What it is
Thymosin alpha-1 is a peptide of 28 amino acids, acetylated at one end, cut from a larger protein called prothymosin alpha. It was first isolated from thymus tissue — the organ where T cells mature — and that is where both its name and its function come from: it acts on how immune cells develop and respond.
Its international nonproprietary name is thymalfasin, and where it is a licensed medicine it is usually sold under the brand name Zadaxin.
The unusual thing about it
Most compounds in this library are unapproved everywhere and thinly studied. This one is a licensed prescription medicine in a number of countries and has 64 randomized controlled trials in PubMed out of 867 records total (E-utilities, 2026-09-08).
And it has no US approval at all. A Drugs@FDA search on 2026-09-08 for thymalfasin as an active ingredient returned nothing; so did a search for the brand name Zadaxin (openFDA). There is no application to point at, approved or withdrawn.
That combination — real medicine there, absent from the record here — is the single most useful thing to know about it, and it is the reverse of the pattern this library usually documents. Where the Russian-developed peptides have a home literature that never crossed into international registries, this one has crossed into them thoroughly; it just never entered the US regulatory system.
What it is used for
Its clinical footing is as an adjunct — a treatment added to another treatment — in chronic hepatitis B and C. It has also been studied in sepsis and alongside cancer treatment.
The common thread is a person whose immune response is impaired or under heavy strain. That is the setting the trials were run in, and it is not the setting implied by consumer immune-support marketing.
Registered work is still going. Studies recruiting as of 2026-09-08 include NCT06598839, on the immune effect of thymosin alpha-1 in stage I non-small-cell lung cancer, and NCT05339529, on its protective effect against immune dysregulation. A compound with trials currently recruiting is in a different position from one whose evidence stopped accumulating years ago.
Not the same as TB-500
The word "thymosin" covers two unrelated things, and the confusion is constant.
| Thymosin alpha-1 | Thymosin beta-4 / TB-500 | |
|---|---|---|
| Comes from | Prothymosin alpha | A separate beta-thymosin protein |
| Length | 28 amino acids | 43 amino acids; TB-500 is a 7-residue fragment of it |
| Function | Immune signalling, T-cell responses | Actin binding, cell motility, tissue repair |
| Regulatory standing | Licensed medicine in several countries | Not approved anywhere; TB-500 is on the WADA list |
They were grouped under one name because both turned up in thymus preparations. Nothing about one carries over to the other.
The library's other seriously-studied immune peptide is the only cathelicidin humans make, and it is the mirror image of this entry: a licensed medicine here with a single national sponsor behind it, against a compound with a large academic literature and no company anywhere.
Not the same as Thymalin either
A second confusion is worth heading off. Thymalin is also derived from thymus tissue and is also licensed outside the United States, but it is a polypeptide complex — a mixture extracted from the organ, not a single defined molecule with a published sequence. Its evidence sits almost entirely in Russian-language journals and it has no entry in the international trial registry. The two are related by origin and purpose; a claim about one is not evidence about the other.
Where it stands
The best-evidenced unapproved compound in this library, and the one where "unapproved" carries the least weight — because it is unapproved here, not unstudied. The evidence supports specific medical uses in ill people, under prescription. It does not address taking it for general immune support while healthy, which is what it is mostly sold for.
Related entries in this library: KPV is the other immune-signalling fragment here, and the contrast is instructive — same broad area, a fraction of the evidence. The eleven-residue fragment of erythropoietin is the compound in this library whose trials are best designed, which is precisely why its posted numbers — no dose-response, placebo ahead on one nerve measure — carry weight. TB-500 shares nothing but the word thymosin — and neither does the beta thymosin it was cut out of, whose eighteen registered trials are in eyes and hearts rather than in immunity.
Last checked
2026-09-08. The Drugs@FDA searches for thymalfasin and Zadaxin were run through the openFDA API on that date, as were the PubMed counts and the ClinicalTrials.gov queries.