Semaglutide vs Tirzepatide: What the Clinical Trials Actually Show (2026)

Abstract laboratory illustration: two descending curves compared side by side, cyan and navy, with data points on a light grid

The short answer, with the numbers

In the only published head-to-head trial in adults with obesity and without diabetes, SURMOUNT-5, tirzepatide produced a mean weight reduction of 20.2% over 72 weeks versus 13.7% for semaglutide — a gap of roughly 6.5 percentage points in favour of the dual agonist. In type 2 diabetes, SURPASS-2 had already shown a larger HbA1c reduction with tirzepatide (up to −2.30 points) than with semaglutide 1 mg (−1.86 points) over 40 weeks.

The picture is not one-sided, however. On the cardiovascular front, semaglutide holds a formal superiority demonstration against placebo in the SELECT trial (a 20% reduction in major adverse events), whereas SURPASS-CVOT, published in late 2025, established tirzepatide's noninferiority against an active comparator, dulaglutide, without statistical superiority. Both molecules share an adverse-event profile dominated by gastrointestinal effects.

This article walks through the published data, trial by trial. It is a factual report on the scientific literature: nothing here constitutes medical advice, a treatment recommendation, or an encouragement to consume these substances.

Two mechanisms: GLP-1 mono-agonist versus dual GIP/GLP-1 agonist

Semaglutide is an analogue of GLP-1 (glucagon-like peptide-1), a gut incretin that stimulates insulin secretion in a glucose-dependent manner, slows gastric emptying and acts on hypothalamic satiety circuits. Engineered to resist degradation and bind albumin, this 31-amino-acid peptide reaches a half-life of about one week, allowing once-weekly administration.

Tirzepatide rests on a different bet: a single 39-amino-acid peptide built on the backbone of GIP (glucose-dependent insulinotropic polypeptide) that activates both the GIP receptor and the GLP-1 receptor. The designers' hypothesis: the additional GIP signal would improve insulin sensitivity, adipose tissue metabolism and, potentially, digestive tolerability.

This distinction — mono-agonism versus dual agonism — is the key to reading the entire comparative literature: the trials below were designed precisely to test whether adding the GIP signal translates into a measurable clinical benefit.

Semaglutide on trial: the SUSTAIN and STEP programmes

Semaglutide's clinical development came in two waves. The SUSTAIN programme, run in type 2 diabetes at doses of 0.5 to 2 mg, chained together more than ten phase 3 trials: HbA1c reductions of 1.5 to 1.8 points at 1 mg, superiority over several active comparators (including dulaglutide in SUSTAIN-7), and a favourable cardiovascular safety signal as early as SUSTAIN-6.

The STEP programme then evaluated the 2.4 mg weekly dose for weight management. STEP 1 (2021) randomised 1,961 adults with overweight or obesity and without diabetes: mean weight loss of 14.9% over 68 weeks versus 2.4% on placebo, with a third of participants achieving at least 20%. Later trials confirmed this order of magnitude, with more modest results in diabetes (STEP 2).

These two programmes established semaglutide as the benchmark of its class and provided the implicit reference point for every competitor trial that followed.

Tirzepatide on trial: the SURPASS and SURMOUNT programmes

The SURPASS programme tested tirzepatide (5, 10 and 15 mg) in type 2 diabetes. The HbA1c reductions, often exceeding 2 points, rank among the largest reported for an injectable glucose-lowering agent. Weight loss, a secondary endpoint, already reached 11 to 12 kg at the higher doses.

SURMOUNT-1, published in 2022, carried the molecule into weight management in 2,539 adults without diabetes: mean losses of 15.0%, 19.5% and 20.9% over 72 weeks at 5, 10 and 15 mg, versus 3.1% on placebo. At 15 mg, more than half of participants lost at least 20% of their baseline weight — an order of magnitude previously associated with bariatric surgery.

Later trials in the programme (SURMOUNT-2 in diabetes, SURMOUNT-3 and 4 on intensification and maintenance) consolidated these results and set the stage for the direct comparison.

SURMOUNT-5: the head-to-head finally published

Until 2025, comparison in obesity relied on indirect juxtapositions of separate trials. SURMOUNT-5, an open-label phase 3b trial published in the New England Journal of Medicine in May 2025, filled that gap: 751 adults with overweight or obesity and without diabetes, randomised between tirzepatide (maximum tolerated dose of 10 or 15 mg) and semaglutide (1.7 or 2.4 mg) for 72 weeks.

The headline result: −20.2% of body weight on tirzepatide versus −13.7% on semaglutide, roughly 22.8 kg versus 15.0 kg in absolute terms. Waist circumference decreased by 18.4 cm versus 13.0 cm, and participants on tirzepatide were more likely to cross each predefined threshold, from 10% to 25% of baseline weight.

Two caveats flagged by the authors: the trial was open-label (the injector pens differ) and funded by Eli Lilly, tirzepatide's manufacturer. Its consistency with earlier indirect comparisons nonetheless strengthens the credibility of the results.

Weight and HbA1c: the numbers side by side

In type 2 diabetes, a direct comparison has existed since 2021: SURPASS-2 randomised 1,879 patients between tirzepatide (5, 10 or 15 mg) and semaglutide 1 mg. HbA1c at 40 weeks: −2.01, −2.24 and −2.30 points versus −1.86 points; weight: −7.6, −9.3 and −11.2 kg versus −5.7 kg. Superiority was significant at every dose, with the caveat that the comparator was the 1 mg dose, not the 2 mg dose approved later.

In obesity without diabetes, the hierarchy observed in SURMOUNT-5 (a gap of about 6.5 percentage points in weight loss) matches what the placebo-controlled trials suggested: mean plateaus around 15% for semaglutide 2.4 mg in STEP 1 and around 21% for tirzepatide 15 mg in SURMOUNT-1.

One important point of interpretation: these averages conceal considerable between-individual variability, with some participants responding little to either molecule. The publications report distributions, not individual guarantees.

Tolerability: similar profiles, dominated by the gut

Across all programmes, the most frequent adverse events are gastrointestinal: nausea, diarrhoea, vomiting and constipation, mostly during dose escalation and most often graded mild to moderate. In STEP 1, 74.2% of participants on semaglutide reported at least one gastrointestinal event; in SURMOUNT-1, nausea affected up to a third of participants at the higher doses.

In SURMOUNT-5, profiles were broadly comparable between arms. Treatment discontinuations for gastrointestinal events were reported in 2.7% of participants on tirzepatide versus 5.6% on semaglutide — a figure that does not support the common assumption that digestive tolerability necessarily worsens as efficacy increases.

The trials also report rarer events monitored across the class: gallstones, pancreatitis (rare), and injection-site reactions. These data come from selected, medically supervised populations — a condition that does not transfer to any context of use outside a trial.

The heart as referee: SELECT and SURPASS-CVOT

SELECT, published in late 2023, remains the landmark cardiovascular trial for semaglutide 2.4 mg: 17,604 adults with overweight or obesity and established cardiovascular disease but no diabetes, followed for a mean of 39.8 months. The result: a 20% reduction in the composite of cardiovascular death, myocardial infarction or stroke (hazard ratio 0.80; 95% CI 0.72-0.90; 6.5% versus 8.0% of events) — the first demonstration that a pharmacological obesity treatment reduces cardiovascular events.

SURPASS-CVOT, presented at the EASD congress and then published in the New England Journal of Medicine in December 2025, evaluated tirzepatide in 13,299 people with type 2 diabetes and atherosclerotic disease, followed for about four years. Its particularity: the comparator was not placebo but dulaglutide, a GLP-1 agonist with an already demonstrated cardiovascular benefit. Primary endpoint: 12.2% of events versus 13.1%, with noninferiority met (p = 0.003) and superiority missed (p = 0.09); tirzepatide clearly outperformed its comparator on weight (−11.6% versus −4.5%) and HbA1c (−1.66 versus −0.88 points).

Methodologically, the two trials are not interchangeable: beating placebo and matching an effective active comparator are two different demonstrations. To date, only semaglutide holds a formal cardiovascular superiority result against placebo in obesity without diabetes; tirzepatide holds robust noninferiority against an active GLP-1 in high-risk diabetes.

Beyond the duel: the next generation takes shape

This duel is only one step in a broader trajectory: the stacking of complementary hormonal signals. Retatrutide, a triple GIP/GLP-1/glucagon agonist, reported a mean weight reduction of 24.2% in phase 2 after 48 weeks, with no apparent plateau at trial end; its phase 3 programme (TRIUMPH) is ongoing.

Other approaches combine or reformulate: CagriSema (cagrilintide, an amylin analogue, combined with semaglutide) reported about 22.7% weight loss over 68 weeks in REDEFINE-1, while orforglipron, an oral non-peptide GLP-1 agonist, reached about 12% in ATTAIN-1 — less than the injectables, but with a daily tablet and no food restrictions.

These molecules are covered in dedicated articles; the point to retain here is that the 2026 comparison between mono- and dual agonist will probably be reread, a few years from now, as a snapshot of a fast-moving field.

The limits of cross-trial comparisons

Outside SURMOUNT-5 and SURPASS-2, every comparison between the two molecules is indirect, and therefore fragile. Populations (baseline weight, diabetes duration, comorbidities), durations (40 to 72 weeks), doses compared, lifestyle co-interventions and the statistical handling of treatment discontinuations differ from trial to trial — enough to shift the headline figures by several points.

The cardiovascular trials illustrate the trap: comparing SELECT's hazard ratio (against placebo, without diabetes) with SURPASS-CVOT's (against dulaglutide, with diabetes) makes little sense. Likewise, the semaglutide 1 mg comparator in SURPASS-2 does not correspond to the 2 mg dose approved since in diabetes.

Finally, nearly all of these trials are manufacturer-funded — standard practice, but one that justifies the attention paid to pre-registered protocols, independent event-adjudication committees and replication of results.

Regulatory status in France and the EU, and the line with research peptides

Both molecules are authorised medicines in the European Union. Semaglutide holds marketing authorisations as Ozempic (type 2 diabetes, 2018), Rybelsus (oral form) and Wegovy (weight management, 2022); tirzepatide is authorised as Mounjaro (2022, with the indication extended to weight management in 2023). In France, Wegovy and Mounjaro have been marketed since late 2024 and, since 15 June 2026, are reimbursed at 65% in severe obesity under strict conditions (documented failure of nutritional management, a BMI of at least 40, or 35 with a comorbidity, initial prescription in specialised centres), per the decrees of the Journal officiel of 28 May 2026.

This regulatory reality draws a clear line with so-called Research Use Only peptides: compounds sold for in vitro research are not medicines, hold no marketing authorisation or pharmaceutical-grade manufacturing control in the medicinal sense, and are not intended for human or veterinary administration. The figures reported here come exclusively from approved medicines administered in supervised trials and cannot be extrapolated to any other product.

In summary: the trials published through August 2026 give tirzepatide the edge on weight loss and HbA1c, including in a direct head-to-head, and give semaglutide the most complete cardiovascular demonstration against placebo. What comes next — secondary SURPASS-CVOT analyses, the triple agonists — will be documented here, published trials in hand.

Sources

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