What percentage of weight loss did retatrutide achieve in trials?

In the Phase 2 trial (Jastreboff et al., NEJM 2023), participants in the highest 12mg weekly dose arm lost a mean of ~24.2% of body weight at 48 weeks, vs ~2.1% in the placebo arm. About 26% of the 12mg cohort lost 25% or more of their starting body weight. These are reported clinical-trial outcomes, not predicted outcomes outside the trial context.

How does retatrutide compare to semaglutide and tirzepatide on weight loss?

Semaglutide 2.4mg achieved ~14.9% at 68 weeks (STEP-1). Tirzepatide 15mg achieved ~22.5% at 72 weeks (SURMOUNT-1). Retatrutide 12mg achieved ~24.2% at 48 weeks (Phase 2). These are separate trials with different populations and durations, so direct comparison is approximate, not statistically equivalent. Phase 3 TRIUMPH-1 will produce more comparable data.

What was the dose-response curve in the retatrutide Phase 2 trial?

The trial tested 1mg, 4mg, 8mg, and 12mg weekly doses. Mean weight loss at 48 weeks scaled roughly with dose: 8.7% (1mg), 17.1% (4mg), 22.8% (8mg), 24.2% (12mg). The curve was steeper at lower doses and flatter at higher ones, suggesting diminishing additional benefit above 8mg - though Phase 3 may revisit dose selection.

Why is retatrutide producing higher weight loss than the dual-agonist class?

The third receptor it activates - the glucagon receptor - increases resting energy expenditure. GLP-1 and GIP reduce calorie intake; glucagon raises calorie output. The combination closes both sides of the energy-balance equation in a way no single- or dual-agonist drug currently does. That's the mechanistic explanation for the larger reported percentages.

Clinical Data · Retatrutide

Retatrutide Weight Loss: 24% at 48 Weeks - What the Trial Data Actually Shows

May 26, 2026 · 9 min read

The number that put retatrutide on every metabolic researcher's radar is ~24.2% mean weight loss at 48 weeks. It comes from the Phase 2 trial published in the New England Journal of Medicine in August 2023 (Jastreboff et al.). It's the largest mean weight-loss figure ever published for a pharmacological agent in trials - bigger than semaglutide (STEP-1), bigger than tirzepatide (SURMOUNT-1). This piece walks through the actual data, the dose-response curve, and why the percentage matters.

RUO framing throughout. Retatrutide is investigational, not approved by FDA / EMA / MHRA. New-U supplies it strictly as a research reagent (GLP-1 RC-R) for laboratory use only - not for human weight-loss application. Trial outcomes describe what happened in the trial, not what happens outside it.

The Headline Number

Phase 2 (Jastreboff AM, Kaplan LM, Frias JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial. N Engl J Med. 2023;389:514-526):

338 adults with obesity (BMI ≥30, or ≥27 with weight-related comorbidity)
48 weeks of weekly subcutaneous injection
Five arms: placebo, retatrutide 1mg, 4mg, 8mg, 12mg
Primary endpoint: percent change in body weight from baseline

The Dose-Response Curve

Arm	Mean weight loss at 48 weeks	vs placebo
Placebo	~2.1%	baseline
Retatrutide 1mg	~8.7%	+6.6 pp
Retatrutide 4mg	~17.1%	+15.0 pp
Retatrutide 8mg	~22.8%	+20.7 pp
Retatrutide 12mg	~24.2%	+22.1 pp

The curve is steep through 8mg and starts to flatten at 12mg. That diminishing-returns shape is important - it suggests Eli Lilly may select 8mg or 12mg as the lead Phase 3 dose rather than pushing higher. The trial also showed weight loss was still declining at week 48 in higher-dose arms (not yet plateaued), so the 72-week Phase 3 trials may produce larger numbers.

The Responder Distributions

Mean is one thing; the proportion of participants who hit specific thresholds is what matters for clinical and consumer interpretation. In the 12mg arm at 48 weeks:

Threshold	% of 12mg arm achieving
≥5% weight loss	~100%
≥10%	~94%
≥15%	~83%
≥20%	~63%
≥25%	~26%

For context: in SURMOUNT-1 (tirzepatide 15mg), ~36% of participants hit ≥25% weight loss at 72 weeks. Retatrutide hit 26% at 25% threshold by week 48 alone. Whether the 72-week Phase 3 readout pulls ahead of tirzepatide on the ≥25% responder rate is a key question to watch.

The Honest Comparison to Semaglutide and Tirzepatide

Drug	Trial	Top dose	Duration	Mean weight loss
Semaglutide	STEP-1 (2021)	2.4mg weekly	68 weeks	~14.9%
Tirzepatide	SURMOUNT-1 (2022)	15mg weekly	72 weeks	~22.5%
Retatrutide	Phase 2 (2023)	12mg weekly	48 weeks	~24.2%

Three important caveats:

Different populations. Retatrutide's Phase 2 was a smaller cohort. Direct comparison requires careful adjustment for baseline characteristics.
Different durations. Comparing 48 weeks (retatrutide) to 68 / 72 weeks (semaglutide / tirzepatide) is apples-to-oranges. Phase 3 TRIUMPH-1 runs 72 weeks, which will produce a comparable readout.
Phase 2 vs Phase 3. Phase 2 cohorts skew toward more-compliant, more-monitored participants. Phase 3 efficacy almost always erodes vs Phase 2. The Phase 2 24.2% should be considered an upper bound, not a baseline.

For the full breakdown of the Phase 3 trials underway, see our companion piece on the TRIUMPH program.

Why the Glucagon Receptor Is the Differentiator

The mechanistic explanation for retatrutide's higher reported percentage is the third receptor it activates: the glucagon receptor. Semaglutide acts on GLP-1 alone. Tirzepatide adds GIP. Retatrutide adds glucagon on top of both.

GLP-1 + GIP reduce caloric intake (appetite suppression, slower gastric emptying, improved satiety signalling).
Glucagon receptor activation increases caloric expenditure (raises resting metabolic rate, mobilises stored fat).

The hypothesis: closing both sides of the energy-balance equation produces more weight loss than addressing only one side. The Phase 2 data is consistent with that hypothesis at the population level. Whether the effect scales in Phase 3 will be answered by TRIUMPH-1.

Other Trial Outcomes Worth Knowing

Weight loss was the primary endpoint, but the Phase 2 trial also reported improvements in:

HbA1c - meaningful reductions in participants with prediabetes; T2D was excluded from this trial but is the subject of TRIUMPH-2
Systolic and diastolic blood pressure - reductions consistent with the broader GLP-1 class
Triglycerides and lipid markers - favourable shifts
Liver fat (MRI-PDFF where measured) - substantial reductions, supporting the MASLD hypothesis behind TRIUMPH-5
Waist circumference - consistent with the weight-loss percentages

The Side-Effect Picture

Reported adverse events were dose-dependent and predominantly gastrointestinal - nausea, diarrhoea, constipation, vomiting. The pattern is consistent with the broader GLP-1 / GIP class but somewhat more pronounced at the highest doses. Discontinuation rates due to adverse events were higher at 12mg than at lower doses but still lower than commonly assumed for the class. Cardiovascular safety signals were not detected at Phase 2 sample size; TRIUMPH-3 will resolve CV outcomes at scale.

Status check. The figures here are reported Phase 2 outcomes in a controlled trial. They are not predictions for individual results outside the trial. Retatrutide is not approved by FDA, EMA, or MHRA. New-U supplies it as a research reagent (GLP-1 RC-R) for laboratory use only, not for human consumption or weight-loss application. It is also on the WADA Prohibited List (S2).

What This Means for Research Buyers

The headline percentage (~24% at 48 weeks) is the largest in the published pharmacological literature. The numbers are real.
The dose-response curve suggests 8mg may be near the efficiency frontier; 12mg pushes the mean up but with diminishing returns and more side-effect burden.
The Phase 3 comparison (TRIUMPH-1 vs SURMOUNT-1) will produce the first apples-to-apples readout. Late 2026 / early 2027 is the window.
The verification step is non-negotiable. Counterfeit and underdosed retatrutide is documented in the research-grade market. Janoshik / Freedom Diagnostics COAs are the only defence - how to read a COA.