Semaglutide vs. Tirzepatide: Single vs. Dual Agonism in the Literature

Semaglutide and tirzepatide are the two peptides that turned incretin pharmacology from a diabetes niche into the center of metabolic medicine. They are frequently compared, and the comparison is genuinely informative because it tests a scientific question: does adding GIP receptor activity to a GLP-1 agonist change what the molecule does? The trial programs, SUSTAIN and STEP for semaglutide, SURPASS and SURMOUNT for tirzepatide, are large, well run and, unusually, include head-to-head studies. This note reviews the two molecules, what the trials found, and what the comparison leaves open. Both compounds are approved drugs manufactured by pharmaceutical companies. Wednesday does not sell them, and the findings below concern those products in clinical populations.

Two molecules, two design philosophies

Semaglutide is a 31-amino-acid analogue of human GLP-1. Three modifications convert a hormone with a two-minute half-life into a weekly drug: a substitution at position 8 that blocks cleavage by DPP-4, a substitution at position 34 to control where the lipid attaches, and a C18 fatty diacid on a hydrophilic spacer that binds albumin tightly and delays renal clearance. It is a faithful GLP-1 receptor agonist with no meaningful activity at other receptors. The background chemistry is covered in What Are GLP-1 Peptides?.

Tirzepatide is a 39-amino-acid peptide whose backbone is based on the GIP sequence, engineered to also bind the GLP-1 receptor, with a C20 fatty diacid for weekly duration. Willard and colleagues characterized its receptor pharmacology in 2020: tirzepatide binds the GIP receptor with affinity similar to native GIP but binds the GLP-1 receptor about five-fold more weakly than native GLP-1, and at the GLP-1 receptor it shows biased signaling, favoring cAMP production over β-arrestin recruitment, which may reduce receptor internalization and prolong signaling.¹ It is therefore not a balanced dual agonist but a GIP-weighted one with a partial, biased GLP-1 component.

Why add GIP at all?

For two decades GIP was considered the wrong incretin to target. Its insulinotropic effect is blunted in type 2 diabetes, and GIP receptor knockout mice were resistant to diet-induced obesity, which suggested that blocking GIP, not activating it, would help with weight. Campbell’s 2021 review lays out the paradox: both GIP receptor agonists and antagonists reduced body weight in rodents, and the mechanism of either is not fully understood.² Proposed explanations for a benefit from agonism include central effects on appetite that are additive to GLP-1, improved adipose tissue insulin sensitivity and lipid buffering, and a reduction in the nausea that limits GLP-1 dosing. The wider receptor biology is in GLP-1 vs. GIP vs. Glucagon Receptors. Tirzepatide is, in effect, the clinical test of whether agonism was the right bet.

The semaglutide record

The SUSTAIN program established semaglutide in type 2 diabetes. SUSTAIN-6, a cardiovascular outcomes trial in 3,297 people with diabetes and high cardiovascular risk, found that semaglutide reduced the composite of cardiovascular death, non-fatal myocardial infarction and non-fatal stroke by 26% relative to placebo over about two years.³ The STEP program then tested a higher dose, 2.4 mg weekly, in obesity. In STEP 1, 1,961 adults without diabetes lost a mean of 14.9% of body weight over 68 weeks against 2.4% on placebo.⁴ SELECT, reported in 2023, enrolled 17,604 people with obesity and cardiovascular disease but no diabetes and found a 20% reduction in major adverse cardiovascular events with semaglutide, the first such result for a weight-management drug.⁵

The tirzepatide record

SURPASS tested tirzepatide in type 2 diabetes. SURPASS-2 is the key comparison: 1,879 participants were randomized to tirzepatide at 5, 10 or 15 mg or to semaglutide 1 mg weekly for 40 weeks. HbA1c fell by 2.01, 2.24 and 2.30 percentage points with the three tirzepatide doses against 1.86 with semaglutide, and body weight fell by 7.6, 9.3 and 11.2 kg against 5.7 kg.⁶ All three doses were superior on both endpoints. The important caveat is the comparator dose: 1 mg was the highest approved semaglutide dose for diabetes at the time, but it is well below the 2.4 mg used in obesity trials.

SURMOUNT-1 then tested tirzepatide in obesity without diabetes: over 72 weeks, mean weight change was −15.0%, −19.5% and −20.9% at 5, 10 and 15 mg against −3.1% on placebo.⁷ SURMOUNT-5, published in 2025, finally compared the two drugs at their obesity doses: 751 adults were randomized to tirzepatide at the maximum tolerated dose (10 or 15 mg) or semaglutide at 2.4 mg for 72 weeks. Weight fell by 20.2% with tirzepatide and 13.7% with semaglutide, a difference of about 6.5 percentage points, with a similar profile of gastrointestinal adverse events.⁸

SURMOUNT-5 settled the weight question at approved doses; it did not settle whether GIP is the reason.

Head-to-head summary

What the comparison leaves open

The trials show that tirzepatide produces larger weight and glycemic changes than semaglutide at the doses tested. They do not show why. Three explanations remain plausible and are not mutually exclusive. First, GIP receptor activation may contribute independently, the hypothesis the molecule was designed to test. Second, biased signaling at the GLP-1 receptor may yield more sustained GLP-1 action for a given exposure.¹ Third, the GIP component may simply make higher effective GLP-1 receptor engagement tolerable by dampening nausea, so that the difference is one of achievable dose rather than mechanism. Distinguishing these requires experiments the clinical trials were not designed to do: selective GIP receptor agonists and antagonists in humans, and receptor-knockout models with tirzepatide. That work is ongoing.

There is also an asymmetry in the evidence. Semaglutide’s cardiovascular record is longer and includes a non-diabetic population; tirzepatide’s outcomes data matured later. And both drugs were compared over 40 to 72 weeks in populations that met trial entry criteria, with structured lifestyle support. Durability beyond that window, and effects after discontinuation, are separate literatures.

Why the comparison matters for research

For researchers, the semaglutide–tirzepatide comparison is a natural experiment in receptor pharmacology conducted at scale. It suggests that engaging a second proglucagon-family receptor changes the magnitude of effect, and it created the template for the triple agonists that add glucagon receptor activity, which are the subject of Triple GLP-1/GIP/Glucagon Agonists: Where the Research Stands. It also raised questions, about biased agonism, receptor trafficking and the tissue-level actions of GIP, that are answerable with in vitro and preclinical tools. Those are the questions research-grade incretin-class peptides exist to address.