Thymosin alpha-1 (thymalfasin, Zadaxin): research, side effects and dosage
In short
Thymosin alpha-1 is a 28-amino-acid peptide isolated from the thymus that has been used as a medicine in some countries, but in the largest placebo-controlled trial it did not reduce deaths from sepsis [1] [5]A. In the EU it has a national marketing authorisation only in Italy, not in Finland [16]regulator.
- As a synthetic medicine the substance is called thymalfasin, with the brand name Zadaxin [3] [4]A.
- In a trial of 1,106 sepsis patients, 28-day mortality was 23.4% with thymosin alpha-1 and 24.1% with placebo, and the difference was not significant [5]A.
- In hepatitis B the certainty of the evidence is low [7]A, and in COVID-19 the results conflict [9, 10]A.
- Thymosin alpha-1 is not the same substance as TB-500 or thymosin beta-4 [12]regulator.
- Even though the Finnish medicines list does not name it, it can still be a medicine under the law [14]regulator.
What is thymosin alpha-1?
Thymosin alpha-1 is a 28-amino-acid peptide. It was the first peptide isolated from thymic tissue, and it acts on both innate and adaptive immunity [1].
The synthetic thymosin alpha-1 used as a medicine is called thymalfasin [4]A. In the research literature it also appears under the brand name Zadaxin [3].
Despite the names, thymosin alpha-1 and thymosin beta-4 are different substances. According to the FDA, TB-500 is a short piece of thymosin beta-4, and the FDA assessed thymosin alpha-1 and TB-500 as separate substances [12]regulator. All substances: Peptides A-Z.
Another peptide on this site that acts on the immune system is LL-37, the human body’s own antimicrobial peptide.
Evidence at a glance
Each cell gives the kind of study and its source. The letter is the evidence level, not the result.
| Claimed effect | Humans | Animals | Cells |
|---|---|---|---|
| Sepsis | 1,106 patients: no difference in mortality [5]A; in a meta-analysis no benefit in the highest-quality trials [6]A | - | - |
| Hepatitis B | Cochrane review, 10 trials: certainty of evidence low or very low [7]A | - | - |
| Hepatitis C | 552 patients: did not improve viral clearance [8]A | - | - |
| COVID-19 | Meta-analyses conflict [9, 10]A; no difference in recovery in a small open-label randomised trial [4]A | - | - |
| Melanoma | 488 patients: more tumour responses, survival difference not significant [11]A | - | - |
| Safety | In the sepsis trial adverse events as common as with placebo [5]A | - | - |
"-" = no such study is known.
What is thymosin alpha-1 used for?
According to a 2009 review, thymosin alpha-1 was approved in over 35 countries for hepatitis B and C, and as an immune stimulant and adjuvant [2]. The review does not list the countries. In the EU a thymalfasin product (Zadaxin) was authorised only in Italy (data as of 21 Sep 2026) [16]regulator.
- Studied uses: sepsis [5, 6]A, hepatitis B [7]A and C [8]A, COVID-19 [9, 10]A and as an add-on to cancer treatment [11]A.
- Flu vaccine adjuvant: according to a review, Zadaxin has been studied as a co-adjuvant in influenza vaccines [3].
Claims online. The claim that it strengthens immunity in serious illness is not supported by the largest trial: deaths among sepsis patients did not fall [5]A.
How does thymosin alpha-1 work?
Thymosin alpha-1 acts on both innate and adaptive immunity [1]. According to a review, it has been used as an immune stimulant [2].
The effect on the immune system has not shown up as a clear treatment benefit in large trials [5, 8, 9]A.
Has thymosin alpha-1 been studied in humans?
Yes, thymosin alpha-1 has been studied a lot, including in large placebo-controlled trials. No clear benefit has been shown in sepsis, hepatitis C or COVID-19 [5, 8, 9]A.
Sepsis
- TESTS (2025). At 22 hospitals in China, 1,106 adults with sepsis received thymosin alpha-1 or placebo for seven days. 28-day mortality was 23.4% and 24.1%, and the difference was not significant [5]A.
- Meta-analysis (2025). The pooled result of 11 randomised trials pointed to lower mortality, but the subgroups of high-quality and multicentre trials showed no benefit, and according to the authors the number of patients is not yet sufficient [6]A.
Hepatitis, COVID-19 and cancer
- Hepatitis B. A Cochrane review included 10 randomised trials with 1,349 participants. The authors could not say whether thymosin alpha-1 reduces deaths or serious harms, because the certainty of the evidence was low or very low [7]A.
- Hepatitis C. In a trial of 552 patients, adding thymosin alpha-1 to peginterferon and ribavirin did not improve sustained viral clearance (12.7% vs 10.5%, difference not significant) [8]A.
- COVID-19. In a meta-analysis of nine studies and 5,352 patients, thymosin alpha-1 had no significant effect on mortality, and according to the authors the results do not support its use [9]A. Another meta-analysis found lower mortality, but results varied widely [10]A. In a small open-label randomised trial of 49 patients there was no significant difference in recovery [4]A.
- Melanoma. In a trial of 488 patients, adding thymosin alpha-1 to chemotherapy increased tumour responses in two groups. Median overall survival was 9.4 months versus 6.6 months, but the difference was not statistically significant (p = 0.08) [11]A.
Who studies thymosin alpha-1?
The largest sepsis trial of thymosin alpha-1 was done at 22 hospitals in China and was partly funded by the drug company SciClone Pharmaceuticals [5]A. The melanoma trial was a multicentre study [11]A, and the hepatitis B trials have been brought together in a Cochrane review [7]A.
Who wrote this page: the editorial team
This page was written by the Molecular Recovery editorial team. The draft was made with the help of AI, and the claims were checked against the sources (AI policy). The page has not been medically reviewed. How we work: About us.
What are the side effects of thymosin alpha-1?
In placebo-controlled trials thymosin alpha-1 did not increase harms compared with placebo [5, 8]A.
Harms in trials
- In the TESTS trial, adverse events occurred in 66.4% of those given thymosin alpha-1 and 67.6% of those given placebo, and serious adverse events in 26.8% and 29.3%. No safety outcome differed significantly between the groups [5]A.
- In the hepatitis C trial there was no significant difference in adverse events between groups [8]A, and in the melanoma trial thymosin alpha-1 did not add toxicity [11]A.
- According to the hepatitis B review, the evidence on serious harms is of low certainty [7]A.
Compounded and online products
These results concern products used in clinical trials. According to the FDA, compounded products containing thymosin alpha-1 may pose a significant risk of immune reactions by certain routes, and the safety information is not enough to judge the risks [12]regulator. More: Side effects and safety.
Interactions and who lacks data
In trials thymosin alpha-1 was given on top of other treatments, for example with peginterferon and ribavirin [8]A or chemotherapy [11]A, and it did not add harms. We know of no research data on interactions with other medicines or on use during pregnancy or breastfeeding. Ask a doctor about interactions.
What speaks against it and what is unknown
The largest and highest-quality trials have not shown a clear benefit [5, 6, 8]A.
Contrary evidence
- In sepsis, 28-day mortality did not differ from placebo [5]A, and the highest-quality trials in the meta-analysis showed no benefit [6]A.
- In hepatitis C viral clearance did not improve [8]A.
- In COVID-19 one meta-analysis found no effect on mortality [9]A.
- In melanoma the survival difference was not statistically significant [11]A.
What the evidence does not support
- "Thymosin alpha-1 is the same substance as TB-500": it is not. Thymosin alpha-1 is a 28-amino-acid peptide [1], whereas TB-500 is, according to the FDA, a short piece of thymosin beta-4 [12]regulator.
- "Thymosin alpha-1 reduces deaths from sepsis": not in the largest trial [5]A.
- "Thymosin alpha-1 is an approved medicine in Finland": it is not. Zadaxin was authorised only in Italy, and an EU orphan designation is not a marketing authorisation [13, 16]regulator.
- "Thymosin alpha-1 is a proven treatment for COVID-19": it is not. One meta-analysis concluded that the results do not support its use [9]A, and the authors of the other said randomised trials were still needed [10]A.
It is not known whether some subgroup of sepsis patients benefits: in the large trial the effect may have differed by age and diabetes, but this is only a hint [5]A. In hepatitis B the certainty of the evidence is very low for almost all outcomes [7]A.
Thymosin alpha-1 dosage in studies
In Finland thymosin alpha-1 has no approved dosage, because it has no marketing authorisation in Finland [16]regulator. Below are the doses used in trials. They were given in hospital or under trial supervision, and they are not recommendations [5, 8]A.
| Study | Who | Dose and route | Duration |
|---|---|---|---|
| TESTS, sepsis (2025) | 1,106 adults in intensive care | 1.6 mg under the skin every 12 hours | 7 days [5]A |
| Hepatitis C (2012) | 552 patients | 1.6 mg under the skin twice a week, added to peginterferon and ribavirin | 48 weeks [8]A |
| Melanoma (2010) | 488 patients | 1.6, 3.2 or 6.4 mg added to chemotherapy | treatment duration not given in the abstract; the main result was assessed at 12 months [11]A |
What the numbers do not tell you. The trials used substance made for the study and given by hospital staff [5]A. The figures say nothing about powder bought online. The dose in the product information of the Zadaxin product authorised in Italy is on the dose page: thymosin alpha-1 dosage. The calculator has no thymosin alpha-1 preset.
Thymosin alpha-1 vs TB-500: what is the difference?
They are different substances, even though the names are similar. Thymosin alpha-1 is a 28-amino-acid peptide [1], and TB-500 is, according to the FDA, a short piece of thymosin beta-4 [12]regulator.
- Human studies: thymosin alpha-1 has large randomised trials [5, 8, 11]A. Studies of thymosin alpha-1 say nothing about TB-500 [12]regulator.
- Doping: the WADA list names thymosin beta-4 and its derivatives, for example TB-500, but not thymosin alpha-1 [15]regulator.
More: TB-500.
Situation in the United States
On the FDA risk page, thymosin alpha-1 is among substances that had been placed in compounding category 2 because of safety risks and whose nomination was later withdrawn by the nominator. The FDA risk assessment remains on the page (updated 22 Apr 2026) [12]regulator.
Is thymosin alpha-1 legal in Finland?
Thymalfasin has no marketing authorisation in Finland. In the EU a thymalfasin product (Zadaxin) was authorised only in Italy [16]regulator.
- EU. There is no EU-wide marketing authorisation, and the orphan designation for liver cancer (EU/3/02/110, 2002) is not a marketing authorisation [13]regulator.
- Article 57. In the EMA Article 57 database, which must list every human medicine authorised in the EU and EEA, Zadaxin was authorised only in Italy, for use into a muscle or under the skin (data as of 21 Sep 2026) [16]regulator.
- Medicines list. Annexes 1 and 1 A of the Finnish medicines list (230/2025) do not name thymosin alpha-1 or thymalfasin. The list is not exhaustive, however, and a substance that meets the definition of a medicine is a medicine even if it is not listed [14]regulator.
- Import. Ordering and importing medicines is covered on Peptides and the law in Finland. Bringing them back from travel: Travelling with peptides.
This is not legal advice. Disclaimer.
Is thymosin alpha-1 doping?
The WADA 2026 Prohibited List does not name thymosin alpha-1. The list names thymosin beta-4 and its derivatives, for example TB-500, in class S2.3, but that is a different substance [15]regulator.
How is a thymosin alpha-1 product assessed?
According to the FDA, peptides such as thymosin alpha-1 may carry peptide-related impurities, and identifying the raw material and confirming its quality is complex [12]regulator.
Clinical trials used lyophilised thymosin alpha-1 made for the study and given by hospital staff [5]A. These trials say nothing about the content or purity of powder sold online "for research use", and a seller's own certificate of analysis (COA) does not show quality by itself. More: How to read a COA and check a test result.
FAQ
Thymosin alpha-1: what is it?
A 28-amino-acid peptide originally isolated from the thymus that acts on the immune system. Its synthetic form is thymalfasin [1] [4]A.
Thymosin alpha-1 vs TB-500: is it the same substance?
No. TB-500 is a piece of thymosin beta-4, and the FDA treats the substances separately [12]regulator. Studies of thymosin alpha-1 do not apply to TB-500.
Does thymosin alpha-1 reduce deaths from sepsis?
Not in the largest trial. 28-day mortality was 23.4% with thymosin alpha-1 and 24.1% with placebo, and the difference was not significant [5]A.
Thymosin alpha-1 experiences: what do studies show?
User accounts of this substance have not been compiled on this site. In the largest sepsis trial mortality did not fall [5]A, and in hepatitis B and COVID-19 the evidence is uncertain [7, 9]A.
Thymosin alpha-1 side effects: what is known?
In a placebo-controlled sepsis trial, adverse events were as common as with placebo [5]A. According to the FDA, compounded products may carry a risk of immune reactions [12]regulator.
Thymosin alpha-1 dosage: what did studies use?
Most trials used 1.6 mg under the skin [5, 8]A. This is not dosing guidance: dosage in studies.
Thymosin alpha-1 in Finland: is it legal?
Thymalfasin has no marketing authorisation in Finland: in the EMA Article 57 database the only thymalfasin product was authorised in Italy [16]regulator. The medicines list does not name the substance, but because the list is not exhaustive, a substance that meets the definition of a medicine is a medicine [14]regulator. This is not legal advice.
Thymosin alpha-1 and doping: is it banned?
It is not named on the WADA 2026 list. Thymosin beta-4 and TB-500, which are on the list, are different substances [15]regulator.
Read the research yourself: sources
Every source opens the original publication or the regulator's page. The level shows what the source itself reports.
- Wei Y, Zhang Y, Li P, Yan C, Wang L. Thymosin α-1 in cancer therapy: Immunoregulation and potential applications. Int Immunopharmacol. 2023;117:109744. PMID 36812669 (review)
- Goldstein AL. From lab to bedside: emerging clinical applications of thymosin alpha 1. Expert Opin Biol Ther. 2009;9(5):593-608. PMID 19392576 (review)
- Panatto D, Amicizia D, Lai PL, et al. Utility of thymosin alpha-1 (Zadaxin) as a co-adjuvant in influenza vaccines: a review. J Prev Med Hyg. 2011;52(3):111-115. PMID 22010537 (review)
- 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. PMID 36056913 A
- 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 A
- Gu B, Zhou Y, Nie Y, 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 A
- Naing C, Ni H, Aung HH, et al. Thymosin-α1 for people with chronic hepatitis B. Cochrane Database Syst Rev. 2026;9(9):CD014610. PMID 42713852 A
- Ciancio A, Andreone P, Kaiser S, et al. Thymosin alpha-1 with peginterferon alfa-2a/ribavirin for chronic hepatitis C not responsive to IFN/ribavirin: an adjuvant role? J Viral Hepat. 2012;19 Suppl 1:52-59. PMID 22233415 A
- 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 A
- Soeroto AY, Suryadinata H, Yanto TA, Hariyanto TI. 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. PMID 37845598 A
- 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 A
- U.S. Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks (content current as of 22 Apr 2026). FDA regulator
- European Medicines Agency. Medicine data: medicines and orphan designations data sets of 12 Oct 2026 (no medicine with thymalfasin; orphan designation EU/3/02/110, thymalfasin, hepatocellular carcinoma, 30 Jul 2002). EMA regulator
- Finland. Lääkeluettelo (Fimea medicines list), 230/2025, section 1 and annexes 1 and 1 A. Finlex regulator
- World Anti-Doping Agency. 2026 Prohibited List, class S2.3. WADA regulator
- European Medicines Agency. Article 57 product data (EMA/518502/2018 Rev.89, data as of 21 Sep 2026): thymalfasin (Zadaxin), Italy only. EMA regulator
Research sources checked 12-13 Oct 2026. Regulator and registry sources checked 12 Oct 2026.
Evidence levels: A = randomised human trial or meta-analysis · B = observational human study or case series · C = animal study · D = cell or laboratory study · E = not peer-reviewed · regulator = a regulator's assessment or decision, not research evidence. The marker official means official product information (summary of product characteristics or label); it is not research evidence. For a review, the level shows what kind of research the statement rests on. No level: statements that something has not been studied, reviews' interpretations, named hypotheses and the general rules in our certificate-of-analysis guide.
Content is general information about research, not medical advice. If you have symptoms, contact a healthcare professional.