Fullness is not one hormone. Cagrilintide, tirzepatide and retatrutide each probe a different set of signals. Of the pairings relevant to these three, only cagrilintide with semaglutide has been tested in people.
After a meal the gut and pancreas release several hormones that tell the brain and body how to handle the fuel arriving. GLP-1 is the best known, but it works alongside GIP, glucagon and amylin. Researchers study more than one hormone because each carries a different message. The three compounds here are tools for asking what those messages do, alone and, in one studied case, together.
Satiety, the feeling of fullness that helps bring a meal to an end, is assembled from several signals. GLP-1 (glucagon-like peptide-1) is released by the gut after eating and is involved in insulin release, slower stomach emptying and appetite. GIP (glucose-dependent insulinotropic polypeptide) is a second gut hormone that also prompts insulin release and acts on fat tissue. Glucagon, from the pancreas, raises blood glucose between meals and is associated with energy expenditure. Amylin is released by the pancreas with insulin and acts on brainstem and hypothalamic receptors involved in fullness.
These are different receptors in different tissues. A molecule that engages one of them is asking a different question from a molecule that engages another, and that is the reason the field did not stop at GLP-1.
Tirzepatide is a single 39-amino-acid peptide that activates the GLP-1 and GIP receptors. It has the largest human evidence base of the three: SURMOUNT-1 followed 2,539 adults with obesity for 72 weeks against placebo (Jastreboff et al. 2022), and the same molecule is registered in Australia as the prescription medicine Mounjaro. Retatrutide adds the glucagon receptor to make three. Its evidence is a 48-week phase 2 trial in 338 adults with obesity (Jastreboff et al. 2023) and phase 3 results emerging through 2026; it remains investigational.
Cagrilintide leaves the incretin pathway altogether. It is a long-acting analogue of amylin, tested on its own in a 26-week phase 2 trial of 706 adults (Lau et al. 2021). Whether activating three receptors produces a different result from two, or whether amylin adds something incretins do not, are hypotheses under test rather than settled findings. No published trial compares the three head to head.
Cagrilintide with semaglutide, a GLP-1 receptor agonist not sold in this catalogue, is the pairing with human evidence. REDEFINE 1 was a 68-week phase 3a trial in 3,417 adults with obesity or overweight, without diabetes, in which the two peptides were given together once weekly by injection and compared with placebo, with smaller arms of each component alone (Garvey et al. 2025). Mean weight change was about 20 per cent with the combination against 3 per cent with placebo; gastrointestinal adverse events affected about four in five participants on the combination. The main comparison was with placebo; the single-agent arms were smaller. REDEFINE 2 repeated the design in adults with type 2 diabetes (Davies et al. 2025).
That is what a studied combination looks like: pharmaceutical products at defined doses, lifestyle support, component arms and a pre-specified endpoint.
REDEFINE 1 is evidence for cagrilintide with semaglutide in that formulation and that population. It is not evidence for cagrilintide with tirzepatide, or with retatrutide, or for any research material. No human trial of either of those pairings was identified. A 2026 study reported additive effects of cagrilintide with retatrutide in obese male rats (Petersen et al. 2026); that is an animal finding about a pairing that has not been tested in people.
The temptation is to reason by analogy: if amylin adds to one GLP-1 agonist it should add to another. Analogy is a hypothesis, and the field’s own history, in which a receptor profile that looked promising in cells has repeatedly needed a trial to confirm it, is the argument for waiting for the trial.
Every human result above comes from sponsor-run trials of a pharmaceutical formulation, given by subcutaneous injection under medical supervision, in adults who met entry criteria. Body weight and HbA1c were the main endpoints; long-term safety and what happens after stopping are less well described. Research-grade peptide has no established equivalence to those products in identity, purity, sterility or dose, so the trial findings describe the medicines, not the material.
Sources checked 2026-09-22