Comparison · 11 min read

Semaglutide vs Tirzepatide vs Retatrutide: The GLP-1 Showdown

One hormone, two hormones, three: how the single-, dual-, and triple-receptor incretins stack up on mechanism, trial evidence, and approval status — and why "more receptors" is a hypothesis the data is still testing.

By PepCue Editorial · evidence-checked · no dosing advice

Key takeaways

The three drugs escalate by receptor count: semaglutide hits GLP-1 only, tirzepatide adds GIP (dual), and retatrutide adds glucagon (triple) — but more receptors is a hypothesis the trial data is still testing, not a guarantee.
Evidence depth differs sharply. Semaglutide and tirzepatide have completed phase 3 obesity programs (STEP and SURMOUNT); retatrutide's reputation rests on a single phase 2 trial.
In SURMOUNT-5 (NEJM, 2025), the first head-to-head obesity trial, tirzepatide produced superior average weight and waist reductions versus semaglutide over 72 weeks — an average advantage, not an individual guarantee.
Semaglutide is the only one of the three with a published cardiovascular outcomes trial (SELECT), reporting roughly a 20% reduction in major adverse cardiovascular events.
Approval status is the key practical line: semaglutide (Wegovy, 2021) and tirzepatide (Zepbound, 2023) are FDA-approved; retatrutide is investigational and not approved for any use.
All three share GLP-1-driven gastrointestinal side effects; the approved drugs have well-characterized profiles, while retatrutide's full long-term safety picture is not yet established.

The one-line summary, and why the receptor count matters

These three drugs are best understood as an escalating bet on gut hormones. Semaglutide activates a single receptor (GLP-1). Tirzepatide activates two (GIP and GLP-1). Retatrutide activates three (GIP, GLP-1, and glucagon). The marketing-friendly story is that stacking receptors stacks benefits — and on average weight loss, the trial data has so far moved in that direction. But the honest scientific story is more interesting: each added receptor is a distinct pharmacological wager with its own mechanism, its own evidence base, and its own open questions, and the three drugs are at very different stages of proof.

That last point is the one most worth holding onto. Semaglutide and tirzepatide are FDA-approved obesity medicines with large, completed, peer-reviewed phase 3 programs and, for semaglutide, a published cardiovascular outcomes trial. Retatrutide is investigational. Its headline numbers come from a single phase 2 trial, and phase 2 results — however eye-catching — are not the same as proven, durable, head-to-head benefit. This article compares mechanism, the named trials behind each drug, approval status, and side-effect profiles. It does not discuss doses, protocols, or how anything is administered.

Mechanism: GLP-1, GIP, and glucagon do different jobs

All three molecules are incretin-based, meaning they mimic or build on hormones the gut releases in response to food. GLP-1 (glucagon-like peptide-1) is the common thread. It slows gastric emptying, enhances glucose-dependent insulin secretion, and acts on appetite centers in the brain to reduce hunger and food intake. Semaglutide is a GLP-1 receptor agonist — a refined, long-acting version of that single signal. Most of what people associate with this drug class, including the appetite suppression and the gastrointestinal side effects, traces back to GLP-1.

Tirzepatide adds GIP (glucose-dependent insulinotropic polypeptide) to the GLP-1 action in a single molecule. GIP's exact contribution to weight loss is still being worked out — it influences insulin secretion and may modulate appetite and nausea signaling — but the practical observation from trials is that the dual agonist produced larger average weight reduction than GLP-1 alone. Retatrutide goes one step further by adding glucagon receptor agonism. Glucagon is best known for raising blood sugar, which sounds counterproductive, but it also increases energy expenditure and can promote fat utilization in the liver. The triple-agonist bet is that carefully balanced glucagon activity boosts metabolic rate on top of the appetite-driven losses from GLP-1 and GIP. That is a plausible, mechanistically grounded hypothesis — and in obesity pharmacology, plausible mechanisms still have to be proven in humans before they count.

Semaglutide: the STEP program and a cardiovascular outcomes trial

Semaglutide for obesity was established by the STEP (Semaglutide Treatment Effect in People with obesity) phase 3 program. The foundational trial, STEP 1, was published in the New England Journal of Medicine in 2021 (Wilding et al.). It randomized 1,961 adults with obesity, or overweight with a weight-related complication, to once-weekly semaglutide 2.4 mg or placebo, both with lifestyle intervention, over 68 weeks. The semaglutide group lost a mean of 14.9% of body weight versus 2.4% with placebo, and 86.4% of those on semaglutide reached at least 5% weight loss versus 31.5% on placebo. Those numbers reset expectations for what a drug could do for obesity.

What sets semaglutide apart from the other two is outcome data beyond the scale. The SELECT trial (NEJM, 2023) enrolled 17,604 adults with established cardiovascular disease and overweight or obesity but without diabetes, and reported roughly a 20% reduction in major adverse cardiovascular events versus placebo. That is the kind of hard-endpoint evidence — fewer heart attacks and strokes, not just smaller waistlines — that tirzepatide and retatrutide have not yet matched in published obesity-outcome trials. When you compare these drugs, depth of evidence is a real axis, not just peak weight loss.

Tirzepatide: the SURMOUNT program and bigger average losses

Tirzepatide's obesity evidence comes from the SURMOUNT program. SURMOUNT-1 (NEJM, 2022; Jastreboff et al.) randomized 2,539 adults with obesity, or overweight with a complication, without type 2 diabetes. Across the studied dose levels, participants lost roughly 16.0% to 22.5% of body weight, compared with about 2.4% on placebo, and the large majority reached at least 5% weight loss. Later analyses from the program reported sustained weight control over multiple years and a markedly lower rate of progression from prediabetes to type 2 diabetes than placebo — a meaningful metabolic signal beyond weight itself.

The most useful comparison, because it is direct, is SURMOUNT-5 (NEJM, 2025; Aronne et al.), the first head-to-head phase 3b trial of tirzepatide versus semaglutide for obesity in adults without diabetes. Over 72 weeks, tirzepatide produced superior average reductions in both body weight and waist circumference. It is worth being precise about what 'superior' means here: it is an average advantage in a randomized comparison, not a guarantee for any individual, and both drugs produced substantial weight loss. Notably, gastrointestinal adverse events leading to discontinuation were somewhat less frequent with tirzepatide than semaglutide in that trial — a reminder that the dual agonist's tolerability profile is not simply 'more drug, more side effects.'

Retatrutide: striking phase 2 numbers, and the preclinical-to-proven gap

Retatrutide is where excitement and evidence are furthest apart, so this section needs the most care. The drug's reputation rests largely on a single phase 2 trial published in NEJM in 2023 (Jastreboff et al.), which enrolled 338 adults with obesity. Over 48 weeks, the highest dose level studied was associated with mean weight reduction in the low-to-mid 20% range, with the trial meeting its primary endpoint at 24 weeks. The most common adverse events were gastrointestinal — nausea, vomiting, diarrhea — consistent with the incretin class.

Here is the crucial framing. A phase 2 trial is designed to explore efficacy and safety signals and guide later studies; it is not the large, long, confirmatory evidence that supports approval or proves durable real-world benefit. Retatrutide has not completed a published phase 3 obesity program, has no published cardiovascular outcomes trial, and is not FDA-approved for any use as of this writing — it remains an investigational compound. The added glucagon mechanism, while promising for energy expenditure, also raises questions that larger trials exist to answer, including effects on heart rate and other parameters. Anyone presenting retatrutide's phase 2 weight-loss figure as if it were an established, head-to-head win over the approved drugs is overstating what the evidence currently supports. The phase 2 signal is genuinely encouraging; it is also, by definition, preliminary.

Approval status: two prescription medicines and one research compound

This is the single most important practical distinction, and it is a matter of public record rather than interpretation. Semaglutide is FDA-approved for chronic weight management (marketed as Wegovy) and, in its diabetes formulations, for type 2 diabetes (Ozempic, Rybelsus); the obesity approval dates to 2021. Tirzepatide is FDA-approved for chronic weight management (Zepbound, approved in late 2023) and for type 2 diabetes (Mounjaro). Both are real, regulated prescription medicines with defined labels, manufacturing oversight, and pharmacovigilance.

Retatrutide has none of that. It is an investigational molecule still in clinical development and is not approved by the FDA for weight management or any other indication. That status matters for safety in a concrete way: approved drugs come from a regulated supply chain with verified identity, purity, and dosing, whereas anything sold as 'retatrutide' outside a clinical trial is unapproved, unregulated, and of unverified content. The gap between 'showed promise in a phase 2 trial' and 'is a vetted medicine you can rely on' is exactly the gap regulatory approval is meant to close — and for retatrutide, that gap is still open.

Side-effect profiles: a shared core, with differences at the edges

The dominant side effects across all three are gastrointestinal and stem from the GLP-1 component they share: nausea, vomiting, diarrhea, constipation, and reduced appetite, typically most pronounced early and during dose increases. Because the class slows gastric emptying and blunts appetite, these effects are mechanistically expected rather than incidental. For the approved drugs, labeling also reflects class-wide cautions established through their trial programs and post-marketing surveillance, and the appropriate place to read those in full is the official prescribing information.

Where the profiles diverge is at the edges. SURMOUNT-5's direct comparison found gastrointestinal events leading to discontinuation were somewhat less common with tirzepatide than semaglutide, which complicates any simple 'more receptors equals worse tolerability' narrative. Retatrutide's added glucagon agonism introduces mechanism-specific considerations — for example, effects on heart rate and glucose handling — that ongoing larger trials are designed to characterize; its full side-effect profile in big populations over long periods simply is not established yet. The fair summary: the three drugs share a common gastrointestinal core, the two approved agents have well-characterized profiles backed by large programs, and the triple agonist's complete safety picture awaits phase 3 and outcomes data.

FAQ

Is retatrutide better than semaglutide and tirzepatide?

Its phase 2 trial showed striking average weight loss, in the low-to-mid 20% range over 48 weeks, which is competitive with or above the approved drugs' figures. But that comes from a single phase 2 study of 338 people — not a completed phase 3 program and not a head-to-head trial against the other two. Retatrutide is also investigational and not FDA-approved. Calling it 'better' overstates what the current evidence supports; 'promising and unproven' is the accurate framing.

What is the actual difference between semaglutide, tirzepatide, and retatrutide?

Mechanism and receptor count. Semaglutide activates one gut-hormone receptor (GLP-1). Tirzepatide activates two (GIP plus GLP-1) in a single molecule. Retatrutide activates three (GIP, GLP-1, and glucagon). They also differ in evidence and status: the first two are FDA-approved with large phase 3 programs, while retatrutide is still in clinical development.

Which one causes the most side effects?

All three share GLP-1-driven gastrointestinal effects — nausea, vomiting, diarrhea, reduced appetite — that are usually most noticeable early. Counterintuitively, the head-to-head SURMOUNT-5 trial found gastrointestinal events leading to discontinuation were somewhat less frequent with tirzepatide than semaglutide. Retatrutide's complete side-effect profile in large, long-term populations is not yet established, and its glucagon component adds mechanism-specific questions that larger trials are designed to answer.

Is retatrutide FDA-approved or available by prescription?

No. As of this writing, retatrutide is an investigational compound not approved by the FDA for weight management or any other use. Semaglutide (Wegovy) and tirzepatide (Zepbound) are FDA-approved prescription medicines. Anything sold as 'retatrutide' outside of a clinical trial is unapproved, unregulated, and of unverified identity and purity.

Does any of these reduce heart attacks and strokes, or just weight?

Semaglutide is the only one of the three with a published cardiovascular outcomes trial. The SELECT trial reported roughly a 20% reduction in major adverse cardiovascular events in adults with cardiovascular disease and overweight or obesity but without diabetes. Tirzepatide and retatrutide do not yet have comparable published obesity cardiovascular-outcome data, so semaglutide currently has the deepest hard-endpoint evidence.

Sources

[1]Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1) — Wilding JPH et al., N Engl J Med, 2021. PMID 33567185
[2]Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1) — Jastreboff AM et al., N Engl J Med, 2022. PMID 35658024
[3]Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial — Jastreboff AM et al., N Engl J Med, 2023. PMID 37366315
[4]Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5) — Aronne LJ et al., N Engl J Med, 2025. PMID 40353578
[5]Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes (SELECT) — Lincoff AM et al., N Engl J Med, 2023. NEJMoa2307563
[6]PubMed search: tirzepatide vs semaglutide obesity comparison trials — NCBI PubMed query (verify current literature)

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