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Thymosin Alpha-1

Thymosin alpha-1 is a peptide of 28 amino acids first isolated from the thymus gland. The drug version, thymalfasin (brand Zadaxin), is made synthetically and is identical to the natural peptide. It is used to support immune responses. It is approved as a medicine in some countries, but not in the US.

Approved in some countriesChecked October 2026
Open the Thymosin Alpha-1 calculatorSources

Approval status

Thymalfasin (Zadaxin) is approved in Italy by the medicines agency AIFA as an add-on to flu vaccination in people with weakened immune systems. Reviews report approvals in more than 30 countries in all, mainly for hepatitis B and C. It is not approved by the FDA. FDA had listed it among compounding substances that may pose significant safety risks; the nomination was withdrawn, and it is not a substance US pharmacies may compound with.35678910

What it’s used or studied for

  • Approved use

    Add-on to flu vaccination in people with weakened immune systems (approved in Italy by AIFA).35

  • Human studies

    Chronic hepatitis B: tested in 10 randomized trials, alone or with antiviral drugs. A 2026 Cochrane review rated the evidence of benefit as very uncertain.11

  • Human studies

    Sepsis: a large placebo-controlled trial in 1,106 adults found no reduction in deaths at 28 days.12

  • Human studies

    COVID-19: two 2023 meta-analyses reached opposite conclusions on whether it lowers deaths in hospitalized patients.1314

  • Human studies

    Metastatic melanoma: in a randomized trial of 488 patients, adding it to chemotherapy suggested some activity, and the authors called for further trials.15

Each label shows how strong the evidence is: an approved use, human studies, animal or lab studies only, or a popular claim with no published studies. Being studied is not the same as being proven safe or effective.

Side effects

Common

  • Mild, short-lived pain or irritation at the injection site

The Italian label lists only occasional mild, short-lived pain at the injection site. In a large sepsis trial, side effects and serious side effects were no more common than with placebo.3124

Cautions

  • The Italian label says not to use it during pregnancy or breastfeeding.3
  • The label advises caution in people with allergies, and a case-by-case decision in people with autoimmune disease.3
  • The label advises caution when it is combined with other drugs that act on the immune system.3
  • FDA has raised concerns about immune reactions and impurities in compounded versions, and said the safety information it had was inadequate.8

Sources

  1. PubChem: Thymalfasin (CID 16130571)
  2. UniProt: Prothymosin alpha, human (P06454), source of thymosin alpha-1
  3. AIFA (Italian Medicines Agency): Zadaxin (thymosin alpha-1) summary of product characteristics, in Italian
  4. Dominari et al. (2020). Thymosin alpha 1: a comprehensive review of the literature. World J Virol
  5. EMA: Orphan designation EU/3/02/110, thymalfasin for hepatocellular carcinoma
  6. Goldstein (2009). From lab to bedside: emerging clinical applications of thymosin alpha 1. Expert Opin Biol Ther
  7. DrugBank: Thymalfasin (DB04900)
  8. FDA: Bulk drug substances used in compounding that may present significant safety risks (current as of April 2026)
  9. FDA: Bulk drug substances nominated for compounding under section 503A, categories 1 to 3 (updated May 14, 2026)
  10. FDA guidance: Interim policy on compounding using bulk drug substances under section 503A (January 2025)
  11. Naing et al. (2026). Thymosin-α1 for people with chronic hepatitis B. Cochrane Database Syst Rev
  12. Wu et al. (2025). The efficacy and safety of thymosin α1 for sepsis (TESTS): phase 3 randomized placebo-controlled trial. BMJ
  13. Shang et al. (2023). Thymosin alpha1 use in adult COVID-19 patients: a systematic review and meta-analysis. Int Immunopharmacol
  14. Soeroto et al. (2023). The efficacy of thymosin alpha-1 therapy in moderate to critical COVID-19 patients: a meta-analysis. Inflammopharmacology
  15. Maio et al. (2010). Large randomized study of thymosin alpha 1, interferon alfa, or both in combination with dacarbazine in metastatic melanoma. J Clin Oncol

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