Immunomodulatory peptide

Thymosin alpha-1

Clinical

Also known as Thymalfasin, Zadaxin

An immune-tuning peptide approved across dozens of countries.

Cited sources: 3
Human-tested claims: 2 of 2
Best evidence: Tier 2 · Clinical
Status: Approved abroad

A 28-amino-acid peptide that boosts T-helper and NK-cell responses. Approved in dozens of countries (not the US) for chronic hepatitis and as a vaccine adjuvant, with randomized trial data in viral hepatitis and severe sepsis — the best-replicated of the immune/longevity group.

immuneantiviralapproved-abroad

How it works

Thymosin alpha-1 is the acetylated N-terminal fragment of prothymosin alpha. It nudges the immune system toward a Th1 (antiviral) response and enhances T-cell and natural-killer-cell activity, partly through Toll-like-receptor signaling — useful where immunity is blunted, as in chronic infection.

Sequence: Ac-SDAAVDTSSEITTKDLKEKKEVVEEAENThe acetylated N-terminal 28 residues of prothymosin alpha; the N-acetylation is required for activity.

What it's studied for

One claim per card, each with its own tier and source. Tiers 1 and 2 are human data. Tiers 3 and 4 are not yet.

Studied in chronic viral hepatitis, alone and combined with interferon.
Clinical
[1]Naylor, 1999 (Expert Opin Investig Drugs)(opens in a new tab)
Improved immune measures and 28-day outcomes in severe sepsis in a multicenter randomized trial (ETASS).
Clinical
[2]Wu et al., 2013 (ETASS, Critical Care)(opens in a new tab)

Safety & limitations

Generally well tolerated in trials. The clinical evidence base is real but heterogeneous, with some studies limited by design quality — read meta-analyses with that caveat.

[2]Wu et al., 2013 (ETASS, Critical Care)

Regulatory & legal status

Approved abroad

Approved in dozens of countries (as Zadaxin / thymalfasin) for chronic hepatitis B/C and as a vaccine adjuvant. Not FDA-approved in the United States.

[1]Naylor, 1999 (Expert Opin Investig Drugs)

Frequently asked

Is thymosin alpha-1 FDA-approved?

No — it is approved in many other countries (as Zadaxin) but not in the United States, where it remains investigational.

References

Written by Twenty Residues editorial. Medical review: pending; a named reviewer is being assigned.

Last updated September 19, 2026.

Spot an error or a better source? Report a correction. Every change is logged below.

Update history

2026-09-19 — Sourced review — every claim checked against primary literature; sequence, regulatory status, safety, and FAQs added.