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Peptide (approved abroad, not in the US)Human trialsNot FDA approved

Thymosin Alpha-1

Thymosin alpha 1 (thymalfasin)

Last reviewed October 27, 2026 · 22 sources · Written for general readers · Compare with another compound →

The bottom line

Thymosin alpha 1 is an approved drug in more than 35 countries, mainly for hepatitis B and C, but FDA has never approved it.[1][2] When it was finally tested in large, well run trials, it did not help people with sepsis survive and did not prevent infections in severe pancreatitis.[3][4] Vials sold online are not the approved medicine, and FDA has raised concerns about purity and immune reactions.[5]

Where the evidence stands

What it is

Thymosin alpha 1 is a small protein chain of 28 amino acids. It was first found in the thymus, the gland in the chest where immune T cells mature. Researchers first obtained it from calf thymus extracts in 1977, and the drug version, called thymalfasin, is made in a lab.[1]

Thymalfasin is sold under the brand name Zadaxin and is approved in more than 35 countries for chronic hepatitis B and C and as an immune booster.[1] FDA says no thymosin alpha 1 drug is approved in the United States. It holds US orphan drug designations, which encourage research on rare diseases but are not approvals.[2]

Online, it is promoted as a general immune, recovery or anti infection peptide. FDA has said that compounded versions may carry impurity and immune reaction risks, and that the safety information is not enough to judge how big those risks are.[5]

Name
Thymosin Alpha-1 (Thymosin alpha 1 (thymalfasin))
Other names
thymalfasin, Zadaxin, Ta1, Tα1
Type
Peptide (approved abroad, not in the US)

What it does in the body

Thymosin alpha 1 is a natural 28 amino acid peptide first found in the thymus, the gland in the chest where immune T cells mature.[1] It is thought to act as an immune tuner: it supports the maturation of lymphocytes and strengthens the Th1 response, the arm of immunity that targets viruses and infected cells.[6]

This is the biology, area by area. A change measured in the body, like a higher blood level, does not always mean a person feels a benefit. The claims check below covers that.

Immune system

Measured in people

In pooled trials of 706 people with severe pancreatitis, those given thymosin alpha 1 had higher levels of CD4 helper T cells and a higher ratio of CD4 to CD8 T cells.[7] In 20 small trials of 1,704 people with stomach cancer, adding it to chemotherapy raised the share of CD3 and CD4 T cells and natural killer cells in the blood.[8] In a look back study of 76 people with severe COVID-19, those given it had higher T cell counts and fewer signs of T cell exhaustion, meaning worn out immune cells.[9] These are measured immune changes in people, but they did not always lead to fewer infections or deaths.[3][4]

Fighting infection

Measured in people

In 1,106 adults with sepsis, deaths at 28 days were the same with or without it, 23.4 versus 24.1 percent.[3] In 508 people with severe pancreatitis, infection of the damaged pancreas was not reduced.[4] In a smaller open label trial of 120 people with sudden liver failure on top of hepatitis B, new infections were 32 versus 59 percent.[10]

Liver

Measured in people

A 2026 Cochrane review of 10 trials with 1,349 adults with chronic hepatitis B found fewer deaths with it, a risk ratio of 0.53, but rated the evidence very low certainty, so the authors could not be sure of any effect.[11] In the liver failure trial, 75 percent of treated patients were alive without a transplant at 90 days versus 53 percent with usual care.[10]

Cancer

Measured in people

Added to chemotherapy, it was linked to more tumor shrinkage in advanced melanoma and stomach cancer trials, but survival gains were not statistically clear in the largest melanoma trial, and the stomach cancer trials were small and of limited quality.[12][8]

Absorption and breakdown

Measured in people

After a shot under the skin it is absorbed quickly, lasts about two hours in the blood, and does not build up with repeated use.[2]

How it works

Thymosin alpha 1 is thought to act as an immune tuner rather than a germ killer.

  • It supports the maturation of lymphocytes, the white blood cells that fight infection.[6]
  • It strengthens the Th1 response, the part of immunity that targets viruses and infected cells.[6]
  • It improves T cell function and may help correct some immune defects.[6]
  • After a shot under the skin it is absorbed quickly, lasts about 2 hours in the blood, and does not build up with repeated use.[2]

These effects come mostly from lab work and immune marker studies, which do not prove people get healthier.

What the research shows

We keep animal and human evidence apart. A result in animals is a reason to study something in people, not proof that it works in people.

Animal and lab studies

Lab work explains the idea, but people have been studied directly

  • Its immune effects, such as stronger T cell responses and support for lymphocyte maturation, come from lab and immune marker studies and are what led to human testing.[6][1]

Limit: For this compound, human trials exist and matter far more than animal or lab results.

Human studies

Approved abroad, but the biggest recent trials were negative

  • Sepsis, TESTS (2025): 1,106 adults with sepsis got shots of thymosin alpha 1 or placebo. At 28 days, 23.4 percent versus 24.1 percent had died, no meaningful difference.[3]
  • Sepsis review (2025): pooling 11 smaller randomized trials showed fewer deaths overall, but not in the better quality and multicenter trials.[13]
  • Severe pancreatitis (2022): 508 patients, most in intensive care, got shots under the skin for two weeks or placebo. Infection of the damaged pancreas occurred in 15.7 percent versus 18.1 percent, not a real difference. New organ failure was 10.6 versus 15 percent and bleeding 6.3 versus 3.5 percent, with no significant differences reported.[4]
  • Hepatitis B liver failure (2022): 120 people with sudden liver failure on top of chronic hepatitis B were randomly given shots under the skin for 12 weeks plus usual care, or usual care alone, without blinding. At 90 days, 75 versus 53 percent were alive without a transplant, and new infections were 32 versus 59 percent. A second type of analysis did not confirm the survival difference.[10]
  • Pancreatitis review (2025): five randomized trials with 706 patients showed fewer infections outside the pancreas and higher CD4 T cells, but no shorter hospital stay.[7]
  • Melanoma (2010): 488 people with advanced melanoma got chemotherapy with or without thymosin alpha 1, with some groups also getting interferon. More tumors shrank with the peptide, and median survival was 9.4 versus 6.6 months, but this was not statistically significant. Adding it caused no extra side effects.[12]
  • Cancer review (2011): a Cochrane review of 26 trials of thymus peptides found no clear benefit for extracts, and only a trend toward longer survival for synthetic peptides like thymosin alpha 1.[15]
  • COVID-19 review (2023): 9 studies with 5,352 hospitalized adults found no effect on deaths overall.[17]
  • Dialysis pilot (2023): 194 people on dialysis were randomly given shots for eight weeks or no treatment. Early results showed 3 versus 7 deaths and 5 versus 7 serious COVID-19 events, without formal statistics.[14]
  • Severe COVID-19, Wuhan (2020): a look back at 76 severe cases found deaths of 11 percent in those given thymosin alpha 1 versus 30 percent in those not given it, with recovery of T cell counts. Patients were not randomly assigned, and a later pooled analysis of 5,352 patients found no effect on deaths.[9][17]
  • Hepatitis B review (2026): a Cochrane review of 10 trials with 1,349 adults, published from 1991 to 2018, found fewer deaths and fewer serious side effects, but rated the evidence low to very low certainty. Quality of life did not differ in the one trial that measured it.[11]
  • Stomach cancer review (2026): 20 small trials from China with 1,704 patients found more tumor responses, higher immune cell counts and fewer chemotherapy side effects, such as gut upset and low blood counts, when thymosin alpha 1 was added. The certainty of the evidence ranged from very low to moderate, and the trials were of limited quality.[8]
  • Hepatitis: it has long been used for chronic hepatitis B and C, but newer antiviral drugs have largely replaced it.[1]
  • Side effects: it is generally well tolerated, and the most common problem is irritation or redness where it is injected.[1]

Limit: Many positive results came from small trials, while the largest, best designed trials found no benefit in sepsis or pancreatitis.

Claims check

The most common claims about this compound, and what studies actually found. Verdicts rate the evidence, not whether you should use it.

Fights sepsis and severe infections

No benefit in trials so far

In the TESTS trial, 1,106 adults with sepsis got thymosin alpha 1 or placebo shots. After 28 days, 23.4 percent had died in the drug group and 24.1 percent in the placebo group, no real difference.[3] Pooling 11 sepsis trials with 1,927 patients suggested fewer deaths at 28 days, but the effect was no longer significant when only the higher quality or multicenter trials were counted.[13]

Boosts the immune system and prevents infections

Mixed results

In 508 people with severe pancreatitis, two weeks of shots under the skin did not reduce infection of the damaged pancreas, 15.7 percent versus 18.1 percent with placebo.[4] A 2025 pooled analysis of five pancreatitis trials with 706 patients reported fewer infections outside the pancreas, a risk ratio of 0.56, and higher CD4 T cell levels, but it mixed small older trials with the large negative one.[7] In 120 people with hepatitis B liver failure, new infections were 32 versus 59 percent in an open label trial.[10] In a pilot study of 194 dialysis patients during COVID-19, 3 people died in the treated group versus 7 in an untreated group, with no placebo.[14]

Treats hepatitis B and C

Mixed results

It is approved for chronic hepatitis B and C in many countries, based on trials done mostly in the 1990s and 2000s.[1] A 2026 Cochrane review of 10 trials with 1,349 adults with hepatitis B found fewer deaths and fewer serious side effects with it, but the evidence was low or very low certainty and the authors said they are not sure it works.[11] FDA reviewers noted flares of liver enzymes in hepatitis B and changes in thyroid tests in hepatitis C trials, and newer antiviral drugs have largely replaced it.[2][1]

Helps fight cancer

Mixed results

In a trial of 488 people with advanced melanoma, adding it to chemotherapy produced more tumor responses (10 and 12 versus 4) and a median survival of 9.4 versus 6.6 months, but the survival difference was not statistically significant.[12] A Cochrane review of six trials of synthetic thymic peptides found only a trend toward longer survival.[15] A 2026 review found 26 mostly low quality reports and said controlled trials are needed.[16] A 2026 pooled analysis of 20 small Chinese trials in stomach cancer found more tumor responses with it added to chemotherapy, a risk ratio of 1.39, and fewer gut side effects, but rated the certainty of the evidence from very low to moderate and found little data on long term survival.[8]

Treats COVID-19

No benefit in trials so far

An early look back study of 76 people with severe COVID-19 in Wuhan reported 11 percent deaths with thymosin alpha 1 versus 30 percent without, along with recovery of T cell counts, but patients were not randomized.[9] A 2023 review pooled 9 studies with 5,352 adults in hospital with COVID-19, 1,152 of whom got thymosin alpha 1. Overall it did not lower deaths, with a risk ratio of 1.03, and the authors concluded the evidence does not support its use.[17] Most of the pooled studies were not randomized.

Known risks and side effects

  • Not approved in the US. FDA has not approved any thymosin alpha 1 drug, and FDA describes its human safety database as limited.[2]
  • Immune reactions to the product. FDA says compounded thymosin alpha 1 may trigger unwanted immune responses for some routes of use, partly because of impurities in the peptide.[5]
  • Reported side effects. Injection site redness and irritation are the most common complaints.[1] Trials also reported liver enzyme flares in hepatitis B and thyroid test changes in hepatitis C, and one transplant patient died of an immune attack on red blood cells, though it is unclear whether the drug caused it.[2] In pooled hepatitis B trials, serious side effects were not more common than in comparison groups.[11]
  • Unregulated online products. Research peptide vials are not Zadaxin. FDA found the raw ingredient was not fully tested for impurities, clumping and bacterial toxins, so the quality of any vial is uncertain.[2]
  • Care with suppressed immunity. A review advises caution in people whose immune system is deliberately suppressed, such as transplant patients.[1]

Regulatory status

Today: Thymosin alpha 1 is not approved in the US.[2] FDA lists it among peptides that were previously in category 2 and whose nominations were withdrawn, and it is not in category 1, the group FDA generally lets pharmacies compound from while under review.[5][18][19] It is not named on WADA's 2026 Prohibited List or on the 2027 list that takes effect January 1, 2027.[20][21]

  1. FDA staff reviewed thymosin alpha 1 for compounding and concluded the evidence weighed against adding it to the 503A bulks list.[2]
  2. FDA's Pharmacy Compounding Advisory Committee met to discuss thymosin alpha 1 along with AOD-9604 and CJC-1295.[22]
  3. The TESTS sepsis trial was published in the BMJ and found no survival benefit.[3]
  4. FDA's safety risk page listed thymosin alpha 1 among peptides previously in category 2 whose nominations were withdrawn.[5]

Sources

  1. Dominari A, Hathaway D 3rd, Pandav K, et al. Thymosin alpha 1: A comprehensive review of the literature. World J Virol. 2020;9(5):67-78. PMID 33362999
  2. US Food and Drug Administration. FDA Briefing Document: Thymosin alpha-1 free base and thymosin alpha-1 acetate, Pharmacy Compounding Advisory Committee meeting of December 4, 2024 (evaluation dated November 15, 2024). FDA briefing document
  3. 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. PMID 39814420
  4. Ke L, Zhou J, Mao W, et al. Immune enhancement in patients with predicted severe acute necrotising pancreatitis: a multicentre double-blind randomised controlled trial. Intensive Care Med. 2022;48(7):899-909. PMID 35713670
  5. 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 safety risks page
  6. Liaw YF. Thymalfasin (thymosin-alpha 1) therapy in patients with chronic hepatitis B. J Gastroenterol Hepatol. 2004;19(S6). DOI 10.1111/j.1440-1746.2004.03633.x
  7. Tian Y, Yao J, Ma Y, et al. Thymosin alpha 1 alleviates inflammation and prevents infection in patients with severe acute pancreatitis through immune regulation: a systematic review and meta-analysis. Front Immunol. 2025;16:1571456. PMID 40599771
  8. Wang Z, Sun T, Qu H, et al. Efficacy and safety of thymosin alpha1 combined with chemotherapy for gastric cancer: a systematic review and meta-analysis of randomized controlled trials. Front Immunol. 2026;17:1884797. PMID 42827862
  9. Liu Y, Pan Y, Hu Z, et al. Thymosin Alpha 1 Reduces the Mortality of Severe Coronavirus Disease 2019 by Restoration of Lymphocytopenia and Reversion of Exhausted T Cells. Clin Infect Dis. 2020;71(16):2150-2157. PMID 32442287
  10. Chen JF, Chen SR, Lei ZY, et al. Safety and efficacy of Thymosin alpha1 in the treatment of hepatitis B virus-related acute-on-chronic liver failure: a randomized controlled trial. Hepatol Int. 2022;16(4):775-788. PMID 35616850
  11. Naing C, Ni H, Aung HH, et al. Thymosin-alpha1 for people with chronic hepatitis B. Cochrane Database Syst Rev. 2026;9(9):CD014610. PMID 42713852
  12. Maio M, Mackiewicz A, Testori A, et al. Large randomized study of thymosin alpha 1, interferon alfa, or both in combination with dacarbazine in patients with metastatic melanoma. J Clin Oncol. 2010;28(10):1780-1787. PMID 20194853
  13. Gu B, Zhou Y, Nie Y, Wang L, et al. Efficacy of thymosin α1 for sepsis: a systematic review and meta-analysis of randomized controlled trials. Front Cell Infect Microbiol. 2025;15:1673959. PMID 40969554
  14. Tuthill CW, Awad A, Parrigon M, Ershler WB. A pilot trial of Thymalfasin (Ta1) to prevent covid-19 infection and morbidities in renal dialysis patients: Preliminary report. Int Immunopharmacol. 2023;117:109950. PMID 36881981
  15. Wolf E, Milazzo S, Boehm K, Zwahlen M, Horneber M. Thymic peptides for treatment of cancer patients. Cochrane Database Syst Rev. 2011;(2):CD003993. PMID 21328265
  16. Kim SD, Kim KR, Kim DH, et al. Clinical Applications and Evidence Landscape of Thymosin Alpha 1 as an Immunomodulatory Adjunct in Cancer Care: A Scoping Review. Pharmaceuticals. 2026;19(9):1492. DOI 10.3390/ph19091492
  17. Shang W, Zhang B, Ren Y, et al. Thymosin alpha1 use in adult COVID-19 patients: A systematic review and meta-analysis on clinical outcomes. Int Immunopharmacol. 2023;114:109584. PMID 36527881
  18. U.S. Food and Drug Administration. Bulk Drug Substances Nominated for Use in Compounding Under Section 503A of the FD&C Act (category lists). Updated May 14, 2026. FDA 503A category lists
  19. US Food and Drug Administration. Bulk Drug Substances Used in Compounding Under Section 503A of the FD&C Act. Content current as of May 14, 2026. FDA 503A page
  20. World Anti-Doping Agency. World Anti-Doping Code International Standard: Prohibited List 2026. Effective 1 January 2026. WADA 2026 List
  21. World Anti-Doping Agency. International Standard: The 2027 Prohibited List. Comes into effect 1 January 2027. WADA 2027 List
  22. US Food and Drug Administration. December 4, 2024 Meeting of the Pharmacy Compounding Advisory Committee. FDA advisory committee page
Medical disclaimer

This page is educational information only. It is not medical advice and does not replace care from a licensed medical provider. Do not use this page to decide whether to take any substance. Talk to a licensed medical provider first.

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