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Immunomodulatory Peptide

Thymosin Alpha-1

Also known as: Tα1, Zadaxin, Thymalfasin

A 28-amino-acid synthetic peptide with the most mature real-world clinical track record of any peptide on this site — approved as a prescription drug (Zadaxin/thymalfasin) in dozens of countries for hepatitis B and as a vaccine/immune adjuvant, though not FDA-approved in the United States.

Educational content, not advice. Compiled from information publicly available on the internet; it may contain inaccuracies and was not written or reviewed by medical professionals. Use it as a starting point for your own research, verify against primary sources, and see our full disclaimer.

Overview

Thymosin alpha-1 is a naturally occurring thymic peptide with a synthetic version (thymalfasin, brand name Zadaxin) that has been an approved prescription drug in roughly 35-40 countries since the 1990s, primarily for chronic hepatitis B and as an adjunct to vaccination or chemotherapy in immunocompromised patients. It has not received FDA approval in the United States, despite its approval elsewhere and an extensive international trial history.

Proposed Mechanism of Action

Thymosin alpha-1 modulates immune function, primarily by supporting T-cell maturation and function; the reviewed literature describes effects on dendritic cell maturation, Toll-like receptor signaling, and enhancement of vaccine immunogenicity, alongside its established antiviral-adjunct role in hepatitis B.

Research Context

The cited review (Naylor, 1999) summarizes clinical trial evidence showing efficacy in chronic hepatitis B, both alone and combined with interferon-alpha, and in hepatitis C combined with interferon-alpha. Beyond hepatitis, thymosin alpha-1 has a substantial published trial history in other contexts — as a vaccine-response enhancer (including a pilot study of H1N1 influenza vaccine response), and more recently in inflammatory and infectious conditions including severe acute pancreatitis and, during the COVID-19 pandemic, as an investigated adjunct therapy.

Regulatory Status

Thymosin alpha-1 stands out among the peptides on this site as having a genuine, decades-long international regulatory approval history (as Zadaxin) — but it's worth being precise about what that does and doesn't mean: approval in other countries for specific indications (chronic hepatitis B, immune support) does not mean FDA approval, and does not extend to unrelated uses.

Limitations of the Current Evidence

  • The cited source is a review rather than a single primary trial; we recommend following its references to the underlying randomized trials if you need primary-source-level detail.
  • FDA approval status varies from most other regulatory bodies that have approved it, which is a notable regulatory gap worth understanding before assuming U.S. regulatory equivalence.

Cited Studies

  • Zadaxin (thymosin alpha1) for the treatment of viral hepatitis

    Naylor PH · Expert Opinion on Investigational Drugs · 1999

    Review of human clinical trialsView source →

Last updated August 1, 2026