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Immune · In Clinical Trials

Thymosin Alpha 1 (Zadaxin)

Awaiting FDA decision

Talk to a clinician about your goal
Written by Peptide Association Editorial TeamLast reviewed Editorial policy

01 · Overview

A 28-amino acid peptide originally isolated from thymic tissue (thymosin fraction 5) and used as an immunomodulator under the name thymalfasin. Thymosin alpha 1 activates dendritic cells through Toll-like receptor signaling and increases expression of tumor, viral, and MHC class I antigens (PMID 17600290).

It has been studied in combination with interferon for chronic viral hepatitis and with chemotherapy in cancer (PMID 17600290).

02 · Research summary & key findings

Not FDA-approved; it is marketed in some other countries as Zadaxin. A 2026 Cochrane review of 10 randomized trials in chronic hepatitis B rated the evidence very low certainty and could not confirm benefit on mortality or liver outcomes (PMID 42713852). A retrospective study of 76 severe COVID-19 patients reported lower mortality with treatment (Liu et al., Clin Infect Dis, 2020) (PMID 32442287), but a larger propensity-matched multicenter study in critically ill patients found no mortality benefit (PMID 33208294). The phase 3 TESTS trial in 1,106 adults with sepsis found no reduction in 28-day mortality (PMID 39814420).

03 · Regulatory status

Thymosin Alpha 1 (Zadaxin) is currently undergoing clinical trials. It is not yet FDA-approved and is available only through clinical research settings or investigational protocols.

Thymosin Alpha 1 (Zadaxin) is not FDA-approved, and as of October 2026 FDA has not finalized whether pharmacies may compound it. Whether a pharmacy will compound it depends on the pharmacy and your state. If you are working on the goal it is studied for, a licensed clinician can tell you what is lawfully available, which may be something else.

04 · Administration routes

  • Subcutaneous injection

References

  1. [1]Thymosin alpha 1: from bench to bedside Ann N Y Acad Sci, 2007. PMID 17600290
  2. [2]Thymosin Alpha 1 Reduces the Mortality of Severe Coronavirus Disease 2019 by Restoration of Lymphocytopenia and Reversion of Exhausted T Cells Clin Infect Dis, 2020. PMID 32442287
  3. [3]The effect of thymosin α1 on mortality of critical COVID-19 patients: A multicenter retrospective study Int Immunopharmacol, 2021. PMID 33208294
  4. [4]The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial BMJ, 2025. PMID 39814420
  5. [5]Thymosin-ɑ1 for people with chronic hepatitis B Cochrane Database Syst Rev, 2026. PMID 42713852

Primary literature

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Disclaimer: This information is provided for educational and research purposes only. It is not intended as medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before starting any peptide therapy. The Peptide Association does not endorse or recommend any specific treatment protocol. See our editorial policy for how this content is produced and reviewed. Clinicians: join the network.