RESEARCH PEPTIDE FUNDAMENTALS / MATRIX
Three Peptides, Three Very Different Silences
What separates these compounds is less their mechanism than the shape of the evidence around them — how wide the tested population is, how long the follow-up ran, and what each record conspicuously does not cover.
The short version
Lining up semaglutide, tesamorelin and PT-141 side by side is useful for one reason: they show three different ways a body of evidence can be incomplete.
Semaglutide has enormous trials — 17,604 participants in one cardiovascular study alone [3] — so its gaps are about individuals, time horizons and events too rare to count. Tesamorelin has consistent, replicated results [6] inside a single patient group, so its gap is about everyone outside that group. PT-141 has two solid phase 3 trials in one narrowly defined population [13] and a much larger public conversation about people the trials never enrolled.
None of those is a weakness in the science. Trials answer the question they were designed to ask. The mistake is reading an answer to one question as an answer to a different one, and that mistake looks the same in all three cases.
The comparison matrix
| Dimension | Semaglutide | Tesamorelin | PT-141 (bremelanotide) |
|---|---|---|---|
| Peptide class | GLP-1 receptor agonist (incretin mimetic), 31 amino acids | GHRH receptor agonist, 44 amino acids | Melanocortin MC3R/MC4R agonist, cyclic heptapeptide |
| Primary site of action | Pancreas, stomach, hypothalamic and brainstem appetite circuits | Anterior-pituitary somatotrophs, then liver and visceral fat | Hypothalamic and limbic melanocortin circuits |
| Approved indication | Type 2 diabetes, chronic weight management, cardiovascular risk reduction, MASH | Excess abdominal fat in HIV-associated lipodystrophy (2010) [7] | Acquired, generalised HSDD in premenopausal women (2019) [15] |
| Population studied | Tens of thousands across multiple diseases [2][3][4] | Antiretroviral-treated adults with HIV lipodystrophy [6][8][10] | Premenopausal women with HSDD; early male dose-ranging [13][17] |
| Headline result | -14.9% body weight at 68 weeks vs -2.4% placebo [4] | Visceral fat -27.71 cm2, hepatic fat -4.28% pooled [6] | Desire +0.35, distress -0.33 vs placebo at 24 weeks [13] |
| Longest cited follow-up | 72 weeks in a head-to-head trial [1] | 52 weeks [10] | 52-week open-label extension [14] |
| Dominant tolerability issue | Gastrointestinal, nausea in about a third [5] | None reported as serious in pooled trials [6] | Nausea in 40.4% over long-term use [14] |
| Effect after stopping | Not quantified in the sources listed here | Visceral fat reaccumulates [10] | As-needed use; no maintenance effect claimed [15] |
| Sport status | Not specifically prohibited by WADA at this revision | Prohibited, WADA category S2 | Falls under WADA's non-approved-substances framework |
| The unresolved question | Who responds, for how long, and what stopping costs | Whether any of it transfers outside HIV lipodystrophy | Whether modest average gains matter to a given person |
Evidence maturity, in three shapes
Broad and deep. Semaglutide's record spans several diseases and several endpoint types: weight [4], cardiovascular events [3], kidney events [2], and a head-to-head ranking against another molecule [1]. Its safety literature is correspondingly detailed, including signals explicitly described as unconfirmed rather than absent [5].
Narrow and replicated. Tesamorelin's evidence is a tight cluster of randomised trials asking one question in one population, with a pooled analysis of five of them agreeing on the answer [6] and a 52-week programme showing durability while treatment continues [10]. Replication inside a narrow frame is a genuine strength; it is simply a different strength from breadth.
Narrow and contested. PT-141 has two identical phase 3 trials that met their co-primary endpoints [13] and a long-term extension confirming the safety profile [14], alongside a mechanistic imaging study [12], supportive animal work [16][17] and an animal study that failed to confirm a reward-circuit mechanism [11]. The effect sizes are small enough that their clinical meaning is argued about openly.
A reader comparing the three is not comparing how well they work. They are comparing how far each result can legitimately travel.
Where each record goes quiet
For semaglutide the silence is about the individual and the long horizon. The trials report means over defined windows; they do not describe who within the enrolled group did well or badly, what happens after the final visit, or whether rare signals resolve in either direction [5].
For tesamorelin the silence is about everyone outside one diagnosis. Every pivotal trial enrolled antiretroviral-treated adults with HIV-associated lipodystrophy [6][8][10], so any question about a different population is unstudied rather than answered. Its own record also shows that the benefit depends on continuing treatment [10], which pushes the question of an indefinite course past where the data stop.
For PT-141 the silence is about men, about postmenopausal women, and about anyone using material that is not the characterised pharmaceutical the trials used. The approved label defines a dose, a maximum frequency, a half-life of about 2.7 hours and a cardiovascular contraindication [15]; none of that describes an unverified product.
The three silences have one property in common. Filling them requires a specific person's history, current medications, comorbidities and priorities — information a licensed clinician holds and a published paper does not. That is what makes them clinical questions rather than gaps in the literature waiting for a website to fill.
How to read the matrix without over-reading it
A comparison table flattens things, and three cautions keep this one honest.
First, the headline results are not comparable to one another. A body-weight percentage [4], a visceral-fat measurement in square centimetres [6] and a desire-scale point change [13] measure different constructs in different populations. Ranking them against each other would be meaningless.
Second, an empty cell is not a good result. No serious adverse events reported in pooled trials [6] describes what five trials observed in one population over their follow-up periods. It is not a claim of general safety.
Third, regulatory status is a statement about evidence submitted for a specific indication, not a verdict on a molecule. Tesamorelin's single approval reflects the trials that were run, and PT-141's narrow approval reflects the population that was enrolled [15].