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Not medical advice. The Atlas is not a substitute for a clinician who can examine you.

Thymosin Alpha-1

Thymalfasin / TA1

Evidence grade 3 of 5, Mixed. Read how the Atlas grades evidence.

Thymosin alpha-1 (thymalfasin) is approved as Zadaxin in roughly 35 countries and carries genuine human-trial data, but the evidence is mixed rather than strong. Chronic hepatitis B trials and meta-analyses (Chan 2001; Wu 2007 vs interferon; individual RCTs showing higher virological response) are small and inconsistent, and the 2022 Cochrane review on the question was published only as a protocol with no completed high-certainty conclusion. Sepsis signals from earlier meta-analyses (RR ~0.59 for mortality) were undercut by the largest trial to date, the phase 3 double-blind TESTS trial (n=1106, 2025), which found no 28-day mortality benefit (HR 0.99), so an honest grade for the specific supported uses is moderate, not strong.

FDA
Not FDA-approved for any US indication; marketed as Zadaxin (thymalfasin) in ~35 countries for chronic hepatitis B/C and as an immune adjunct. In the US it has been available only via compounding, and the FDA Pharmacy Compounding Advisory Committee (Dec 2024) reviewed TA1 as a 503A bulk drug substance and flagged immunogenicity/aggregation concerns, moving toward barring it from 503A compounding.
WADA
Not on the WADA Prohibited List

Reference only. Not medical advice.

What can go wrong

Generally well tolerated in trials (mild injection-site reactions), but US supply is unapproved compounded or research-chemical material of uncertain purity, and FDA flagged immunogenicity/aggregation risk in injectable form.

Talk to a clinician before taking this. The Atlas can tell you what has been studied about Thymosin Alpha-1. It cannot tell you how it would interact with your body, your history, or anything else you are taking, and nothing on this page is a reason to start.

A 28-amino acid naturally occurring thymic peptide, approved in 35+ countries (including Italy and China) as Zadaxin, with FDA IND status in the US. It carries substantial human-trial data for immune dysfunction, chronic hepatitis, and post-COVID recovery.

The other Goldstein thymic peptide. Approved in 35+ countries. Strong post-COVID research.

What people take it for

These are the reasons people give for taking it. They are not effects it has been shown to have.

  • Chronic hepatitis B/C
  • Post-COVID immune recovery
  • Cancer immunotherapy adjunct
  • Immune deficiency

How it is supposed to work

Thymosin Alpha-1 is an immune modulator, which means it recalibrates the immune system rather than simply revving it up. Originally a product of the thymus, the gland that trains immune cells, it promotes the maturation of T-cells, restores a healthy balance between the different arms of the immune response, and activates dendritic cells (through a sensor called TLR9), natural killer cells and antibody production. The result is a better-tuned immune system, which is why it is used to help the body fight chronic infections and to strengthen weak vaccine responses.

Thymosin Alpha-1 (also called thymalfasin, brand name Zadaxin) is a synthetic 28-amino-acid peptide identical to a fragment of the larger thymic protein prothymosin alpha. It was first isolated from the thymus by Allan Goldstein’s laboratory in 1977, during the era when the thymus was still a poorly understood organ. Its identity as a natural immune-regulating molecule, rather than a designed drug, underlies both its broad effects and its excellent tolerability.

Type
Immunomodulator (28 aa)
Acts via
TLR9, T-cell maturation
Brand
Zadaxin (thymalfasin)
Approved in
~30 countries
FDA
Not approved
Dose
1.6 mg twice weekly

Approved Abroad, Unapproved At Home

Thymosin Alpha-1 is the rare case in this library where the honesty runs the other way. Most gray-market peptides are overhyped relative to their evidence; this one is better evidenced than its US status suggests. It is a genuinely approved medicine, sold as Zadaxin in roughly 30 countries, with real randomized-trial data, especially for hepatitis B, where it rivals interferon while being far better tolerated. Its quirk is regulatory, not scientific: it is not FDA-approved, so in the United States it is a research compound, which is why most of its clinical literature comes from Italy and China and why American users obtain it gray-market. The genuine caveats are narrower than usual: the booming off-label use for general anti-aging and immune-boosting still reaches beyond its proven uses, and US gray-market sourcing carries the usual quality risks. But the core molecule is a legitimately approved immune drug, not a speculative one.

Safety in detail

Approved-abroad Zadaxin is a regulated drug product, but material sold in the US is either compounded (now facing 503A restriction) or sold as a research chemical not for human use; purity, dose accuracy, and immunogenicity are not FDA-verified for those sources.

  • Its tolerability is excellent and is a defining advantage: unlike interferon, it causes no flu-like syndrome and no significant blood toxicity.
  • As an immunomodulator that recalibrates rather than overstimulates, it has a benign profile across decades of approved use in large numbers of patients.
  • It is approved abroad but a research compound in the US, so American gray-market sourcing carries the usual identity and purity risks.
  • Its off-label use for general anti-aging and immune-boosting goes beyond the indications it is actually proven for.
  • Dosing is unusually consistent (1.6 mg twice weekly) across approved and research protocols, reflecting established pharmaceutical standards.

Children and teenagers

In countries where it is approved, Thymosin Alpha-1 has clinical use that can include pediatric contexts such as hepatitis B and immune support, under medical supervision. That is distinct from gray-market anti-aging or immune-boosting use in healthy minors, which has no rationale or oversight. Any pediatric use belongs strictly within approved clinical care, not self-sourced research material.

What people get wrong

Nothing recorded here yet. There is almost certainly something people get wrong about this one, and the blank space means we have not written it down, not that the record is clean.

Who profits, and who pushes back

Nobody is named here yet, because we have not done that work on this entry. Read that as unfinished. Somebody almost certainly sells this, and somebody almost certainly disputes it. Assume both until we have checked.

Thymosin Alpha-1 has a real pharmaceutical owner, SciClone Pharmaceuticals, which markets Zadaxin as an approved drug, particularly across Asia where the hepatitis B burden is high. Around the unapproved US market sits the familiar gray-market and clinic economy: research-peptide vendors and integrative or longevity clinics offering it for immune support and anti-aging. So its money splits between a legitimate approved-drug business abroad and an off-label research-chemical market in countries like the US where it is not approved.

How we got here

Thymosin alpha 1 is a 28 amino acid thymic peptide originally isolated from thymosin fraction 5, an extract of calf thymus tissue characterized in the 1970s. The synthetic version, given the International Nonproprietary Name thymalfasin, was developed and marketed by SciClone Pharmaceuticals under the brand name Zadaxin. It has been approved in roughly 35 countries as a treatment for chronic hepatitis B and C and as an immune modulator, but it has never received FDA approval in the United States.

Thymosin Alpha-1 grew out of 1970s research into the immune role of the thymus, with Allan Goldstein’s lab isolating it in 1977. SciClone Pharmaceuticals licensed it and pursued approval, and controlled hepatitis B trials established it as effective and well tolerated, leading to approval as Zadaxin across Asia, Italy and Latin America. Over the following decades it accumulated research in sepsis, cancer, vaccination and, later, COVID-19, with most of that work coming from Italy and China, the two places where it has long been both a marketed medicine and a research tool. In the US it has remained, to date, an unapproved research compound used off-label.

  1. 1977Thymosin Alpha-1 isolated from the thymus by Goldstein’s lab.
  2. 1980s-1990sSciClone licenses it and runs hepatitis B trials; approvals follow in Asia.
  3. 1995-2010Controlled trials establish hepatitis B efficacy comparable to interferon with better tolerability.
  4. 2000s-2010sResearch expands into sepsis, cancer adjuvant use and vaccine enhancement, mainly in Italy and China.
  5. 2020sUsed in severe COVID-19 in China; remains an unapproved research compound in the US.

The numbers behind the grade

Type
Immunomodulator (28 aa)
Brand
Zadaxin
Approved in
~30 countries
FDA
Not approved
Best evidence
Hepatitis B
Tolerability
Excellent

The people taking it

One of the most clinically documented peptides in existence and a genuinely approved drug abroad (Zadaxin), a major name in immune support, yet a research compound in the US.

Thymosin Alpha-1’s approved users are largely hepatitis B patients, concentrated in Asia, plus cancer and sepsis patients receiving it as an adjunct and elderly or immunocompromised people using it to improve vaccine response. Its off-label users, especially in the US, are anti-aging and immune-support seekers and people with chronic infections such as Lyme disease. The approved population is genuinely clinical; the off-label population is wellness-oriented.

Approved abroad for chronic hepatitis B and C, immunodeficiency and vaccine enhancement, and used off-label, especially in the US, for anti-aging and immune support. Genuinely evidence-backed for its approved uses, beyond most peptides here.

Set against the alternatives

Against interferon, the standard comparison in hepatitis B, Thymosin Alpha-1 achieves similar viral-control results with dramatically better tolerability, which is its clinical niche. Against Thymosin Beta-4 (and its fragment TB-500), with which it is often confused, it is entirely different: that peptide is about tissue repair, while Thymosin Alpha-1 is about immune modulation, and they share little beyond a thymic name. And against the general category of immune boosters and supplements, Thymosin Alpha-1 stands apart by recalibrating the immune system through a defined mechanism and by having real regulatory approval and trial data. Its strength is genuine evidence; its main limitation is the US regulatory gap and the off-label stretch.

What we do not know

  • Whether Thymosin Alpha-1 ever gains FDA approval in the US.
  • Whether the off-label anti-aging and immune-boosting use is justified beyond its proven indications.
  • Whether its benefits in sepsis and as a cancer adjunct hold up in larger, well-designed trials.
  • How valuable immune recalibration is for age-related immune decline.

What could change

Possible US approvalWhether it ever clears the FDA remains an open and consequential question.Sepsis and critical careLarger trials could firm up its role as an immune adjunct.Immune agingIts effect on weak vaccine responses keeps it relevant to immunosenescence research.Cancer immunotherapyContinued interest as an adjunct to modern immunotherapy.

The market for this

What follows describes how this is bought and sold. It is not a buying guide and it is not an endorsement of any seller.

Where approved, it is obtained as the prescription drug Zadaxin or a local thymalfasin equivalent. In the US and other unapproved markets it is available only as research-use-only material, which is not an approved route; anyone using it that way should insist on third-party testing and recognize that the anti-aging claims exceed its proven uses.

As an approved drug it is affordably and routinely used in markets like China. In the US it is sold as a research peptide at gray-market prices, without the oversight of its approved-drug status elsewhere. The dosing is consistent between approved and off-label use.

The approved drug comes from SciClone under full regulation in authorized markets. Gray-market material for unapproved markets comes from Chinese research synthesis, where a certificate of analysis is the only quality check.

SciClone (Zadaxin, approved)Manufactures and sells the approved drug under regulation in the roughly 30 countries where it is authorized, a controlled chain.China and research vendorsSupply gray-market Thymosin Alpha-1 as research-use-only material in markets like the US where it is unapproved.Clinic channelIntegrative and longevity clinics distribute it off-label for immune and anti-aging use.

At a glance

Approval
35+ countries (Zadaxin)
Indications
Hepatitis, immune dysfunction
Origin
Thymus gland
Safety
Among the best of any non-FDA peptide

What this entry does not yet cover

What follows is unfinished work on our side, not a finding about Thymosin Alpha-1.

Nothing outstanding on this one.

Sources

The studies that matter

  • Goldstein isolation1977

    First isolated Thymosin Alpha-1 from the thymus and established its role in T-cell development, the foundation of the field.

  • Chronic hepatitis B trials1995-2010

    Controlled trials showed roughly 25 to 40% HBeAg seroconversion with twice-weekly Thymosin Alpha-1, comparable to interferon but with far better tolerability, the basis for approval across Asia.

  • Sepsis randomized trials2010s-2021

    Including a 2021 trial of 361 patients that reported reduced 28-day mortality, and systematic reviews suggesting lower ICU mortality, though some results are limited by study size and design.

  • Cancer adjuvant trials2000s-2010s

    More than 40 trials as an add-on to chemotherapy and immunotherapy, with a notable lung-cancer study reporting improved survival and preserved immune cell counts.

  • Vaccine enhancement2000s-2010s

    Improved antibody responses to vaccination in elderly and immunocompromised people, relevant to age-related immune decline.