# Thymosin Alpha-1

> Evidence tier A — Controlled human trials · Longevity & Immune · reviewed 2026-11-08
> Source page: https://peptidemeta.com/p/thymosin-alpha-1/

Also known as: Thymalfasin, Zadaxin, Tα1

## Quick answers

**What is Thymosin Alpha-1?**

A thymic peptide that enhances T-cell maturation and dendritic cell function.

**How strong is the evidence for Thymosin Alpha-1?**

Tier A (controlled human trials) on the peptidemeta evidence scale. Many randomized trials, including the phase 3 TESTS trial in sepsis (BMJ 2025, no mortality benefit shown) and earlier RCTs in chronic hepatitis B. It is approved outside the US and EU but not by the FDA or EMA.

**Is Thymosin Alpha-1 FDA-approved?**

No. Thymosin Alpha-1 is not approved by the FDA for any use. Outside the US: China (1996) — Zadaxin (thymalfasin); Singapore — Zadaxin injection, prescription-only, for chronic hepatitis B and C.

**Is Thymosin Alpha-1 banned in sport?**

No. Thymosin Alpha-1 is not on the World Anti-Doping Agency Prohibited List.

**How does Thymosin Alpha-1 work?**

Thymosin alpha-1 is a 28-amino-acid thymic peptide that activates toll-like receptors (TLR2, TLR9) on dendritic cells, promoting MHC class I expression and antigen presentation. It enhances CD4+ and CD8+ T-cell maturation, stimulates NK cell cytotoxicity, and shifts the immune balance toward a Th1 response, improving antiviral and antitumor immunity.

**What is known about the safety of Thymosin Alpha-1?**

Approved outside the US/EU (Zadaxin) with clinical safety data from hepatitis trials; in the 1,106-patient TESTS sepsis RCT no safety outcome differed from placebo. FDA orphan drug designation (not an approval).

## Key facts

- **Evidence tier:** A (Controlled human trials)
- **FDA:** not approved
- **Other approvals:** China (1996) — Zadaxin (thymalfasin); Singapore — Zadaxin injection, prescription-only, for chronic hepatitis B and C
- **WADA:** not on the Prohibited List
- **Peptide:** yes
- **Sequence:** [Ac]SDAAVDTSSEITTKDLKEKKEVVEEAEN (28 residues)
- **Formula:** C129H215N33O55
- **Molecular weight:** 3108.3 g/mol
- **Routes studied:** Subcutaneous, Intramuscular

## Overview

A thymic peptide that enhances T-cell maturation and dendritic cell function. Sold as Zadaxin (thymalfasin), it is approved outside the US and EU (e.g. China, Singapore) for chronic hepatitis B, but not by the FDA or EMA.

## Mechanism

Thymosin alpha-1 is a 28-amino-acid thymic peptide that activates toll-like receptors (TLR2, TLR9) on dendritic cells, promoting MHC class I expression and antigen presentation. It enhances CD4+ and CD8+ T-cell maturation, stimulates NK cell cytotoxicity, and shifts the immune balance toward a Th1 response, improving antiviral and antitumor immunity.

## Effects reported in research

_Reported outcomes, not promises; see the evidence tier._

- Enhanced T-cell and NK cell activity
- Improved vaccine response rates
- Modulated Th1/Th2 immune balance
- Supported immune function in immunocompromised patients

## Safety & unknowns

Approved outside the US/EU (Zadaxin) with clinical safety data from hepatitis trials; in the 1,106-patient TESTS sepsis RCT no safety outcome differed from placebo. FDA orphan drug designation (not an approval). Generally well-tolerated; reported side effects include mild injection site discomfort. Has been used in immunocompromised populations including cancer and hepatitis patients.

## Studies

- Garaci E. (2007). *Thymosin alpha1: a historical overview*. Annals of the New York Academy of Sciences. Review. https://pubmed.ncbi.nlm.nih.gov/17567941/
  Historical overview of thymosin alpha 1 since its 1977 characterization, focusing on its use combined with cytokines and chemotherapy in cancer and infectious disease.
- Wu J, Pei F, Zhou L, et al. (2025). *The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial*. BMJ. Clinical trial. https://pubmed.ncbi.nlm.nih.gov/39814420/
  Phase 3 RCT in 1,106 adults with sepsis: 28-day mortality was 23.4% with thymosin α1 versus 24.1% with placebo, with no significant difference.

## Status sources

- [TESTS trial — thymosin α1 for sepsis, phase 3 RCT (BMJ 2025)](https://pubmed.ncbi.nlm.nih.gov/39814420/)
- [PubChem CID 16130571 (thymalfasin)](https://pubchem.ncbi.nlm.nih.gov/compound/16130571)
- [Singapore National Drug Formulary — Zadaxin injection (SIN07483P)](https://www.ndf.gov.sg/about-drugs/product-information/sin07483p/)
- [SciClone prospectus (HKEX 2021) — Zadaxin approved for sale in China in 1996](https://www1.hkexnews.hk/listedco/listconews/sehk/2021/0219/9626734/2021021900023.pdf)
- [WADA 2026 Prohibited List (only thymosin-β4 is named; S0 does not apply to an approved drug)](https://www.wada-ama.org/sites/default/files/2025-09/2026list_en_final_clean_september_2025.pdf)

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Educational reference only — not medical advice. No dosing information is published. Tiers rate the strength of published evidence, not effectiveness or safety.