Thymosin Alpha-1

An immune-modulating peptide not approved in the US

Not FDA-approved · not available for sale or prescription

Stylised model of the real sequence

Overview.

Thymosin alpha-1 is not FDA-approved. As thymalfasin, it is a synthetic version of a naturally occurring thymic peptide that modulates innate and adaptive immunity. Its maker reports approvals (brand Zadaxin) in more than 30 countries, mainly for chronic hepatitis B (also hepatitis C and as a vaccine or chemotherapy adjuvant), though FDA says it could not verify them, and the certainty of evidence for hepatitis B outcomes is rated very low to low. In the US it has no lawful compounding pathway, and product comes through compounding or research-chemical channels.

Also known as Tα1 · Thymalfasin · Zadaxin

The molecule, decoded.

Sequence: Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN-OH
Length: 28 residues
Molecular weight: 3,108.3 g/mol
Formula: C129H215N33O55
CAS number: 62304-98-7
Half-life: About 2 hours after subcutaneous injection (healthy men)
Routes studied: subcutaneous injection (Zadaxin's route and the route used in clinical trials)
Modifications: N-terminal acetylation; free C-terminal acid

How it works.

Thymosin alpha-1 acts on toll-like receptors (particularly TLR9) and modulates dendritic cell maturation, T-cell differentiation (increasing CD4+ counts and the CD4+/CD8+ ratio), and cytokine balance, generally shifting an over-active inflammatory response toward resolution while boosting antigen-specific immune responses -- a dual immunomodulatory profile studied both for chronic viral infection and for hyperinflammatory states such as sepsis or severe pancreatitis.

Evidence · Approved outside the US (several countries) · limited evidence

What the research shows

Cochrane meta-analysis2026

A 2026 Cochrane systematic review and meta-analysis of 10 RCTs (1,349 participants) in chronic hepatitis B rated the certainty of evidence very low to low for nearly all outcomes; within that limit, thymosin-α1 may reduce all-cause mortality (RR 0.53, 95% CI 0.29-0.96) and serious adverse events (RR 0.72, 95% CI 0.53-0.99) versus control, and effects on quality of life and histological improvement were uncertain.

1349 adults with chronic hepatitis B across 10 RCTs (Bangladesh, China, Italy, Korea, Singapore, Taiwan)

Source

meta-analysis2025

A meta-analysis of 5 RCTs (706 patients) found thymosin-α1 significantly increased CD4+ cell percentage and CD4+/CD8+ ratio, reduced extrapancreatic infection incidence (RR 0.56), and lowered APACHE II scores in patients with severe acute pancreatitis, though it did not significantly shorten hospital stay.

706 patients with severe acute pancreatitis across 5 RCTs

Source

open-label RCT2026

In a small open-label RCT (NCT03082885) of 73 patients with HBV-related acute-on-chronic liver failure, adding thymosin-α1 to standard medical therapy significantly increased 90-day transplant-free survival and was associated with reduced regulatory T-cell frequencies and moderated late-stage hyperinflammation.

73 adults with HBV-related acute-on-chronic liver failure

Source

What to know first.

Based on: randomized trials and meta-analyses, mostly rated low or very low certainty.

Reported side effects

Most common in clinical studies: local irritation, redness or discomfort at the injection site
Reported in studies: ALT flares in chronic hepatitis B, TSH abnormalities in chronic hepatitis C, and fatal immune hemolytic anemia and engraftment failure in hematopoietic stem-cell transplant recipients
Cochrane (2026): serious adverse events RR 0.72 (95% CI 0.53-0.99) vs control in chronic hepatitis B, low-certainty evidence; the reviewers were not sure Tα1 reduces serious adverse events

Cautions

Not FDA approved, and the PCAC voted 4-17 (Dec 2024) against adding Tα1 to the 503A Bulks List, so US compounding with it falls outside the 503A pathway; research-channel product has quality/consistency concerns
FDA flagged risks in patients undergoing deliberate immunosuppression (e.g., graft-versus-host disease or engraftment failure after stem-cell transplant) and too little safety data for use as a vaccine adjuvant
Ex-US labels carry warnings or contraindications for children, pregnancy and lactation, autoimmune disease and immunosuppressed patients (per FDA review)
Marketed outside the US since 1997, but current systematic-review evidence for hepatitis B outcomes is rated very low to low certainty

Checked 2026-09

Where it stands.

Not FDA-approved · not available for sale or prescription. Thymosin Alpha-1 is not FDA-approved and is not available for sale or prescription. We profile it because people ask about it.

FDA: Not FDA approved; investigational in the US. Marketed ex-US as thymalfasin (Zadaxin, SciClone Pharmaceuticals). SciClone reports approvals in 30+ countries, mainly in Asia, Latin America and the Middle East, chiefly for chronic hepatitis B (also hepatitis C and use as a vaccine or chemotherapy adjuvant). FDA says it was unable to verify these approvals and that Tα1 is not approved in the US, Japan or Europe except Italy; the Italian approval SciClone acquired in 1998 was as an influenza-vaccine adjuvant. SciClone's own first launch of Zadaxin was in China in January 1997. Tα1 was not on the July 2026 PCAC agenda, but FDA took Tα1 free base and acetate to the PCAC on 4 Dec 2024, and the committee voted 4 yes, 17 no on each form against adding them to the 503A Bulks List.
Approved products: Zadaxin (China, Italy and others): chronic hepatitis B (principal indication); hepatitis C and vaccine or chemotherapy adjuvant in some countries (Italy: influenza-vaccine adjuvant)
Compounding: Nominated for the 503A Bulks List by Wells Pharmacy Network (FDA-2015-N-3534-0288) and held in 503A Category 2; Wells withdrew on 18 Sep 2024 (FDA-2015-N-3534-0470), and FDA's Category 2 page (updated 22 Apr 2026) lists thymosin alpha-1 only under 'nominated but withdrawn'. FDA still evaluated Tα1 free base and acetate for 12 uses and, at the 4 Dec 2024 PCAC, proposed NOT adding them; the committee agreed, voting 4-17-0 on each form. Tα1 is not on 21 CFR 216.23 and not in Category 1.
Sport (WADA): Not named anywhere in the WADA 2026 Prohibited List; the only thymosin-related entry is thymosin-β4/TB-500 under S2.3, a different molecule. Its long-standing drug approvals outside the US (Zadaxin) likely place it outside the S0 'non-approved substance' criterion.

References (5)

Thymosin Alpha-1

Overview.

The molecule, decoded.

How it works.

What the research shows

What to know first.

Reported side effects

Cautions

Where it stands.

More in Immunity