Peptides in fitness: what they are, and what the evidence actually says
A plain, non-promotional reference. We explain what each compound is, separate what’s claimedfrom what’s actually shown, and are honest about how thin the human data usually is.
What are peptides?
Peptides are short chains of amino acids, the same building blocks as proteins, just fewer of them. Your body makes and uses thousands of them as signalling molecules. The compounds people ask about in fitness are synthetic versions designed to mimic or trigger some of that signalling: to nudge growth-hormone release, to influence tissue repair, or to act on appetite or pigmentation pathways.
A few terms get used loosely. A peptide hormone is a signalling peptide like insulin or growth hormone. A secretagogue is something that makes your body release more of its own hormone. And some compounds sold alongside peptides (MK-677 is the common example) are not peptides at all, a distinction we flag on the individual pages.
The “research chemical” and legality problem
This is the part the marketing skips. Most of these compounds are not approved medicinesfor the uses people want them for. They are typically sold as “research chemicals”, made outside pharmaceutical quality control, where the purity, the actual dose in the vial, and the absence of contamination cannot be assumed. Several are prohibited in sport under the WADA code. Buying and injecting an unlicensed substance of unknown composition is a genuinely different proposition from taking an approved medicine.
Claimed vs shown, how we grade the evidence
We grade the evidence behind a compound’s fitness claims like medals. The stronger the evidence, the higher the medal, and we flag whether it comes from humans, animals, or the lab bench. Claims that are merely oversold earn no medal at all:
- Gold· human data, strong, replicated human trials (usually because the compound went through drug development).
- Silver· human data, real but limited human evidence; promising is not the same as proven.
- Bronze· animal data only, early or animal-only evidence: a lead worth noting, not a result to rely on.
- Unproven, heavily promoted, but the human evidence is weak, absent, or actively didn’t pan out. No medal.
A useful thing happens when you line them up: the compounds with the strongest evidence (tesamorelin, the GLP-1 medicines, bremelanotide) are precisely the ones that were developed and approved as medicines for specific conditions, not as fitness aids. They show what real evidence looks like, and how far most “research peptides” fall short of it.
What this section will not do
We do not give doses, protocols, routes, cycles, stacks, or sourcing information for any of these compounds, deliberately. This is a reference for understanding what they are and what the evidence says, not a how-to. If you are considering any of them, that is a conversation for a qualified clinician who knows your history.
Calculator
- Peptide Reconstitution Calculator
Arithmetic only, on values you supply, concentration, draw volume and U-100 syringe units. Enhanced disclaimer; no dosing guidance, ever.
Growth-hormone secretagogues
- TesamorelinGold· human data
An evidence-tiered explainer on tesamorelin (Egrifta): the one GH-releasing peptide with FDA approval and robust trials, for a medical use, not fitness.
- MK-677 (Ibutamoren)Silver· human data
An evidence-tiered explainer on MK-677 (ibutamoren): an oral ghrelin-receptor agonist that raises GH and IGF-1, with modest human trial data and no approval.
- IpamorelinUnproven· mixed evidence
An evidence-tiered explainer on ipamorelin: a GH secretagogue marketed for muscle, fat loss and anti-ageing, with unproven real-world benefits.
- CJC-1295Unproven· mixed evidence
An evidence-tiered explainer on CJC-1295: a long-acting GHRH analogue marketed for muscle and anti-ageing, with only early data and no proven fitness benefits.
- SermorelinSilver· human data
An evidence-tiered explainer on sermorelin: a GHRH analogue with a real medical history, now marketed for anti-ageing and muscle on evidence that falls short.
- GHRP-6Unproven· mixed evidence
An evidence-tiered explainer on GHRP-6: an older GH-releasing peptide known for strong appetite stimulation, marketed for muscle without real-world evidence.
- GHRP-2Silver· human data
An evidence-tiered explainer on GHRP-2 (pralmorelin): a growth-hormone releasing peptide that reliably raises GH, and why raising GH is not the same as changing your body.
- HexarelinSilver· human data
An evidence-tiered explainer on hexarelin: one of the most potent growth-hormone releasing peptides, and the one that stops working fastest.
“Healing” / recovery peptides
- BPC-157Bronze· animal data only
An evidence-tiered explainer on BPC-157: marketed for tendon and gut healing on almost entirely animal evidence, with no controlled human trials.
- TB-500 / Thymosin Beta-4Bronze· animal data only
An evidence-tiered explainer on TB-500 / thymosin beta-4: marketed for recovery and repair on preclinical evidence, with no controlled human fitness trials.
- GHK-CuSilver· mixed evidence
An evidence-tiered explainer on GHK-Cu (copper peptide): a real cosmetic history for skin, versus the speculative injectable 'healing' and anti-ageing claims.
- Thymosin alpha-1Unproven· human data
An evidence-tiered explainer on thymosin alpha-1: an immune peptide with genuine approved uses in some countries, distinct from its wellness marketing.
Melanocortin peptides
- PT-141 (Bremelanotide)Gold· human data
An evidence-tiered explainer on PT-141 (bremelanotide): FDA-approved for one specific sexual-health condition, a useful contrast to the unapproved peptides.
- Melanotan IIUnproven· human data
An evidence-tiered explainer on Melanotan II: an unlicensed melanocortin agonist sold for tanning, with documented safety concerns and no approval anywhere.
- Melanotan-1Gold· human data
An evidence-tiered explainer on Melanotan-1 (afamelanotide): a licensed medicine for a rare light-sensitivity disorder, and the instructive contrast with its unlicensed cousin.
Metabolic & other
- GLP-1 receptor agonistsGold· human data
How GLP-1 receptor agonists (semaglutide, tirzepatide) differ from unregulated 'research peptides': large-scale trials, approval and prescription-only status.
- HGH Fragment 176-191Unproven· human data
An evidence-tiered explainer on HGH Fragment 176-191: a growth-hormone fragment marketed for fat loss whose largest human trial failed and was abandoned.
- AOD-9604Not supported· human data
An evidence-tiered explainer on AOD-9604: a growth-hormone fragment that failed to beat placebo for weight loss in human trials, now resold for fat loss.
- EpithalonUnproven· mixed evidence
An evidence-tiered explainer on epithalon: marketed for longevity and telomere lengthening on human evidence that is limited, dated and far from proof.
- IGF-1 LR3Unproven· in-vitro only
An evidence-tiered explainer on IGF-1 LR3: a modified growth factor sold as a muscle builder that was created as a cell-culture reagent and never tested in people.
- MOTS-cUnproven· animal data only
An evidence-tiered explainer on MOTS-c: a mitochondrial-derived peptide with striking mouse data, sold as exercise in a vial on the strength of it.
- SemaxSilver· human data
An evidence-tiered explainer on Semax: a Russian-approved nootropic peptide whose entire evidence base sits outside the literature the rest of the world reads.
- SelankSilver· human data
An evidence-tiered explainer on Selank: a Russian anxiolytic peptide with a small domestic evidence base and no independent replication.
- DSIPUnproven· mixed evidence
An evidence-tiered explainer on DSIP (delta sleep-inducing peptide): named for an effect that fifty years of research has not reliably reproduced.
- KisspeptinSilver· human data
An evidence-tiered explainer on kisspeptin: a genuinely important reproductive hormone under active clinical research, and not the libido peptide it is sold as.
- Follistatin-344Unproven· animal data only
An evidence-tiered explainer on Follistatin-344: the myostatin-blocking peptide sold on the back of a photograph of a very muscular mouse.