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Thymosin Alpha-1: What Lakeland Readers Should Know

Also known as: Tα1, Thymalfasin, Zadaxin, Thymosin alpha 1, TA1

Not reviewed by a licensed provider. Read our disclaimer

Last updated

At a glance

Type
Immune-signaling peptide (28 amino acids)
Human evidence
Some human trials Studied in people, but not FDA-approved for the uses it is marketed for.
FDA status
Not FDA-approved; reported as approved in more than 35 other countries (Zadaxin) (as of November 13, 2026)
How it is given
Injection under the skin (as studied)

Thymosin alpha-1 is promoted in the U.S. as an “immune” peptide. Outside the U.S., a version of it is a licensed prescription drug. This page explains what the human trials found, why it is not FDA-approved, and what to ask. It is general information, not advice for your situation.

What it is and how it works

Thymosin alpha-1 is a peptide made of 28 amino acids. It was first isolated in 1977 from an extract of calf thymus, a gland in the chest where immune cells called T cells mature. Thymalfasin is the lab-made version, sold as Zadaxin by SciClone Pharmaceuticals.1

Reviews describe it as an immune modulator, meaning it adjusts how the immune system responds. They report that it acts on sensors on immune cells (toll-like receptors), helps T cells mature and raises some immune signaling chemicals, such as interferons.1 These descriptions come from reviews, not from one defining study, and the exact mechanism is not settled.

It is not the same as thymosin beta-4 or TB-500, which are different peptides.

What the research shows (and where evidence is weak)

Thymosin alpha-1 has more human data than most peptides on this site. But the largest, best-designed trials did not find clear benefits on their main goals.

Sepsis. Sepsis is a life-threatening reaction to infection. The TESTS trial, published in the BMJ in 2025, enrolled 1,106 adults at 22 hospitals in China. It was double-blind and placebo-controlled, meaning neither patients nor doctors knew who got the drug or a dummy treatment. Deaths within 28 days were 23.4% with thymosin alpha-1 and 24.1% with placebo, which is no meaningful difference.2 Earlier small trials, pooled together, had suggested fewer deaths. The TESTS authors noted those trials had design weaknesses and their positive results “might have been chance findings.”2 The trial was partly funded by the drug’s maker.2

Hepatitis B. In a U.S. phase III trial (a late-stage test) of 97 patients, 14% on thymosin alpha-1 had a complete response, compared with 4% on placebo. That difference was not statistically significant, meaning it could have been chance. The authors wrote that the results “do not confirm” earlier reports.3 A Japanese trial of 316 patients compared two strengths of the drug, with no placebo group, so it cannot show how the drug compares with no treatment.4

COVID-19. A 2023 analysis pooling 8 studies found lower death rates, but the results varied widely between studies, and the authors called for proper randomized trials.5 A small U.S. randomized trial of 49 hospitalized patients found no significant difference in recovery.6

Vaccine response. A 2007 review of small trials suggested thymosin alpha-1 might improve flu-vaccine responses in older adults. The reviewer said this deserves more study. It is not established.7

Cancer. A 2011 Cochrane review (a careful summary of all trials) looked at thymic peptides added to cancer treatment. For thymosin alpha-1 it found only a trend toward longer survival that was not statistically significant. Most trials had at least a moderate risk of bias.8

Gaps. Many positive studies are small, unblinded or from a single country. The maker funds much of the research. No trials in healthy people seeking an “immune boost” were found in the research behind this page.

FDA and regulatory status

Not FDA-approved. A November 2026 search of FDA’s Drugs@FDA database for thymalfasin and Zadaxin found no matches.9 Some published articles wrongly describe it as FDA-approved.

Approved abroad. Reviews report that thymalfasin is approved in more than 35 countries, mainly for hepatitis B and C.1 That count has not been checked against each country’s regulator.

Compounding. FDA had placed thymosin alpha-1 on its “Category 2” list of bulk ingredients that may pose significant safety risks in compounding, citing possible immune reactions, impurities and incomplete product characterization. By April 22, 2026, the nominations were withdrawn; FDA still lists its safety concerns.10 It does not appear on FDA’s compounding nominations list updated May 14, 2026.11 According to law-firm summaries, it is not among the six peptides in HHS’s October 2026 draft compounding enforcement policy.12 Ask any provider how their product is legally sourced.

Sport. Thymosin alpha-1 is not named on WADA’s 2026 Prohibited List.13 Because it is approved in other countries, WADA’s ban on unapproved substances may not apply. Athletes should check with their anti-doping organization.

Products sold online as “research peptides” are not made or checked as medicines. See our guide to research chemical peptide risks.

Common side effects and safety

  • In the large sepsis trial, side effects (66.4% vs 67.6%) and serious side effects (26.8% vs 29.3%) were about as common as with placebo.2
  • Injection-site reactions. Reviews describe local irritation, redness or discomfort where the shot is given as the most common effect.1
  • Who should avoid it. Reviews say it should not be used by people allergic to it, or by people whose immune system is deliberately suppressed, such as organ-transplant recipients, unless the benefits outweigh the risks.1
  • Compounded or unregulated products. FDA is concerned about immune reactions and impurities.10

If you have used thymosin alpha-1 and notice any symptom that worries you, tell a licensed provider.

Typical cost range

There is no reliable U.S. price data. The drug is not marketed in the U.S., so there is no U.S. list price, and no published cost analysis was found. Cost depends on where a product comes from and on clinic visit or lab fees. Prices vary between clinics and pharmacies. Ask any provider for a written price breakdown before you start.

Our Polk County cost guide covers what to ask about pricing.

Questions to ask a provider

  • Since thymosin alpha-1 is not FDA-approved, where does your product come from?
  • Is it made by a state-licensed pharmacy, and how is that possible given FDA’s compounding concerns?
  • What evidence supports using it for my situation, given that the large sepsis and hepatitis B trials were negative?
  • Do I take any medicines, or have any condition, that affect my immune system?
  • What side effects should I watch for, and how do I reach you?
  • How will we judge whether it is helping?
  • What is the total cost, and what does it include?

See also our guides on compounded vs FDA-approved peptides, how to vet a peptide clinic and your first peptide consultation.

Footnotes

  1. Dominari A, Hathaway D III, Pandav K, et al. Thymosin alpha 1: a comprehensive review of the literature. World J Virol. 2020;9(5):67-78. doi:10.5501/wjv.v9.i5.67. https://pmc.ncbi.nlm.nih.gov/articles/PMC7747025/ ↩ ↩2 ↩3 ↩4 ↩5

  2. Wu J, Pei F, Zhou L, et al. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. BMJ. 2025;388:e082583. doi:10.1136/bmj-2024-082583. https://pmc.ncbi.nlm.nih.gov/articles/PMC11780596/ ↩ ↩2 ↩3 ↩4

  3. Mutchnick MG, Lindsay KL, Schiff ER, et al. Thymosin alpha1 treatment of chronic hepatitis B: results of a phase III multicentre, randomized, double-blind and placebo-controlled study. J Viral Hepat. 1999;6(5):397-403. doi:10.1046/j.1365-2893.1999.00181.x. https://pubmed.ncbi.nlm.nih.gov/10607256/ ↩

  4. Iino S, Toyota J, Kumada H, et al. The efficacy and safety of thymosin alpha-1 in Japanese patients with chronic hepatitis B; results from a randomized clinical trial. J Viral Hepat. 2005;12(3):300-306. doi:10.1111/j.1365-2893.2005.00633.x. https://pubmed.ncbi.nlm.nih.gov/15850471/ ↩

  5. Soeroto AY, Suryadinata H, Yanto TA, et al. The efficacy of thymosin alpha-1 therapy in moderate to critical COVID-19 patients: a systematic review, meta-analysis, and meta-regression. Inflammopharmacology. 2023;31(6):3317-3325. doi:10.1007/s10787-023-01354-2. https://pubmed.ncbi.nlm.nih.gov/37845598/ ↩

  6. Shehadeh F, Benitez G, Mylona EK, et al. A pilot trial of thymalfasin (thymosin-α-1) to treat hospitalized patients with hypoxemia and lymphocytopenia due to coronavirus disease 2019 infection. J Infect Dis. 2023;227(2):226-235. doi:10.1093/infdis/jiac362. https://pubmed.ncbi.nlm.nih.gov/36056913/ ↩

  7. Ershler WB, Gravenstein S, Geloo ZS. Thymosin alpha 1 as an adjunct to influenza vaccination in the elderly: rationale and trial summaries. Ann N Y Acad Sci. 2007;1112:375-384. doi:10.1196/annals.1415.050. https://pubmed.ncbi.nlm.nih.gov/17600281/ ↩

  8. Wolf E, Milazzo S, Boehm K, et al. Thymic peptides for treatment of cancer patients. Cochrane Database Syst Rev. 2011;(2):CD003993. doi:10.1002/14651858.CD003993.pub3. https://pubmed.ncbi.nlm.nih.gov/21328265/ ↩

  9. U.S. Food and Drug Administration. openFDA Drugs@FDA API query: active ingredient THYMALFASIN; brand name ZADAXIN. Queried November 2026. https://api.fda.gov/drug/drugsfda.json?search=products.active_ingredients.name:THYMALFASIN ↩

  10. U.S. Food and Drug Administration. Certain bulk drug substances for use in compounding that may present significant safety risks. Content current as of April 22, 2026. Accessed November 2026. https://www.fda.gov/drugs/human-drug-compounding/certain-bulk-drug-substances-use-compounding-may-present-significant-safety-risks ↩ ↩2

  11. U.S. Food and Drug Administration. Bulk drug substances nominated for use in compounding under section 503A of the FD&C Act (categories 1-3). Updated May 14, 2026. Accessed November 2026. https://www.fda.gov/media/94155/download ↩

  12. Venable LLP. HHS announces interim enforcement discretion for section 503A compounding with certain peptide bulk drug substances. November 6, 2026. https://www.venable.com/insights/publications/2026/11/hhs-announces-interim-enforcement-discretion ↩

  13. World Anti-Doping Agency. The Prohibited List (2026 List). Accessed November 2026. https://www.wada-ama.org/en/prohibited-list ↩

Frequently asked questions

Is thymosin alpha-1 FDA-approved?

No. A search of FDA's drug approval database found no record for thymalfasin or Zadaxin. Some articles wrongly say it is FDA-approved. Reviews report it is approved in more than 35 other countries, mainly for hepatitis B and C.

Does thymosin alpha-1 boost the immune system?

It is described as an immune modulator, but the best trials did not show clear benefits. In a large 2025 sepsis trial, death rates were nearly the same as with placebo. No trials in healthy people looking for an immune boost were found.

Is thymosin alpha-1 the same as thymosin beta-4 or TB-500?

No. They are different peptides with different structures and research histories.

What are the common side effects?

Reviews describe irritation, redness or discomfort where the shot is given as the most common effect. In a large sepsis trial, side effects were about as common as with placebo.

Related goals

Related peptides

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