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October 16, 2026

Not FDA-Approved3/10 · Low

Thymosin Alpha-1: What the Science Really Says About the “Immune Peptide”

Thymosin alpha-1 is an immune peptide made naturally in the thymus, and it’s an approved medicine in dozens of countries, but not the U.S. Here’s what the research shows, including a large sepsis trial that changed the picture, what’s known about safety, and where it stands with the FDA.

Thymosin alpha-1: What the science really says about the "immune peptide"

Thymosin alpha-1 is a peptide your thymus gland makes naturally, and a lab-made version is an approved medicine in dozens of countries. That gives it a stronger track record than most peptides in our series. Here's what the research shows, including a large trial that didn't pan out, and why the FDA still hasn't approved it.

Peptide Advocate · Research explainer · Updated October 2026

What it isA 28-amino-acid peptide first isolated from the thymus. The drug version is called thymalfasin (brand name Zadaxin)2,3
Why people are excitedIt helps train and activate immune cells, and it's approved abroad for hepatitis B2,3
Human evidenceMore than most peptides, but the largest trial, in 1,106 sepsis patients, found no survival benefit4
Legal statusNot FDA-approved. FDA advisers voted 17–4 against allowing compounding in 20246

What is thymosin alpha-1?

Your thymus is a small gland behind the breastbone where T cells, a key type of immune cell, mature. In 1977, Allan Goldstein and colleagues isolated and sequenced thymosin alpha-1 from a thymus extract called thymosin fraction 5. It's a chain of 28 amino acids, much larger than most peptides we've covered.2

How thymosin alpha-1's size compares with other peptides Number of amino acids: Epitalon 4, Selank 7, Semax 7, BPC-157 15, thymosin alpha-1 28, tesamorelin 44. A bigger peptide than most Number of amino acids in peptides we've covered Epitalon 4 Selank 7 Semax 7 BPC-157 15 Thymosin alpha-1 28 Tesamorelin 44 Thymosin alpha-1 is made naturally in the thymus, a small gland behind the breastbone.
Amino-acid counts from each peptide's published structure; thymosin alpha-1 per Costantini et al., 2019.2 Illustration: Peptide Advocate.

Researchers later developed a lab-made version, thymalfasin, sold as Zadaxin. China approved it in 1995 for hepatitis B, and by 2009 it was approved in 36 countries. In parts of Asia it's also used for hepatitis C and liver cancer, and in Italy alongside flu and hepatitis B vaccines.3

Why researchers are excited

Thymosin alpha-1 is an immune "modulator," meaning it helps tune the immune response rather than simply boosting it. Research suggests it:2,7

  • Supports T cells. It helps T cells mature and function, and increases signals such as interleukin-2 and interferon-gamma that rally the immune system.
  • Activates early-warning cells. It acts on dendritic cells, which spot invaders and alert the rest of the immune system, partly through sensors called Toll-like receptors.
  • Boosts natural killer cells, which attack virus-infected and cancer cells.

It also has a long safety track record. A review of its use in infectious diseases described it as "virtually devoid of toxicity" after use in thousands of patients.5

What about people?

Unlike most peptides in our series, thymosin alpha-1 has been tested in many human trials. The results are a mix of encouraging and disappointing:

ConditionStudyWhat happenedBig caveat
Hepatitis BRandomized trial, 98 patients40.6% cleared the virus after a 26-week course vs. 9.4% of controls7A longer 52-week course wasn't significantly better than control
Sepsis
Wu et al., 2013		361 patients, single-blind26.0% died within 28 days vs. 35.0% with standard care8Just missed statistical significance; no placebo
Sepsis
TESTS trial, 2025		1,106 patients, double-blind, placebo-controlled23.4% died vs. 24.1% on placebo4No clear benefit in the most rigorous trial
COVID-19
Review, 2023		8 studies pooledLinked to lower death rates in moderate to critical COVID-199Mostly observational studies with very mixed results
Cancer
Review, 2019		Melanoma, lung and liver cancer studiesBetter response rates in some trials when added to standard treatment2Mostly small or early-stage studies

The sepsis story is a good lesson in how science works. A smaller 2013 trial hinted that thymosin alpha-1 might save lives. A much larger, more rigorous trial published in The BMJ in 2025 found no clear difference in survival. The authors noted possible differences in some subgroups, such as older patients and people with diabetes, but said those need confirmation.4,8

Death rates in two sepsis trials of thymosin alpha-1 In a 2013 single-blind trial of 361 patients, 28-day death rates were 26.0% with thymosin alpha-1 and 35.0% with standard care, a difference that fell just short of statistical significance. In a larger 2025 double-blind, placebo-controlled trial of 1,106 patients, death rates were 23.4% with thymosin alpha-1 and 24.1% with placebo, no meaningful difference. Sepsis: a promising signal that didn't hold up Share of patients who died within 28 days Thymosin alpha-1 Standard care or placebo 26.0% 35.0% 2013 trial (ETASS) 361 patients, single-blind Difference just missed statistical significance 23.4% 24.1% 2025 trial (TESTS) 1,106 patients, double-blind, placebo No meaningful difference
Sources: Wu et al., Critical Care, 2013; TESTS study group, BMJ, 2025.8,4
28amino acids in thymosin alpha-12
36countries where it was approved as of 20093
1,106sepsis patients in the largest trial, which found no survival benefit4
17–4FDA advisory vote against allowing compounding, December 20246

Is thymosin alpha-1 safe?

As peptides go, thymosin alpha-1 has a reassuring safety record from decades of use as an approved drug abroad.5 Still, there are things to know:

  • Usually well tolerated. Reactions at the injection site are the most common side effect.10 In the large sepsis trial, no safety outcome differed meaningfully from placebo.4
  • Immune conditions. Because it acts on the immune system, some non-U.S. labels carry warnings for people with autoimmune diseases or those taking immune-suppressing drugs, such as organ transplant recipients.10
  • FDA concerns. The FDA says compounded versions may trigger unwanted immune reactions and that the available safety information is inadequate for it to understand the risks.1
  • Product quality. Thymosin alpha-1 sold online in the U.S. is typically "research use only" and isn't made to drug standards.7

Athletes: Thymosin alpha-1 isn't named on the World Anti-Doping Agency's prohibited list, but anti-doping rules on unapproved and immune-related substances are complex. Check with your anti-doping authority before using it.

Where thymosin alpha-1 stands with the FDA

Thymosin alpha-1 has never been approved in the U.S. American trials for hepatitis B began in the 1990s, but the results were too limited to win approval.3 Its compounding status has also shifted. Unlike BPC-157 and TB-500, it already had its FDA advisory review in 2024, and the committee voted against it.6

How thymosin alpha-1's status has changed

  1. 1977Isolated and sequenced from the thymus2
  2. 1995China approves it as Zadaxin for hepatitis B; dozens of countries follow3
  3. 2023FDA places it on its "Category 2" list, effectively halting compounding11
  4. 2024Removed from Category 2 after its nomination is withdrawn; FDA lists it as "nominated but withdrawn"1,12
  5. December 4, 2024FDA advisory committee votes 17–4 against adding it to the list of ingredients pharmacies may compound, citing a lack of convincing evidence. FDA staff also recommended against it6
  6. July 2026Not on the agenda when FDA advisers review seven other peptides13
  7. October 15, 2026HHS sends an "interim access policy" for certain peptides to the White House. Which peptides it covers hasn't been made public14
  8. NextNot currently eligible for U.S. compounding; no public timeline for further review7

Why the mixed signals? You may see claims online that thymosin alpha-1 is "back" for compounding. As of September 2026, independent trackers report it isn't eligible, and its 2024 advisory vote was negative.7,11 Check with a licensed pharmacist. We'll update this article as the rules change.

If you're curious about thymosin alpha-1

  • For hepatitis B or C, see a liver specialist. FDA-approved antivirals are highly effective, and hepatitis C is usually curable.
  • Sepsis is a medical emergency. Fever, confusion, a racing heart or fast breathing during an infection needs immediate care.
  • Stay up to date on vaccines. They remain the best-proven way to train your immune system.
  • "Boosting" immunity isn't always good. If you have an autoimmune condition or take immune-suppressing drugs, talk to your doctor first.
  • Skip "research use only" products, and tell your doctor about any peptides or supplements you take.

The bottom line

Thymosin alpha-1 is one of the most legitimate peptides in our series. It's a natural immune signal, an approved medicine in dozens of countries, and it has an encouraging safety record. Its hepatitis B results are promising, and its cancer and COVID-19 research is intriguing. But the biggest, most rigorous trial so far, in sepsis, found no survival benefit, and FDA advisers weren't convinced by the evidence for compounding. Thymosin alpha-1 shows that a peptide can be real medicine somewhere and still need better trials to prove what it does best.

References

  1. 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. fda.gov
  2. Costantini C, Bellet MM, Pariano M, et al. A reappraisal of thymosin alpha1 in cancer therapy. Front Oncol. 2019;9:873. frontiersin.org
  3. AUTM Better World Project. Therapeutic developed in United States benefits many in Asia (Zadaxin); 2009. autm.net
  4. TESTS Study Collaborator Group. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. BMJ. 2025;388:e082583. doi.org
  5. Historical review of thymosin α1 in infectious diseases. Expert Opin Biol Ther. 2015;15 Suppl 1:S117–27. pubmed.ncbi.nlm.nih.gov
  6. U.S. Food and Drug Administration. Pharmacy Compounding Advisory Committee, December 4, 2024: summary minutes. fda.gov
  7. Superpower. Thymosin alpha-1: immune peptide uses and evidence; updated October 5, 2026. superpower.com
  8. Wu J, Zhou L, Liu J, et al. The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial. Crit Care. 2013;17(1):R8. pmc.ncbi.nlm.nih.gov
  9. 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;31:3317–3325. springer.com
  10. JumpstartMD. Thymosin alpha-1: evidence and FDA status; updated October 2026. jumpstartmd.com
  11. Meto. PCAC peptide review 2026: what the July advisory panel decision means for BPC-157, TB-500, and thymosin alpha-1; 2026. meto.co
  12. Rethink Peptides. Thymosin alpha-1 reclassification; updated March 25, 2026. rethinkpeptides.com
  13. U.S. Food and Drug Administration. July 23–24, 2026 meeting of the Pharmacy Compounding Advisory Committee. fda.gov
  14. Associated Press. Trump administration advances plan for peptide drugs favored by RFK Jr. ABC News; October 15, 2026. abcnews.com

This article is for general education and is not medical advice. Thymosin alpha-1 is not approved by the FDA to diagnose, treat, cure or prevent any condition, and this article does not recommend using it or provide dosing information. Talk with a licensed healthcare provider before starting any new treatment. Regulatory information is current as of October 16, 2026 and is changing quickly. Charts and illustrations are original works by Peptide Advocate.

Peptides Referenced

Thymosin Alpha-1

Immune Peptide

Supports immune function, T-cell response, and modulates inflammation for overall immune resilience.