No peptide class has more human evidence behind it than the incretin mimetics — the GLP-1 receptor agonists and their multi-receptor successors. These are not obscure research chemicals; several are FDA-approved drugs whose trials enrolled thousands of participants and were published in the world's most scrutinized journals. That makes this class the right place to start a series on what the research actually says, because the evidence is strong enough that nobody needs to exaggerate it.
It is also the right place to learn the difference between what a trial showed and what a seller implies.
What the pivotal trials reported
Semaglutide. In the STEP 1 trial (Wilding et al., New England Journal of Medicine, 2021), 1,961 adults with obesity received weekly semaglutide 2.4 mg or placebo alongside lifestyle intervention for 68 weeks. The semaglutide group's mean body-weight change was −14.9%, against −2.4% for placebo. Cardiovascular outcomes were later examined in SELECT (Lincoff et al., NEJM, 2023), which reported a 20% relative reduction in major adverse cardiac events in adults with established cardiovascular disease and overweight.
Tirzepatide. A dual GIP/GLP-1 receptor agonist. In SURMOUNT-1 (Jastreboff et al., NEJM, 2022), 2,539 adults received weekly tirzepatide or placebo for 72 weeks; the highest dose group's mean weight reduction reached −20.9% in the treatment-regimen analysis, with the efficacy analysis reporting up to −22.5%.
Retatrutide. A triple-receptor agonist (GIP, GLP-1, glucagon), still investigational. A phase 2 trial (Jastreboff et al., NEJM, 2023) reported mean weight reduction of −24.2% at 48 weeks at the highest dose — phase 2 data, in several hundred participants, not yet a completed phase 3 program.
The investigators' own framing across these papers is consistent: substantial mean reductions, gastrointestinal adverse events as the dominant tolerability finding, and — a point the trial authors make repeatedly — regain after discontinuation in follow-up studies.
How to read those numbers
Three details separate an accurate reading from a marketing one.
They are means, not promises. A −14.9% mean contains participants who lost far more and participants who lost little. Distribution matters, and every one of these papers publishes it.
They describe the trial's conditions. Sixty-eight to seventy-two weeks, titrated dosing under medical supervision, lifestyle intervention included, defined populations. A number generated under those conditions describes those conditions.
They belong to the drug that was tested. This is the detail this publication exists to make: STEP 1 tested Novo Nordisk's manufactured semaglutide — a specific molecule at verified purity, in a specific formulation, at controlled doses. The trial evidence attaches to what was actually in the vials. Research-grade material of the same nominal sequence is only connected to that evidence to the degree that its identity and purity are actually established — which is a certificate question, not a marketing question. An unverified vial borrows the molecule's name, not its trial record.
What is not established
For the approved members of the class, the human evidence is deep. What remains genuinely open, per the literature itself: long-term outcomes beyond the trial windows, durability after discontinuation (the STEP 1 extension reported most weight regained within a year of stopping), and the profile of the investigational members — retatrutide's phase 3 program is ongoing, and phase 2 results have a history of shrinking in phase 3.
None of the above is dosing guidance, and none of it transfers automatically to any vial that has not been verified to contain what its label claims. Research use only.
Sources: Wilding et al., NEJM 2021 (STEP 1); Jastreboff et al., NEJM 2022 (SURMOUNT-1); Jastreboff et al., NEJM 2023 (retatrutide phase 2); Lincoff et al., NEJM 2023 (SELECT). All trial figures are the papers' reported means.
For research use only · Not for human or veterinary use · No compound discussed here is FDA-approved for any indication