Quick answer: Peptides are short amino-acid chains that signal the hair follicle — but they are not interchangeable, and only one has independent human trial evidence for density. Capixyl (acetyl tetrapeptide-3 + biochanin A) matched 3% minoxidil in an independent 24-week trial. Redensyl has trial data (usually in a blend); Procapil, GHK-Cu and palmitoyl peptides rest mostly on in-vitro or supplier data. And a "multi-peptide" serum with 6+ actives often can't deliver the trial-proven dose of any single one. Here's the evidence, peptide by peptide.
The Ordinary's Multi-Peptide Serum made "peptides for hair" a category. But "peptides" is an umbrella, and the marketing rarely tells you which peptide carries the claim — or what kind of study backs it. This guide ranks the major hair peptides by the strength of their evidence, names each by its INCI ingredient, and explains the one thing no product label shows: the blend-dilution problem.
What peptides actually do at the follicle
Peptides are short chains of amino acids — the building blocks of proteins. In hair care, some act as signals: they tell follicle and dermal-papilla cells to stay in the growth (anagen) phase longer, or to build extracellular matrix and support the follicle's blood supply. Others inhibit enzymes tied to hair loss — chiefly 5-alpha-reductase, which converts testosterone to DHT, the hormone behind male- and female-pattern thinning. The key idea: a different peptide does a different job, so "contains peptides" tells you almost nothing on its own.
The evidence-tier table no product page shows you
Here is what the marketing flattens — each major hair peptide, what it does, its strongest study, and the type of evidence (a human randomised controlled trial is worth far more than an in-vitro dish or a supplier brochure):
| Peptide (INCI) | What it signals | Strongest evidence | Evidence tier |
|---|---|---|---|
| Capixyl (acetyl tetrapeptide-3 + Trifolium pratense/biochanin A) | Inhibits 5-alpha-reductase (anti-DHT); supports follicle matrix | Independent 24-week triple-blind RCT, n=32: +8.3% terminal hair vs +8.7% for 3% minoxidil, no significant difference (Lueangarun 2020). Plus a placebo-controlled RCT of 5% Capixyl (Loing 2013, manufacturer-funded) | Human RCT — strongest |
| Redensyl (DHQG + EGCG2 blend) | Targets hair-follicle stem cells | Comparative RCT, n=54, showing improvement — but tested inside a combination (Pavithra 2023) | Human RCT (in a blend) |
| Procapil (biotinoyl tripeptide-1 + apigenin + oleanolic acid) | Follicle anchoring; tolerability | Apigenin elongated the follicle in organ culture (Huh 2009, in-vitro); a small RCT added Procapil to minoxidil (Fahim 2024, details limited) | Mostly in-vitro / adjunct |
| AnaGain (Pisum sativum/pea-sprout) | Prolongs the anagen growth phase (not anti-DHT) | Open-label clinical report, no placebo arm (Grothe 2020) | Open-label human |
| GHK-Cu (copper tripeptide-1) | Tissue remodeling; follicle size in reviews | Narrative reviews authored by the molecule's commercial owner; no independent human hair-density RCT (Pickart 2008/2018) | In-vitro / review (conflicted) |
| Palmitoyl peptides (palmitoyl tripeptide-1, pentapeptide-4) | Signal extracellular-matrix proteins | No peer-reviewed in-human hair-density RCT located — supplier dossiers and in-vitro only | In-vitro / supplier |
Read top to bottom, the evidence thins fast. One peptide — Capixyl — has an independent head-to-head against the reference drug. Everything below it is weaker, and the bottom two are not backed by any independent human trial for hair density. That is not an attack on those molecules; it is what the published record currently shows.
Why Capixyl's evidence is the molecule, not the trademark
The load-bearing part of Capixyl is biochanin A, a red-clover isoflavone. Its 5-alpha-reductase inhibition is documented in two of the most-cited papers in the field (Hiipakka 2002, cited 230 times; Evans 1995, cited 370 times), and the relevant isoform is type-2 — the one concentrated in the scalp follicle. That matters because it means the science survives a brand change: you are buying a molecule with a mechanism, not a trademark.
The multi-peptide blend problem the label won't show
Here is the gap no editorial page documents. The trials above used a specific peptide at a specific concentration — Capixyl's RCT used it at a defined dose. A serum that lists six or more peptides is making a breadth claim, but a formula has only so much room: pack in 6+ actives and each is likely present below its trial-proven dose. "Multi-peptide" can mean a meaningful dose of one or two effective peptides — or a sprinkle of many. The label rarely discloses the percentage of each, so you cannot tell. Breadth is not the same as efficacy. The honest question to ask any density serum is: which peptide carries the claim, and is it at the dose the study used?
How to use a peptide serum
Peptide serums are leave-on: apply to a clean, towel-dried scalp, target the thinning area, and use daily as directed. They are a gentle, consistent routine — not a medical procedure. And to be clear, a topical peptide serum is not a substitute for PRP (platelet-rich plasma), an in-clinic injectable; they operate at completely different levels of intervention. Give any serum the timeframe the trials used: Capixyl's RCT measured its result at 24 weeks.
The bottom line
"Peptides for hair" is a real category with one standout: Capixyl/biochanin A is the only peptide active with an independent, minoxidil-comparable human trial for density. Judge a serum by which peptide carries its claim and whether it's dosed like the study — not by how many peptides crowd the label. You'll find Capixyl at a disclosed dose in Follicle-Stimulating Scalp Serum and Follicle Boost Serum, with the full active breakdown on the peptide comparison page.
References: Lueangarun S, Panchaprateep R. J Clin Aesthet Dermatol 2020;13(10):32-37 (PMID 33584955). · Loing E, et al. J Cosmet Sci 2013 (PMID 23449130). · Hiipakka RA, et al. Biochem Pharmacol 2002 (PMID 11931850). · Evans BA, et al. J Endocrinol 1995 (PMID 7490559). · Pavithra TR, et al. Cureus 2023 (PMID 37292551). · Huh S, et al. Arch Dermatol Res 2009 (PMID 19277688). · Grothe T, et al. Phytother Res 2020 (PMID 31680356). · Pickart L, Margolina A. Int J Mol Sci 2018 (PMID 29986520).
