Retatrutide Phase 2 Results: What 24% Weight Loss Means for the Future of Obesity Treatment

Key takeaways:

• Retatrutide produced up to 24.2% body weight loss in 48 weeks in a Phase 2 trial
• It is the first triple agonist (GLP-1 + GIP + glucagon receptor) to reach late-stage clinical trials
• The glucagon receptor component appears to drive significant liver fat reduction -- a major differentiator
• Phase 3 trials are ongoing with results expected by late 2026 or 2027
• Retatrutide is NOT FDA-approved and is currently only available through clinical trials

Important: This article is for educational and informational purposes only. It is not medical advice. Retatrutide is an investigational compound that has not been approved by the FDA for any indication. Always consult a qualified healthcare provider before making decisions about your health. See our full medical disclaimer.

Why retatrutide matters

Semaglutide changed obesity treatment by targeting one receptor (GLP-1). Tirzepatide pushed things further by targeting two (GLP-1 + GIP). Retatrutide targets three.

That third receptor -- the glucagon receptor -- is what makes this compound different from everything currently available. It does not just reduce appetite. Early data suggests it actively increases energy expenditure and drives fat burning in the liver, something the existing medications do not do as effectively.

The Phase 2 trial results, published in the New England Journal of Medicine in 2023 (PMID: 37351564), got the attention of researchers, clinicians, and patients for good reason. The weight loss numbers are the highest ever recorded in a controlled obesity trial.

The Phase 2 trial: study design

The trial enrolled 338 adults with obesity (BMI 30+) or overweight (BMI 27+) with at least one weight-related condition. Participants were randomized to receive either retatrutide at various doses or placebo, administered as a once-weekly subcutaneous injection for 48 weeks.

Here is how the groups broke down:

Group	Dose	Participants
Placebo	--	70
Retatrutide	1 mg	54
Retatrutide	4 mg (escalated)	55
Retatrutide	4 mg (fixed)	51
Retatrutide	8 mg (escalated)	53
Retatrutide	12 mg (escalated)	55

The escalated groups started at a lower dose and titrated up over several weeks, similar to how semaglutide and tirzepatide are prescribed today. This titration approach is standard practice to reduce gastrointestinal side effects.

The results: 24.2% weight loss at 48 weeks

The headline number: participants on the highest dose (12 mg) lost an average of 24.2% of their body weight in 48 weeks.

Here is the dose-response data:

Dose	Weight Loss at 24 Weeks	Weight Loss at 48 Weeks
Placebo	-2.1%	-2.1%
1 mg	-7.2%	-8.7%
4 mg (escalated)	-12.9%	-17.1%
4 mg (fixed)	-9.0%	-12.9%
8 mg (escalated)	-16.3%	-22.8%
12 mg (escalated)	-16.8%	-24.2%

Source: Jastreboff AM, et al. N Engl J Med. 2023; PMID: 37351564

A few things stand out.

First, the dose-response curve is steep. Higher doses produced substantially more weight loss, and the weight loss curves had not plateaued at 48 weeks. The 12 mg group was still losing weight when the trial ended. That suggests a longer treatment period could produce even greater reductions.

Second, the escalated dosing groups consistently outperformed the fixed-dose group at the same dose level (4 mg). This reinforces the importance of gradual titration, which is now standard for all incretin-based medications.

Third, more than 90% of participants in the 12 mg group achieved at least 5% weight loss, and over 60% achieved at least 20% weight loss. For context, the threshold most clinicians consider "clinically meaningful" is 5%.

How triple agonism works

To understand why retatrutide produces these results, you need to understand what each receptor does.

GLP-1 receptor: Reduces appetite by acting on the brain's hunger centers. Slows gastric emptying so you feel full longer. Improves insulin sensitivity. This is the same target that semaglutide hits.

GIP receptor: Works with GLP-1 to enhance insulin secretion and may improve fat tissue function. The GIP + GLP-1 combination is what makes tirzepatide more effective than semaglutide alone. Compare semaglutide vs tirzepatide for a detailed breakdown of dual vs single agonism.

Glucagon receptor: This is the new addition. Glucagon increases energy expenditure (your body burns more calories at rest), promotes fat oxidation (breaking down stored fat for energy), and drives lipid metabolism in the liver. Glucagon receptor activation is why retatrutide shows such strong effects on liver fat.

Think of it this way: GLP-1 reduces how much energy comes in. Glucagon increases how much energy goes out. GIP enhances the metabolic environment for both to work better. Triple agonism attacks obesity from three directions at once.

The liver fat data

One of the most clinically significant findings from the Phase 2 trial was the effect on liver fat. A subset of participants underwent MRI-based proton density fat fraction (MRI-PDFF) measurements to assess hepatic steatosis, commonly known as fatty liver.

At the 12 mg dose, participants saw an average relative reduction in liver fat of approximately 82% by week 48. For comparison, tirzepatide has shown liver fat reductions of around 50-55% in its SYNERGY-NASH trial data (PMID: 37840095).

This matters because metabolic dysfunction-associated steatohepatitis (MASH, formerly NASH) affects an estimated 5-12% of the global adult population and currently has very limited treatment options. If the Phase 3 data confirms these liver fat reductions, retatrutide could become a significant option for patients with both obesity and liver disease.

The glucagon receptor is likely responsible for much of this liver-specific benefit. Glucagon directly stimulates hepatic fat oxidation and reduces lipogenesis in the liver. Neither GLP-1 nor GIP has the same direct hepatic effect.

Side effects from the trial

The side effect profile in Phase 2 was broadly similar to other incretin-based medications, with GI symptoms being the most common.

Side Effect	12 mg Group	Placebo
Nausea	25%	6%
Diarrhea	22%	6%
Vomiting	13%	1%
Constipation	11%	1%
Decreased appetite	9%	1%

Discontinuation due to adverse events was relatively low at 6% across all retatrutide groups, compared to 4% for placebo. No cases of pancreatitis were reported. No serious safety signals emerged that were unexpected for this drug class.

The slow titration schedule appeared to mitigate GI symptoms effectively. The escalated dosing groups reported lower peak nausea rates than would be expected from a direct start at higher doses.

It is worth noting that Phase 2 trials are small (338 participants total). Rare side effects may only become apparent in larger Phase 3 trials or post-market surveillance. The long-term safety profile remains unknown.

How retatrutide compares to semaglutide and tirzepatide

Direct head-to-head trials between retatrutide and existing medications have not been conducted. Comparing across different trials has serious limitations -- patient populations, study designs, and endpoints can all differ. With that caveat, here is how the numbers line up.

Metric	Semaglutide 2.4mg (STEP 1)	Tirzepatide 15mg (SURMOUNT-1)	Retatrutide 12mg (Phase 2)
Weight loss (%)	14.9% at 68 weeks	22.5% at 72 weeks	24.2% at 48 weeks
Trial size	1,961	2,539	338
Trial phase	Phase 3 (approved)	Phase 3 (approved)	Phase 2
FDA status	Approved	Approved	Investigational
Receptors	GLP-1	GLP-1 + GIP	GLP-1 + GIP + Glucagon

Sources: STEP 1 (PMID: 33567185), SURMOUNT-1 (PMID: 35658024), Retatrutide Phase 2 (PMID: 37351564)

The retatrutide numbers are striking, especially considering the 48-week time frame. Both semaglutide and tirzepatide trials ran longer (68-72 weeks), and the retatrutide weight loss curve had not yet flattened. A 72-week retatrutide trial could potentially show even greater weight loss.

But the trial size matters. STEP 1 and SURMOUNT-1 each enrolled 5-7 times more participants. Larger trials provide more reliable effect size estimates. Phase 2 results sometimes look better than Phase 3 results because of smaller, more selective patient populations.

For a detailed comparison of how retatrutide stacks up against existing options, see our semaglutide vs retatrutide comparison and tirzepatide vs retatrutide comparison.

Phase 3 timeline and what to expect

Eli Lilly (retatrutide's developer) launched multiple Phase 3 trials under the TRIUMPH program. These trials are significantly larger, enrolling thousands of participants across multiple countries.

Key Phase 3 trials include:

• TRIUMPH-1: Retatrutide for obesity in adults without type 2 diabetes
• TRIUMPH-2: Retatrutide for obesity in adults with type 2 diabetes
• TRIUMPH-3: Retatrutide for MASH (fatty liver disease)

Results from these trials are expected in late 2026 or 2027. If the Phase 3 data confirms the Phase 2 findings, an FDA submission could follow in 2027, with potential approval in 2028.

Until then, retatrutide is only available through enrollment in clinical trials. It is not approved for any indication and should not be obtained through unregulated channels.

What this means for patients

If you are currently considering or using semaglutide or tirzepatide for weight management, the retatrutide data is encouraging but not immediately actionable. Here is a realistic perspective.

Retatrutide is not available yet. Phase 3 trials are ongoing. Even optimistic timelines put FDA approval at 2028 at the earliest. The approved options today -- semaglutide and tirzepatide -- are well-studied and effective.

Phase 2 results are promising but preliminary. The trial was small. Phase 3 data may show different efficacy or safety results. History has examples of drugs that looked excellent in Phase 2 and failed or underperformed in Phase 3.

The liver fat data is the real story. For patients with both obesity and fatty liver disease, the glucagon receptor component could be a meaningful advancement over current options. This is the area where retatrutide most clearly differentiates itself.

More options are better. Not everyone responds to the same medication. Having a third mechanism of action (glucagon) provides another tool. Some patients who do not respond adequately to semaglutide or tirzepatide may respond to triple agonism.

Use our dosage calculator to understand the dosing math for currently available peptide medications while the research continues.

FAQ

What is retatrutide?

Retatrutide (LY3437943) is an investigational triple-receptor agonist developed by Eli Lilly. It activates three receptors simultaneously: GLP-1, GIP, and glucagon. It is currently in Phase 3 clinical trials for obesity and metabolic dysfunction-associated steatohepatitis (MASH). It is not FDA-approved for any indication.

How much weight loss did retatrutide produce in trials?

In the Phase 2 trial published in the New England Journal of Medicine (PMID: 37351564), participants on the highest dose (12 mg) lost an average of 24.2% of their body weight over 48 weeks. This is the highest weight loss recorded in a controlled obesity trial to date, though the data is from a relatively small Phase 2 study.

Is retatrutide better than semaglutide or tirzepatide?

It is too early to say definitively. Cross-trial comparisons have significant limitations. The Phase 2 weight loss numbers (24.2% at 48 weeks) are higher than published semaglutide (14.9% at 68 weeks) and tirzepatide (22.5% at 72 weeks) data, but the retatrutide trial was smaller and shorter. Head-to-head trials would be needed for a fair comparison.

When will retatrutide be available?

Phase 3 trials under the TRIUMPH program are currently enrolling, with results expected in late 2026 or 2027. If the data supports it, an FDA submission could follow in 2027, with potential approval in 2028 at the earliest. There is no guaranteed timeline -- regulatory approval depends on Phase 3 results and the FDA review process.

What makes the glucagon receptor different?

The glucagon receptor, when activated, increases energy expenditure (your body burns more calories at rest), stimulates fat oxidation, and promotes lipid metabolism in the liver. This is different from GLP-1 (which primarily reduces appetite and slows gastric emptying) and GIP (which enhances insulin sensitivity). The glucagon component is believed to be responsible for the strong liver fat reduction seen in the Phase 2 trial.

Bottom line

Retatrutide's Phase 2 data represents a meaningful step forward in obesity research. The 24.2% weight loss at 48 weeks, combined with significant liver fat reduction, suggests that triple agonism offers advantages over current single and dual agonist options.

But it is Phase 2 data from 338 people. The Phase 3 trials will determine whether these results hold up in larger, longer, more diverse populations. Until those results are in and the FDA has reviewed them, the approved options -- semaglutide and tirzepatide -- remain the evidence-based standard of care.

The science is moving fast. We will update this article as Phase 3 data becomes available.

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This article is for educational purposes only and is not medical advice. Retatrutide is an investigational compound not approved by the FDA for any indication. Always consult a qualified healthcare provider before making decisions about your health. See our full medical disclaimer.

Sources

1. Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity -- A Phase 2 Trial. New England Journal of Medicine. 2023;389(6):514-526. PMID: 37351564
2. Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). New England Journal of Medicine. 2021;384(11):989-1002. PMID: 33567185
3. Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). New England Journal of Medicine. 2022;387(4):327-340. PMID: 35658024
4. Loomba R, et al. Tirzepatide for Metabolic Dysfunction-Associated Steatohepatitis with Liver Fibrosis (SYNERGY-NASH). New England Journal of Medicine. 2024;391(4):299-310. PMID: 37840095