Second-Generation GLP-1: The SURMOUNT-5 Head-to-Head and What It Settled

The second-generation GLP-1 (a dual GIP/GLP-1 agonist) is, by most objective measures, the most studied anti-obesity medication of its class. For years the comparison against first-generation GLP-1 rested on cross-trial data — same direction, same magnitude, never head-to-head. In May 2025 the actual head-to-head trial finally read out: SURMOUNT-5 (NEJM, May 11 2025) compared the two generations directly over 72 weeks in adults with obesity, and the results tracked what the indirect data had long suggested.

This article walks through the full picture: SURMOUNT-5 in detail, the four-trial SURMOUNT program that came before it, the GIP receptor debate that's still unresolved, and the labeling question — why the same second-generation GLP-1 is marketed under different labels for type 2 diabetes versus obesity — that confuses a lot of people.

The second-generation GLP-1 hits both GLP-1 and GIP receptors, which makes it the first approved dual incretin agonist. SURMOUNT-1 studied it over 72 weeks in people with obesity. SURMOUNT-5 then put it head-to-head against first-generation GLP-1 over the same period. Four SURMOUNT trials total now cover obesity, type 2 diabetes, and weight maintenance. The GIP receptor question — should you activate it or block it? — is still one of the most interesting open arguments in metabolic pharmacology. The same compound is marketed under different labels for diabetes versus obesity: same molecule, different labels, different insurance conversations.

The dual agonist mechanism

The second-generation GLP-1 is a 39-amino acid peptide based on the GIP sequence but engineered to also activate the GLP-1 receptor. The binding profile is intentionally asymmetric: it activates the GIP receptor with roughly equal potency to native GIP, but activates the GLP-1 receptor at about one-fifth the potency of native GLP-1. This isn't a limitation — it's by design.

The GLP-1 side

The GLP-1 receptor activation does what GLP-1 agonists do: suppresses appetite through hypothalamic signaling, slows gastric emptying, enhances glucose-dependent insulin secretion, and suppresses inappropriate glucagon release. This is the well-understood half of the molecule — decades of research on first-generation GLP-1, early first-generation GLP-1, and other GLP-1 agonists have mapped this pathway thoroughly.

The GIP side — and the controversy

This is where things get genuinely interesting. GIP (glucose-dependent insulinotropic polypeptide) was historically considered an "obesogenic" hormone — some older research suggested that GIP signaling promoted fat storage. This led to a competing hypothesis: maybe you should block GIP, not activate it.

The agonism camp, supported by the second-generation GLP-1's clinical data, argues that GIP receptor activation improves fat metabolism, enhances insulin sensitivity through pathways independent of GLP-1, and may contribute to lean mass preservation during weight loss. The reasoning is straightforward: the dual agonist, a potent GIP agonist, has shown greater effects on body weight in trials than first-generation GLP-1, which has no GIP activity. If GIP agonism promoted fat storage, the dual agonist should perform worse, not better.

The antagonism camp points to preclinical data showing that GIP receptor knockout mice are protected from diet-induced obesity, and that GIP antagonist antibodies (like AMG 133, now maridebart cafraglutide) also produce significant weight loss when combined with GLP-1 agonism. Their argument: maybe the dual agonist works well despite GIP agonism, not because of it.

The resolution may be more nuanced. Some researchers propose that GIP receptor activation in the brain may function differently than in peripheral tissues. Central GIP signaling may suppress appetite and improve energy homeostasis, while peripheral GIP signaling has tissue-specific effects that depend on metabolic context. The same receptor, different outcomes depending on where it's activated.

This debate isn't just academic — it has implications for next-generation drug design. The honest answer is that we don't know yet, and anyone who tells you definitively is getting ahead of the data.

The SURMOUNT trials

The manufacturer ran four major Phase 3 trials under the SURMOUNT banner, each designed to answer a different clinical question.

SURMOUNT-1: the flagship trial

Published in the New England Journal of Medicine, 2022. This was the trial that put the second-generation GLP-1 on the map for obesity.

The trial enrolled 2,539 adults with BMI of 30 or higher (or 27+ with at least one comorbidity) without type 2 diabetes, and ran for 72 weeks at doses up to 15mg. The published results reported changes in body weight from baseline alongside significant improvements in waist circumference, blood pressure, triglycerides, and fasting insulin.

For context on why these results drew attention: the FDA has historically considered 5% weight loss from baseline to be the threshold for a clinically meaningful anti-obesity effect. The magnitude reported in SURMOUNT-1 was studied well above that mark.

SURMOUNT-2: obesity with type 2 diabetes

People with type 2 diabetes typically lose less weight on anti-obesity medications — the metabolic environment is different, and diabetes medications can promote weight gain. SURMOUNT-2 tested whether the second-generation GLP-1 could overcome this.

The trial ran at the 15mg dose over 72 weeks and reported changes in body weight alongside marked improvements in HbA1c. The effect was more modest than SURMOUNT-1 — expected given the population — but the trial framed it as favorable relative to existing diabetes and anti-obesity medications.

SURMOUNT-3: after lifestyle intervention

This trial was clever in its design. Participants first completed a 12-week intensive lifestyle intervention (structured diet and exercise) before being randomized to the second-generation GLP-1 or placebo.

Participants on the second-generation GLP-1 continued to lose weight on top of the lifestyle-induced loss, while the placebo group regained most of what they'd lost during the lifestyle phase. The trial was read as evidence that the dual agonist and lifestyle intervention have additive effects — they're not redundant.

SURMOUNT-4: the maintenance question

Perhaps the most important trial for long-term clinical planning. All participants received open-label second-generation GLP-1 for 36 weeks, then were randomized to continue the dual agonist or switch to placebo.

Those who continued maintained their weight loss; those switched to placebo regained a substantial portion of what they'd lost over the following 52 weeks. This confirms what the field suspected: the second-generation GLP-1, like other GLP-1-based therapies, requires ongoing treatment to maintain benefit. The regain on discontinuation reflects the biology of obesity, not a flaw in the therapy.

Cardiovascular outcomes

For any metabolic therapy, the cardiovascular question is paramount — weight loss drugs that increase heart attacks or strokes are worse than useless. The second-generation GLP-1's cardiovascular outcomes data comes from the SURPASS-CVOT trial.

The trial demonstrated that the second-generation GLP-1 was non-inferior to placebo for major adverse cardiovascular events (MACE) in patients with type 2 diabetes — meaning it doesn't increase cardiovascular risk. Additional analyses suggested potential benefit, including reductions in blood pressure, atherogenic lipids, and inflammatory markers, though the trial wasn't powered to demonstrate superiority on hard cardiovascular endpoints.

This matters because first-generation GLP-1's SELECT trial demonstrated cardiovascular superiority — actual reduction in heart attacks and strokes. Whether the second-generation GLP-1 can match that in a dedicated outcomes trial is an open question. The metabolic improvements it produces are at least as comprehensive as first-generation GLP-1's, so the biological plausibility is there.

SURMOUNT-5: the head-to-head finally read out

For years the second-generation-vs-first-generation GLP-1 question had to be answered with cross-trial comparisons (SURMOUNT-1 vs STEP 1) and a head-to-head in type 2 diabetes (SURPASS-2). Both pointed the same direction. But the obesity-specific head-to-head trial people kept asking for didn't exist — until May 11, 2025, when SURMOUNT-5 was published in the New England Journal of Medicine and presented at the European Congress on Obesity.

Here's how the trial was set up. 751 adults with obesity (BMI ≥30, or ≥27 with weight-related comorbidities), no type 2 diabetes, randomized 1:1 to either the second-generation GLP-1 or first-generation GLP-1 — same once-weekly injection cadence for both. Each compound was titrated up to its maximum tolerated approved obesity dose: the dual agonist at 10 or 15 mg, first-generation GLP-1 at 1.7 or 2.4 mg, alongside behavior support. Seventy-two weeks total. It was open-label, which isn't ideal — but the two injection devices look different enough that you can't really blind it, so that's an unavoidable limitation rather than a design choice.

The trial compared the two agents head-to-head over 72 weeks for their effects on body weight, and the direction favored the second-generation GLP-1. A larger share of the dual-agonist group reached the higher weight-change thresholds the trial tracked than the first-generation group, and waist circumference changed further on the dual agonist too.

What's almost more interesting is the tolerability side, which went the same direction. In the trial, participants discontinued for GI side effects more often on first-generation GLP-1 (5.6%) than on the second-generation GLP-1 (2.7%) — the dual agonist showed the larger effect on body weight while fewer participants stopped because of nausea. There's a working hypothesis for why: GIP receptor activation may have an anti-emetic effect that takes some of the edge off the nausea GLP-1 is driving. If that's right, the dual mechanism is buying more benefit per unit of side effect, not less.

The data tracked what the cross-trial comparisons had been pointing to all along. Where SURMOUNT-1 vs STEP 1 only allowed an indirect comparison of the two generations at their top doses, SURMOUNT-5 compared them directly — head-to-head, same trial, same population, same titration approach. Whether the difference reflects the GIP receptor contribution specifically, differences in GLP-1 receptor engagement kinetics, or some combination is still debated — but the direction of the effect is no longer in question.

The lean mass hypothesis

One of the more intriguing aspects of second-generation GLP-1 research is the body composition data. In SURMOUNT-1, participants maintained a favorable lean-to-fat mass ratio, with the majority of weight lost coming from fat mass rather than lean tissue.

Some researchers attribute this to GIP receptor activation. The hypothesis is that GIP signaling in muscle tissue may promote protein synthesis or inhibit protein breakdown, effectively sparing lean mass during rapid weight loss. If true, this would be a significant advantage of dual agonism over pure GLP-1 therapy, because muscle loss during aggressive weight reduction is a genuine clinical concern — especially in older adults, where sarcopenia is already a problem.

The evidence is suggestive but not conclusive. Preclinical models show GIP receptors on muscle tissue and demonstrate effects on muscle protein turnover. The clinical body composition data is consistent with the hypothesis. But a definitive causal link between the dual agonist's GIP activation and lean mass preservation hasn't been established in humans.

Same molecule, two labels: diabetes vs. obesity

A source of persistent confusion. The second-generation GLP-1 is marketed under two different labels — one for type 2 diabetes, one for obesity — but it's the same molecule, manufactured by the same company at the same facilities. The difference is purely regulatory.

The diabetes label was approved first, receiving FDA approval in 2022 based on the SURPASS trial program. The obesity label was approved subsequently for chronic weight management in adults with obesity (BMI 30+) or overweight (BMI 27+) with at least one weight-related comorbidity, receiving FDA approval in 2023 based on the SURMOUNT program.

The dosing is identical. The injection device is identical. The molecule is identical. The distinction exists because the FDA requires separate approvals for different indications, and insurance coverage often differs between the two labels. This has created access challenges — the same patient taking the same compound might have coverage under one label but not the other, depending on their diagnosis.

The Dose-Response Story

The second-generation GLP-1 uses a gradual dose escalation that's become standard in the GLP-1 class — each step up increases efficacy but also increases the likelihood of gastrointestinal side effects. The specific titration schedule is set by the FDA label and managed by the prescribing physician, not something to design independently.

Meaningful effects on body weight were observed even at the lower doses. In SURMOUNT-1, the 5mg and 10mg dose arms both showed substantial changes from baseline, not just the 15mg arm — so not every patient needs to reach the top dose to see results the trial considered meaningful.

The side effect conversation

The side effect profiles of the second-generation and first-generation GLP-1 are broadly similar — both are incretin-based therapies that share GLP-1 receptor activation — but there are nuances worth understanding.

In the SURMOUNT trials, the most common adverse events were gastrointestinal. Nausea affected roughly 25-30% of participants at higher doses, diarrhea 15-20%, constipation 10-15%, and vomiting 5-12%. These rates are comparable to first-generation GLP-1's STEP trials, though some analyses suggest the second-generation GLP-1 may have a slightly lower incidence of nausea at comparable efficacy.

Why might that be? One hypothesis relates to the GIP component. GIP receptor activation may exert an anti-emetic effect that partially counteracts the nausea driven by GLP-1 receptor activation. Preclinical data shows GIP receptor signaling in brain regions that regulate nausea and vomiting. If true, the second-generation GLP-1's dual mechanism gives it a tolerability advantage — more benefit per unit of nausea, so to speak.

The more serious adverse events — pancreatitis, gallbladder disease, thyroid effects — occur at low rates in both programs. The FDA label for both carries a boxed warning about thyroid C-cell tumors based on rodent data, though no causal link has been established in humans. Gallbladder-related events occur in approximately 1-2% of participants, likely related to rapid weight loss rather than the drugs specifically.

Injection site reactions are generally mild — some patients report redness or itching that resolves on its own and doesn't require discontinuation.

The compounding landscape

The compounding situation is relevant and evolving. During the FDA-recognized shortage of the second-generation GLP-1, compounding pharmacies were able to produce it under the 503B exemption, which significantly expanded access for patients who couldn't obtain or afford the branded versions.

The compounding landscape remains dynamic and subject to regulatory developments. The key considerations are purity, potency testing, and the regulatory framework under which a compounding pharmacy operates. This is an area where the regulatory environment is actively evolving, and the details matter for anyone involved in prescribing or research.

Cost and access

I won't quote specific prices because they change frequently, but the access situation for GLP-1 agonists is one of the most significant challenges in metabolic medicine today. The branded products carry premium pricing, insurance coverage is inconsistent (many plans exclude "weight loss drugs" despite the clinical evidence), and demand exceeds supply.

This creates a situation where a transformative class of medications is available primarily to those who can afford out-of-pocket costs or who happen to have favorable insurance coverage. It's a systemic issue that the field is actively working to address through expanded insurance coverage mandates, manufacturer assistance programs, and the compounding pharmacy ecosystem.

Frequently asked questions

Is the second-generation GLP-1 better than the first-generation?

For weight loss specifically, the evidence points that way — SURMOUNT-5 (NEJM, May 2025) was the first direct head-to-head trial in obesity, comparing the two generations over 72 weeks for their effects on body weight, and the direction favored the second-generation GLP-1, with lower rates of treatment discontinuation due to GI side effects. Whether "better" extends to cardiovascular outcomes is a separate question — first-generation GLP-1 has SELECT (a 20% relative risk reduction in major adverse cardiovascular events demonstrated in the NEJM 2023 trial), while the second-generation GLP-1's SURPASS-CVOT showed non-inferiority but is not yet powered to demonstrate superiority on hard cardiovascular endpoints.

What happens when you stop taking it?

SURMOUNT-4 showed that participants who discontinued the second-generation GLP-1 regained a substantial portion of the weight they'd lost within the following year. This is consistent with first-generation GLP-1 discontinuation data and reflects the biology of obesity — a chronic condition that, for many people, requires ongoing treatment to maintain remission.

What are the most common side effects?

Gastrointestinal effects dominate: nausea (20-30%), diarrhea (15-20%), constipation (10-15%), and vomiting (5-10%). These are most frequent during dose escalation and typically resolve within the first few weeks of each dose level. The gradual titration protocol is designed specifically to minimize them.

How does the second-generation GLP-1 affect blood sugar?

The second-generation GLP-1 was first approved for type 2 diabetes because its metabolic effects are comprehensive. In the SURPASS trials, it showed HbA1c reductions of 2.0-2.3%, among the largest of available glucose-lowering agents. Even in people without diabetes, it significantly improves fasting glucose and insulin sensitivity.

Can the second-generation GLP-1 be combined with other therapies?

Research on combination approaches is ongoing. The trial programs studied the second-generation GLP-1 as monotherapy or alongside metformin for diabetes. Combining it with other GLP-1 agonists would be redundant, but combining it with lifestyle intervention (as in SURMOUNT-3) produces additive benefits. Any combination approach should be guided by the evolving clinical evidence.

Why is the same drug sold under two different labels?

Nothing pharmacologically. Same molecule, same manufacturer, same doses, same injection device. The difference is the FDA-approved indication: one label for type 2 diabetes, the other for chronic weight management. Insurance coverage and pricing may differ between the two labels depending on diagnosis and plan.