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US Peptide Guide

Last reviewed May 22, 2026·Editorial Board

Thymosin Alpha-1

Also known as Tα1, Thymalfasin, Zadaxin, Ta1

Thymosin Alpha-1 is a 28-amino-acid acetylated peptide identical to the N-terminal fragment of prothymosin α, a polypeptide produced by the thymus. It is the active ingredient of Zadaxin (thymalfasin), an immunomodulator approved in over 35 countries — but not in the United States — primarily for chronic viral hepatitis.[1]

Tier 2 — Clinically StudiedNot FDA-ApprovedCompounding: Category 1(2026-04-23)
Class
Thymic peptide
Sequence length
28 amino acids
Half-life
~2 hours SC
Approved abroad
35+ countries (Zadaxin)
FDA US status
Not approved
Largest RCT
ETASS, 361 patients

What thymosin alpha-1 is

Thymosin Alpha-1 (Tα1, thymalfasin) is a 28-residue acetylated peptide identical in sequence to a fragment of prothymosin α, an endogenous thymic polypeptide. The synthetic version is marketed internationally under the trade name Zadaxin and is approved in over 35 countries — including most of Europe, China, Japan, and several Latin American nations — for chronic hepatitis B (monotherapy and in combination), chronic hepatitis C (combination with interferon and/or ribavirin), and as an immune-enhancing adjuvant for vaccines in immunocompromised patients.[1]

It has never received FDA approval as a finished drug product in the United States. The reason is regulatory and economic — the molecule's longest history of clinical study has been outside the US — not a safety or efficacy verdict.

Mechanism of action

The proposed mechanism centers on Toll-like receptor 9 (TLR-9) activation on plasmacytoid dendritic cells, which initiates a cascade promoting:

  • T-cell maturation from CD4⁻CD8⁻ precursors
  • Th1-skewed cytokine response (IFN-γ, IL-2)
  • Restoration of monocyte HLA-DR expression in sepsis and other immune-paralysis states
  • Augmented natural killer cell activity
  • Re-balancing of Th1/Th2 ratios disturbed by chronic viral infection[1]

The clinical pattern across the published literature is that thymosin alpha-1 acts as an immune modulator rather than a pure immune stimulator: it tends to push deficient immune function toward normal rather than driving runaway activation. This profile underpins its use across both immunosuppressed (sepsis, severe viral infection) and chronic infection (HBV, HCV) contexts.

Human evidence

The clinical dataset is several thousand patients across two decades, primarily in chronic hepatitis and critical care.

Chronic hepatitis B — Phase III multicenter RCT
Multicenter, double-blind, placebo-controlled

Chan et al. (1999) reported a Phase III multicenter, randomized, double-blind, placebo-controlled trial of thymosin alpha 1 in chronic hepatitis B. The trial supported the Zadaxin approvals in multiple jurisdictions. Subsequent meta-analyses of approximately five RCTs (n=353) have reported thymosin alpha 1 monotherapy roughly three-fold superior to placebo for virological response at 12 months post-treatment.

Sources:23
Severe sepsis — ETASS multicenter RCT
361 patients, six tertiary hospitals

Wu et al. (Critical Care 2013) randomized 361 severe sepsis patients to thymosin alpha 1 (1.6 mg subcutaneously twice daily for 5 days, then daily for 2 days) plus standard care or standard care alone. 28-day all-cause mortality was 26.0% with Tα1 vs. 35.0% in the control group (relative risk 0.74, 95% CI 0.54–1.02). Monocyte HLA-DR recovery was greater in the Tα1 arm by day 3 and day 7.

Sources:4
Severe COVID-19 — retrospective controlled study
76 severe COVID-19 patients, 2 Wuhan hospitals

Liu et al. (Clinical Infectious Diseases 2020) reviewed 76 severe COVID-19 patients across two Wuhan hospitals. Mortality in the thymosin alpha-1-treated group was 11.1% vs. 30.0% in the untreated group (p=0.044). Treatment was associated with restoration of lymphocyte counts and reversion of T-cell exhaustion markers.

Sources:5

The combined picture: a peptide with substantial RCT support in chronic viral hepatitis (supporting the international approvals), one large RCT in severe sepsis (ETASS) with a trend toward mortality reduction, and observational/retrospective signal in severe COVID-19. The evidence is real but neither uniformly positive nor at the level of multiple-replicated Phase 3 trials in any single non-hepatitis indication.

Marketed claims vs. published evidence

Marketed claims
  • Boosts the immune system in healthy adults
    no data
  • Cures chronic infections
    no data
  • Universal cancer adjuvant
    anecdotal
  • General "anti-aging" benefits
    no data
Published evidence
  • Increases sustained virological response in chronic hepatitis B[2]
    human RCT
  • Trend toward reduced 28-day mortality in severe sepsis[4]
    human RCT
  • Restores monocyte HLA-DR in immune-paralyzed sepsis patients[4]
    human RCT
  • Reduces mortality in severe COVID-19 (observational)[5]
    case series

The evidence supports thymosin alpha-1 as a clinically-active immunomodulator in specific disease contexts. It does not support generic "immune boosting" claims in healthy adults, "cures" for chronic infection, or broad anti-aging effects.

Regulatory status

Thymosin Alpha-1 has one of the more complex regulatory pictures in this database:

  • Approved abroad as Zadaxin in over 35 countries including most of Europe, China, Japan, Brazil, Mexico, and others. The approvals are typically for chronic hepatitis B, chronic hepatitis C combination therapy, and immune-enhancing adjuvant use in immunocompromised patients.[1]
  • Never FDA-approved in the United States as a finished drug product. This is a regulatory-and-commercial outcome, not a verdict on safety or efficacy.
  • 2023 FDA Category 2 placement effectively halted US compounding pharmacy access.[6]
  • April 23, 2026 reclassification to Category 1 alongside BPC-157, TB-500, AOD-9604, MOTS-c, Selank, Semax, and KPV. Licensed 503A compounding is again permissible. Final PCAC review scheduled for July 2026.

WADA status: Thymosin alpha-1 is not currently on the WADA Prohibited List (it does not fit the GH/IGF-1 or peptide growth factor categories used to designate prohibited substances in S2).[7] Athletes should consult their sport's specific anti-doping authority — status interpretations evolve.

Safety considerations

Thymosin alpha-1 has decades of clinical exposure across the international markets where Zadaxin is approved. The reported adverse event profile is mild: injection site reactions are the most common; transient fever and headache occur infrequently. The molecule has not been associated with the immune-overactivation patterns seen with some cytokine therapies.[1]

The safety dataset in the ETASS sepsis trial showed no excess in adverse events with thymosin alpha-1 vs. standard care.[4]

Dosing is not addressed on this page. Decisions about thymosin alpha-1 use belong with a licensed prescriber working from the specific clinical indication and patient context.

Frequently asked questions

BPC-157peptideTB-500peptideImmune peptideshub

Citations

  1. Chien RN, Liaw YF. Zadaxin (thymosin alpha 1) for the treatment of viral hepatitis. Expert Opinion on Biological Therapy. 2005. PMID 15992078
  2. Chan HL, Tang JL, Tam W, Sung JJ. Thymosin alpha 1 treatment of chronic hepatitis B: results of a phase III multicentre, randomized, double-blind and placebo-controlled study. Alimentary Pharmacology and Therapeutics. 1999. PMID 10607256
  3. Andreone P, Cursaro C, Gramenzi A, et al. Efficacy of thymosin alpha 1 in patients with chronic hepatitis B: a randomized, controlled trial. Hepatology. 1996. PMID 9581695
  4. Wu J, Zhou L, Liu J, et al. The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial. Critical Care. 2013. PMID 23327199
  5. Liu Y, Pan Y, Hu Z, et al. Thymosin alpha 1 reduces the mortality of severe COVID-19 by restoration of lymphocytopenia and reversion of exhausted T cells. Clinical Infectious Diseases. 2020. PMID 32442287
  6. Amanecia Health editorial. FDA peptide reclassification 2026. amaneciahealth.com. 2026. ↗ Source industry-analysis
  7. World Anti-Doping Agency. The 2026 Prohibited List — S2 Peptide Hormones, Growth Factors, Related Substances and Mimetics. wada-ama.org. 2026. ↗ Source wada
Revision history
  • 2026-05-22Initial page. Five primary clinical citations across hepatitis B (Chan 1999, Andreone 1996, Chien 2005 review), severe sepsis (ETASS 2013), and severe COVID-19 (Liu 2020). Notes April 2026 Category 1 reclassification and the unusual US/international approval asymmetry.