Not all weight loss peptides work the same way. Some are FDA-approved drugs with thousands of patients in controlled trials. Others are research compounds where the evidence comes almost entirely from rats. Grouping them together, as most guides do, is what leads people to make bad decisions. This guide keeps them separate.
Tier 1 — FDA-approved, Phase 3 trial data
Tirzepatide (Mounjaro / Zepbound)
The most effective weight loss drug ever approved. In the SURMOUNT-1 trial, tirzepatide 15mg produced 20.9% average body weight reduction over 72 weeks. In SURMOUNT-5, a direct head-to-head against semaglutide, it won at every dose. The mechanism is why: tirzepatide activates both the GLP-1 and GIP receptors simultaneously. GLP-1 suppresses appetite. GIP appears to improve fat metabolism and amplify the GLP-1 effect. One pill pushing two levers produces more than either alone.
The practical barrier is cost. LillyDirect self-pay vials run $299-449/month. Insurance coverage is improving but inconsistent. Compounded tirzepatide availability tightened significantly in 2025 after the FDA resolved the shortage designation.
Semaglutide (Wegovy / Ozempic)
The proven standard. 15% average weight loss in the STEP trials. An additional FDA label for cardiovascular risk reduction via the SELECT trial — the only weight loss drug that has demonstrated MACE reduction in people with obesity who don't have diabetes. That cardiovascular data is the reason many cardiologists still reach for semaglutide first even though tirzepatide produces more weight loss on average.
The January 2026 oral formulation (Wegovy tablet, up to 25mg daily) changes the access picture for needle-averse patients. The OASIS 4 trial showed 14.4% weight loss at 64 weeks, comparable to the injectable. Wegovy HD at 7.2mg (approved March 2026) narrows the efficacy gap with tirzepatide further.
Tier 2 — FDA-approved for adjacent indications, strong off-label evidence
Tesamorelin (Egrifta)
FDA-approved for visceral fat reduction in HIV-associated lipodystrophy. It produces about 18% reduction in visceral adipose tissue — the metabolically active fat around the organs that GLP-1 drugs also target. The mechanism is different: tesamorelin is a GHRH analogue that stimulates the pituitary to release growth hormone, which then drives lipolysis in visceral fat specifically.
Two things make tesamorelin genuinely interesting beyond its approved indication. First, a randomised controlled trial showed significant cognitive improvements in mild cognitive impairment — an outcome no GLP-1 drug has demonstrated. Second, the combination of visceral fat reduction and cognitive benefit makes it relevant to longevity-oriented protocols in a way that pure weight loss drugs are not.
Tier 3 — Research-grade, some human data
MK-677 (Ibutamoren)
Not a weight loss compound in the GLP-1 sense. MK-677 raises GH and IGF-1 by mimicking ghrelin at the pituitary. The result is improved body composition — more lean mass, better bone density, slower fat accumulation — rather than the scale weight reduction that GLP-1 drugs produce. The two-year randomised controlled trial is real and the data is solid. The trade-off is significant: MK-677 stimulates appetite. Without disciplined eating it causes fat gain alongside lean mass gain.
Relevant for people optimising body composition, not for people trying to lose weight on a scale.
Retatrutide (Phase 3 pending)
The next drug in the class. Triple agonist: GLP-1, GIP, and glucagon receptors simultaneously. Phase 2 trials showed 24.2% average weight loss — the highest number ever recorded for a pharmacological weight loss intervention. Phase 3 data is expected in 2026-2027. Not available yet, but worth knowing it's coming. If Phase 3 replicates Phase 2, the weight loss drug landscape changes again.
Tier 4 — Research-grade, primarily animal data
AOD-9604 and HGH Fragment 176-191
Both are fragments of human growth hormone engineered to isolate its fat-burning properties without the growth-promoting effects. The animal data is consistent and the mechanism is real — beta-3 adrenergic receptor activation drives lipolysis in fat cells. The human data is the problem.
AOD-9604 completed Phase 2 trials. The first Phase 2 study showed 2.6kg fat loss versus 0.8kg for placebo. The pivotal Phase IIb trial did not replicate this. The development programme was halted. That outcome is usually what gets omitted from the marketing.
HGH Fragment 176-191 has no completed human trials at all. The evidence is extrapolated from animal data and from the AOD-9604 clinical programme.
Neither is going to produce GLP-1-class fat loss. The realistic expectation from community experience is modest — a marginal lipolytic signal that works alongside caloric deficit, not instead of it.
MOTS-c
A 16-amino acid peptide encoded in mitochondrial DNA. Not a weight loss compound in the conventional sense — MOTS-c improves insulin sensitivity and activates AMPK, the cellular energy sensor, which produces improvements in metabolic health and body composition over time. Mouse studies show it reverses age-related metabolic decline. Human evidence consists of observation that circulating MOTS-c levels decline with age and rise with exercise. The mechanism is compelling. The clinical evidence for deliberate supplementation is early.
The honest framework
If you want meaningful weight loss with clinical evidence behind it, the GLP-1 drugs are the only tier where the data supports that expectation. Tirzepatide produces more than semaglutide on average. Semaglutide has better cardiovascular data. Everything else is either adjunct body composition support or early-stage research.
The research-grade compounds are not useless, but they are not substitutes for the FDA-approved tier. People who use AOD-9604 expecting Ozempic-level results are going to be disappointed. People who use MK-677 alongside a caloric deficit and resistance training may genuinely improve their body composition. Context matters more than compound choice at that tier.