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Synthetic thymic peptide, immune modulator

Thymosin alpha-1

Thymosin alpha-1 (thymalfasin) is a synthetic version of a thymus peptide studied as an immune modulator, sold abroad as Zadaxin and not FDA-approved.

Medical review in progressLast reviewed

US regulatory status

Not approved for human use in the US

Thymosin alpha-1 (thymalfasin, sold abroad as Zadaxin) is not approved by the FDA. A 2009 review reported approvals in over 35 countries, mainly for hepatitis B and C. In the US it is on the FDA’s list of Category 2 nominations that were withdrawn, which cites a significant risk of immune reactions, and it sits in Category 3 (nominated without adequate support) for outsourcing facilities. It is not scheduled for advisory committee review.

As of

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On this page
  1. US regulatory status
  2. At a glance
  3. What the research shows
  4. Side effects and safety
  5. Vial math
  6. Questions people ask
  7. Sources
  8. Related compounds

At a glance

Also known as
Thymalfasin, Zadaxin
Supplied as
Freeze-dried powder, mixed before use
Research
Mixed evidence
US status
US status: Not approvedchecked

What the research shows

Evidence grade

Mixed evidence

Studies in people disagree, or strong evidence for one use sits beside weak evidence for another.

Thymosin alpha-1 has been tested in randomized human trials, and the findings disagree. A 2001 review reported it was evaluated in 195 people with hepatitis B across four trials: in one randomized controlled trial, hepatitis B virus DNA cleared in 40.6% of people given 6 months of treatment and 25.6% given 12 months, compared with 9.4% of untreated controls. In a hepatitis C trial, normal liver enzyme (ALT) levels were no more common than with placebo. The largest trial, TESTS (2016 to 2020, 22 centers in China), randomized 1,106 adults with sepsis to injections under the skin or placebo for up to 7 days: 28-day death rates were 23.4% and 24.1% (hazard ratio 0.99, 95% CI 0.77 to 1.27), and the authors found no clear evidence that it lowers deaths. In an exploratory subgroup, the hazard ratio was 1.67 (1.04 to 2.67) in people under 60 and 0.81 (0.61 to 1.09) in people 60 and over. A 49-patient open-label pilot in people hospitalized with COVID-19 found higher recovery with thymalfasin, but the differences were not statistically significant.

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How we grade evidence

  1. Strong human evidence

    Consistent results from large randomized controlled trials in people.

  2. Limited human evidence

    Some studies in people, but they are small, short, early-phase or uncontrolled.

  3. Mixed evidence · this page

    Studies in people disagree, or strong evidence for one use sits beside weak evidence for another.

  4. Animal or lab studies only

    Studied in cells or animals, not properly in people. Animal results may not apply to humans.

The grade describes how much research exists and in whom. It is not a judgment that a compound works or is safe.

Side effects and safety

A 2001 review describes thymosin alpha-1 as well tolerated, with most studies observing only local irritation at the injection site. In the 1,106-person TESTS sepsis trial, no safety outcome differed significantly between thymosin alpha-1 and placebo, and in the COVID-19 pilot, nine serious adverse events among treated patients were judged unrelated to it. The FDA’s concern is about compounded products: it says compounded thymosin alpha-1 “may pose significant risk for immunogenicity for certain routes of administration”, may have complexities with peptide-related impurities and characterization of the active ingredient, and that the safety information is “inadequate” for the agency to understand the extent of any safety issues.

Sources48910

Vial math

Pre-filled calculators turn your vial, water and prescribed dose into syringe units.

Calculator

These pages do the arithmetic for a vial you already have. They don’t suggest a dose.

This is arithmetic, not medical advice. Confirm with your prescriber or pharmacist.

Questions people ask

What is thymosin alpha-1?

A synthetic version of a peptide that comes from the thymus. A 2009 review calls it the first of the thymosins in clinical use. It is thought to modulate the immune system by strengthening T-cell function. Its generic name is thymalfasin, and outside the US it is sold under the trade name Zadaxin. It is given by injection; after a dose, blood levels peak within two hours.

Sources

Is thymosin alpha-1 FDA approved?

No. Drugs@FDA lists no approved thymalfasin product, although a 2009 review reported approvals in over 35 other countries. Early US studies ran under investigational new drug applications, first with an impure peptide mixture in people with primary immune deficiency such as DiGeorge syndrome, and later phase 3 trials of the synthetic peptide included the United States. For compounding, the FDA lists thymosin alpha-1 among Category 2 nominations that were withdrawn, and in Category 3, “nominated without adequate support”, for outsourcing facilities. It was not on the agenda of the July 2026 compounding advisory committee meeting or the one planned before the end of February 2027.

Sources

What is thymosin alpha-1 used for?

Where it is approved, mainly hepatitis B and C, and as an immune stimulant and adjuvant, according to a 2009 review. Researchers have also tested it in sepsis and COVID-19. The findings are mixed: one hepatitis B trial found more virus clearance than in untreated controls, a hepatitis C trial found no difference from placebo, and the 1,106-person TESTS trial found no clear effect on deaths from sepsis. In the US it has no approved use, so no official label tells you what it treats.

Sources

What are the side effects of thymosin alpha-1?

In published trials, mainly local irritation where it was injected. In the TESTS sepsis trial, safety outcomes were similar to placebo. The bigger unknown is product quality: the FDA warns that compounded thymosin alpha-1 may carry a significant risk of immune reactions for some routes and may contain peptide-related impurities. The contents of unapproved vials are not verified, so talk to a licensed prescriber or pharmacist before using one.

Sources

What is the half-life of thymosin alpha-1?

About 2 hours, according to a 2001 review. It is absorbed quickly, reaching peak blood levels within two hours, and blood levels return to baseline within 24 hours.

Source

Is thymosin alpha-1 the same as TB-500?

No. TB-500 is a seven-amino-acid fragment of a different protein, thymosin beta-4, and the FDA notes that even thymosin beta-4 and TB-500 are not the same substance. WADA’s 2026 list bans thymosin beta-4 and its derivatives, naming TB-500, at all times. See the TB-500 page for its own status and evidence.

Sources

Sources

  1. Drugs@FDA: FDA-Approved DrugsU.S. Food and Drug Administration · Accessed Oct 16, 2026 (opens in a new tab)
  2. From lab to bedside: emerging clinical applications of thymosin alpha 1Expert Opin Biol Ther, 2009 · Accessed Oct 16, 2026 (opens in a new tab)
  3. First Line Bio-immunotherapy With Thymosin Alpha 1 in Patients With Sensitizing EGFR Mutation, NCT02906163ClinicalTrials.gov, 2016 · Accessed Oct 16, 2026 (opens in a new tab)
  4. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety RisksU.S. Food and Drug Administration, 2026 · Accessed Oct 15, 2026 (opens in a new tab)
  5. Bulk Drug Substances Nominated for Use in Compounding Under Section 503B of the Federal Food, Drug, and Cosmetic ActU.S. Food and Drug Administration, 2025 · Accessed Oct 16, 2026 (opens in a new tab)
  6. July 23-24, 2026: Meeting of the Pharmacy Compounding Advisory CommitteeU.S. Food and Drug Administration, 2026 · Accessed Oct 15, 2026 (opens in a new tab)
  7. Meeting of the Pharmacy Compounding Advisory Committee (to be held before the end of February 2027)U.S. Food and Drug Administration, 2026 · Accessed Oct 15, 2026 (opens in a new tab)
  8. Thymosin alpha-1Am J Health Syst Pharm, 2001 · Accessed Oct 16, 2026 (opens in a new tab)
  9. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trialBMJ, 2025 · Accessed Oct 16, 2026 (opens in a new tab)
  10. A Pilot Trial of Thymalfasin (Thymosin-α-1) to Treat Hospitalized Patients With Hypoxemia and Lymphocytopenia Due to Coronavirus Disease 2019 InfectionJ Infect Dis, 2023 · Accessed Oct 16, 2026 (opens in a new tab)
  11. Thymosin alpha 1: past clinical experience and future promiseAnn N Y Acad Sci, 2010 · Accessed Oct 16, 2026 (opens in a new tab)
  12. FDA Briefing Document for TB-500-Related Bulk Drug Substances (PCAC, July 23-24, 2026)U.S. Food and Drug Administration, 2026 · Accessed Oct 15, 2026 (opens in a new tab)
  13. World Anti-Doping Code International Standard: Prohibited List 2026World Anti-Doping Agency, 2026 · Accessed Oct 16, 2026 (opens in a new tab)

Related compounds

Guides

Medical review in progressLast reviewed

Every medical and regulatory statement on this page links to a checkable source, listed above. Peptinote does arithmetic and education, not medical advice, and never sells or links to sellers of peptides.

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