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Retatrutide vs Tirzepatide vs Semaglutide: How the Trial Results Compare

One target, two targets, three targets: semaglutide, tirzepatide and retatrutide represent three generations of the same drug idea. Here is what their landmark trials actually reported, what can and cannot be compared across them, and where each stands with regulators.

Abstract lattice illustration for the retatrutide, tirzepatide and semaglutide comparison
Contents
  1. One target, two targets, three targets
  2. What the landmark trials reported
  3. The regulatory line that actually matters
  4. Side effects: same family, same pattern
  5. The honest bottom line

Key findings

  1. 1The three drugs are one design idea in three generations - semaglutide activates one gut-hormone receptor (GLP-1), tirzepatide two (GLP-1 + GIP), retatrutide three (GLP-1 + GIP + glucagon).
  2. 2Landmark-trial weight loss: roughly 15% (semaglutide, 68 weeks), 21% (tirzepatide 15 mg, 72 weeks), and 24% (retatrutide 12 mg, 48 weeks) - but these come from different trials with different populations and durations, so they are not head-to-head results.
  3. 3No published head-to-head trial includes retatrutide; comparing its numbers to the others is indirect by definition.
  4. 4Semaglutide and tirzepatide are FDA-approved and pharmacy-dispensed; retatrutide is investigational, so anything sold under its name outside a trial is unregulated gray-market product.
  5. 5All three share the same dominant side-effect pattern in trials - gastrointestinal symptoms, mostly during dose escalation.

Evidence level

Promising evidence

All three drugs have strong randomized-trial evidence for substantial weight loss. Semaglutide and tirzepatide are FDA-approved with published phase 3 programs; retatrutide has a striking published phase 2 result but remains investigational, with phase 3 trials ongoing. Cross-trial number comparisons are indirect because the trials used different populations, durations and designs.

Regulatory status

Status requires verification

Semaglutide and tirzepatide are FDA-approved (for type 2 diabetes and for chronic weight management under separate brand names). Retatrutide is not approved anywhere and is available only inside clinical trials. The overall status field is marked for verification because the three drugs' statuses differ and change over time.

Semaglutide, tirzepatide and retatrutide are usually presented as rivals. It is more accurate to call them three generations of the same idea: activate the gut-hormone receptors that curb appetite - first one of them, then two, then three. Each generation's landmark trial reported bigger average weight loss than the one before. But the way those numbers get compared online is sloppier than the science allows, and only two of the three drugs can legally be dispensed at all.

One target, two targets, three targets

All three are weekly injections built on the same backbone concept:

  • Semaglutide (Ozempic, Wegovy) activates one receptor: GLP-1, a gut hormone that curbs appetite and helps control blood sugar.
  • Tirzepatide (Mounjaro, Zepbound) activates two: GLP-1 plus GIP, a second gut hormone with related metabolic effects.
  • Retatrutide (Eli Lilly, experimental) activates three: GLP-1, GIP, and glucagon, which at these doses appears to increase energy expenditure - burning more, not just eating less.

An agonist simply means a molecule that switches a receptor on. The design bet across the three generations is that each added receptor buys additional weight loss. So far, the trial numbers have moved in that direction[3].

What the landmark trials reported

Each drug has one defining placebo-controlled obesity trial in adults without diabetes:

  • Semaglutide - STEP 1 (2021). In 1,961 adults over 68 weeks, semaglutide 2.4 mg produced about 15% average weight loss versus 2.4% on placebo[1].
  • Tirzepatide - SURMOUNT-1 (2022). In 2,539 adults over 72 weeks, the top 15 mg dose produced about 21% average weight loss versus 3.1% on placebo[2].
  • Retatrutide - phase 2 (2023). In 338 adults over 48 weeks, the top 12 mg dose produced about 24% average weight loss versus 2.1% on placebo - the largest figure reported for any obesity drug trial at publication[3].

The regulatory line that actually matters

The sharpest difference between the three is not a percentage - it is legal status:

  • Semaglutide and tirzepatide are FDA-approved, prescribed and dispensed through pharmacies, with published phase 3 programs and years of post-market safety monitoring.
  • Retatrutide is approved nowhere. It is available only to participants inside Lilly's ongoing phase 3 TRIUMPH program[4]. Full phase 3 results are not yet published.

That means every "retatrutide" vial sold online today is gray-market product with no regulator checking what is in it - the same problem we document for the whole category in our counterfeit GLP-1 investigation and in the retatrutide profile. Demand created by trial headlines, supply created by no one accountable.

Side effects: same family, same pattern

Across all three trials, the dominant side effects were gastrointestinal - nausea, vomiting, diarrhea, constipation - concentrated during dose escalation and mostly rated mild to moderate[1][2][3]. Retatrutide's long-term safety profile is the least characterized of the three simply because it has the least accumulated human exposure; that is what the phase 3 program exists to establish.

The honest bottom line

The generational trend is real: more receptor targets have, so far, meant more average weight loss in trials. Semaglutide and tirzepatide are proven, approved medicines. Retatrutide's phase 2 result is genuinely striking - and it remains an investigational drug whose defining trials are still running. "Promising" is the right grade for the comparison question itself, and the ranking everyone wants will only be trustworthy when phase 3 data and head-to-head trials exist. When they publish, this page gets updated.

References

Numbered in order of first use. Study type is shown for every source; see our methodology for how we rank evidence.

  1. 1.

    Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1) New England Journal of Medicine (Wilding JPH, Batterham RL, Calanna S, et al.), 2021.

    Randomized controlled trialAdults with obesity, or overweight plus a weight-related condition, without diabetesn = 1961

    Result: 68 weeks of semaglutide 2.4 mg weekly produced a mean weight change of -14.9% versus -2.4% with placebo, alongside lifestyle intervention. Gastrointestinal events were the most common side effects.

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  2. 2.

    Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1) New England Journal of Medicine (Jastreboff AM, Aronne LJ, Ahmad NN, et al.), 2022.

    Randomized controlled trialAdults with obesity, or overweight plus a weight-related condition, without diabetesn = 2539

    Result: 72 weeks of tirzepatide produced mean weight changes of -15.0% (5 mg), -19.5% (10 mg) and -20.9% (15 mg) versus -3.1% with placebo. Gastrointestinal events were the most common side effects, mainly during dose escalation.

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  3. 3.

    Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial New England Journal of Medicine (Jastreboff AM, Kaplan LM, Frías JP, et al.), 2023.

    Randomized controlled trialAdults with obesity (BMI 30 or higher, or 27 or higher with a weight-related condition)n = 338

    Result: 48 weeks of retatrutide 12 mg produced a least-squares mean weight reduction of 24.2% versus 2.1% with placebo in a phase 2 trial. Adverse events were mostly mild-to-moderate gastrointestinal symptoms, mainly during dose escalation.

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  4. 4.

    TRIUMPH-1: A Study of Retatrutide (LY3437943) in Participants With Obesity or Overweight (NCT05929066) ClinicalTrials.gov (Eli Lilly and Company), 2023.

    Trial registryAdults with obesity or overweight

    Result: Registered phase 3 trial of retatrutide for chronic weight management; part of the ongoing TRIUMPH program that will determine whether retatrutide reaches approval.

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Advertising disclosure: ads on this page are for NuNu Research, a research-use-only chemical storefront that shares an owner with PeptidesInfo. Ads never influence the evidence grades, regulatory statuses, or conclusions above, which come solely from the cited primary sources.

Review status: Editorially reviewed against primary sources. This article was fact-checked against the primary sources listed in the references by our editorial team, and it has not been reviewed by a licensed clinician. It is educational content, not medical advice. Read our editorial policy and methodology. Spotted an error? Tell us.

retatrutidetirzepatidesemaglutideGLP-1weight loss drugs

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