzepatide does not?

Retatrutide activates the glucagon receptor in addition to GLP-1 and GIP. Glucagon activation raises resting energy expenditure and pushes the liver toward fat oxidation, which adds a calorie-burn effect on top of appetite suppression.

Why am I not losing weight on retatrutide?

The most common reasons are skipped or delayed doses, calorie compensation between injections, undiagnosed conditions that blunt weight loss (hypothyroidism, certain psychiatric drugs), or being on a sub-therapeutic dose. Genuine non-response at high doses is uncommon in trial data.

Will I regain weight if I stop retatrutide?

Likely yes. Trials of related drugs show 10 to 15% regain within a year of stopping. Retatrutide should be planned as a long-term medication if and when it becomes available.

Does retatrutide work for BMI 30 to 35?

Yes. Phase 2 enrolled participants from BMI 27 upward, and percentage weight loss was similar across BMI strata. Higher BMI groups lost more pounds; lower BMI groups lost a similar percentage of starting weight.

Is retatrutide safe?

Trial safety signals look similar to tirzepatide and semaglutide, dominated by GI side effects during titration. Some independent reviews have flagged possible bone fracture and kidney signals at high doses, which the Phase 3 label discussions will address.

How do I know if retatrutide is working?

Appetite suppression in the first 2 weeks is the earliest sign. Steady weekly weight loss by week 4 to 6 is the next. If 8 to 12 weeks pass with no scale movement and dosing has been consistent, the issue is usually adherence, calorie intake, or an unrelated medical condition rather than the drug.

What this article does not cover

This page is the weight-loss data and the access reality. Side-effect management, injection technique, vial reconstitution, the legality of research-peptide imports in specific countries, and head-to-head dose conversion against tirzepatide each have separate dedicated pages on this site. The data here is current to the public Lilly press release and the Jastreboff NEJM 2023 publication. If Lilly files for FDA approval and a label appears, the dosing in that label will supersede the trial protocols described above.