Peptide Evidence Index

Thymosin Alpha-1

Evidence last reviewed: September 2026

Quick answers

What is it?
Thymosin alpha-1 is a synthetic 28-amino-acid peptide, originally identified in thymus gland extracts, studied as an immune modulator to help the body respond to infection, cancer, or vaccination.
Why are people interested in it?
It has one of the largest human trial records of any compound in this library and is an approved prescription medicine in a number of countries, even though it has never been approved by the FDA.
What has actually been studied?
Sixty-four registered trials, including large multinational randomized, placebo-controlled Phase 3 studies in chronic hepatitis and sepsis.
How strong is the evidence?
Genuinely mixed: the largest and most rigorous trial to date, a 1,106-patient Phase 3 sepsis trial, found no mortality benefit, after an earlier, smaller trial had suggested one.
What don't we know yet?
Why the earlier sepsis trial showed a benefit that the larger, more rigorous trial did not, and whether the modest hepatitis C benefit affects long-term liver outcomes, are both unresolved.
Where can I read the original research?
Sources at the bottom of this page: Wu et al., BMJ 2025; Wu et al., Critical Care 2013.

Plain-English overview

Thymosin alpha-1 is a synthetic 28-amino-acid peptide, originally identified in extracts of thymus gland tissue and now manufactured chemically. It is studied mainly as an immune modulator, intended to help the immune system respond to infection, cancer, or vaccination rather than to act on a pathogen or tumor directly. Its regulatory status is split by geography more than any other compound in this library: it is not approved by the FDA in the United States or by EU regulators, but in a number of other countries it is an approved prescription medicine, marketed as Zadaxin, chiefly as an add-on treatment for chronic hepatitis B and C. It has one of the largest human trial records of any peptide reviewed here, with genuinely mixed results rather than a uniform positive or negative signal.

What researchers are studying it for

Current evidence level

Mixed human evidence

Regulatory status: Not an approved medicine. Evidence strength and approval are scored separately; approval is a regulatory decision about a specific use, not a measure of how much research exists.

Human research

In chronic hepatitis C, a large Phase 3 trial conducted across Europe in patients who had not responded to standard interferon-based therapy found that adding thymosin alpha-1 did not improve the rate of sustained virologic response, the trial's primary goal, but did significantly reduce the relapse rate in patients who completed the full course of treatment (Ciancio & Rizzetto, 2010, Annals of the New York Academy of Sciences, reviewing a Phase 3 trial first described while still recruiting by Camerini et al., 2007, Annals of the New York Academy of Sciences). In sepsis, results reversed over time: an earlier, smaller, single-blind trial in 361 patients with severe sepsis found a borderline reduction in 28-day mortality with thymosin alpha-1 (26.0% versus 35.0%, log-rank P=0.049) and improved recovery of an immune-function marker (Wu et al., 2013, Critical Care). That result helped justify a much larger, more rigorous follow-up: a multicenter, double-blind, placebo-controlled Phase 3 trial in 1,106 adults with sepsis across 22 centers in China, the largest ever run on this compound, which found no difference in 28-day all-cause mortality between thymosin alpha-1 and placebo (23.4% versus 24.1%, hazard ratio 0.99, P=0.93), a clear null result on the primary outcome (Wu et al., 2025, BMJ). Trials in various cancers have generally tested thymosin alpha-1 as an add-on to chemotherapy, radiotherapy, or immune checkpoint inhibitors, aiming to counter treatment-related immune suppression rather than treat the cancer directly; many of these are small, single-center studies, and several remain incomplete or have unreported outcomes.

Animal and preclinical research

The material reviewed for this entry does not describe a distinct animal-only preclinical literature for thymosin alpha-1 separate from its extensive human trial program, which spans chronic hepatitis, sepsis, cancer, HIV, and COVID-19.

Major findings

What has not been established

Important limitations

Regulatory status

Thymosin alpha-1 is not approved by the FDA in the United States or by the EMA in the European Union. It is marketed as an approved prescription medicine under the brand name Zadaxin in a number of other countries, principally as an adjunct treatment for chronic hepatitis B and C.

Primary research sources

Wu J, 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
Wu 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. PMID: 23327199
Ciancio A, Rizzetto M. Thymalfasin in the treatment of hepatitis B and C
Ann N Y Acad Sci 2010;1194:141-6. PMID: 20536462
Camerini R, et al. Studies of therapy with thymosin alpha1 in combination with pegylated interferon alpha2a and ribavirin in nonresponder patients with chronic hepatitis C
Ann N Y Acad Sci 2007;1112:368-74. PMID: 17600286
Efficacy and safety of thymosin alpha 1 for sepsis (TESTS)
ClinicalTrials.gov NCT02867267, Phase 3, n=1,106, completed 2021
Efficacy of thymosin alpha 1 for severe sepsis (ETASS)
ClinicalTrials.gov NCT00711620, n=361, completed 2010
Full evidence entry (complete data, registry records, dosing evidence)
Mixed human evidence
Controlled human trials exist and disagree

At a glance

Also known as
Thymalfasin, Zadaxin (brand name), Tα1
Class
Synthetic 28-amino-acid peptide, immune modulator
Derived from
A synthetic copy of a peptide originally isolated from thymus gland tissue extract (thymosin fraction 5), manufactured chemically rather than extracted from animal glands
Registered human trials
64
…with published results
Dozens, including large multinational Phase 3 trials in chronic hepatitis and sepsis
Largest human trial
1,106 adults with sepsis in the Phase 3 TESTS trial (22 centers in China, 2016-2021), the largest and most rigorous trial of thymosin alpha-1 to date
Studied for
chronic hepatitis B and C (adjunct to antivirals), sepsis and severe sepsis, adjunct to chemotherapy, immunotherapy, or radiotherapy in several cancers, COVID-19 prevention and treatment, vaccine response enhancement, HIV infection
Dosing evidence
Prescribed by a clinicianWhere thymosin alpha-1 is an approved medicine, dosing is set by that country's approved label and a prescriber; large trials in sepsis and hepatitis have tested other regimens with mixed results, and no dose has been validated for the immune-support or longevity use it is marketed for in the unregulated US research-chemical market.
Regulatory status
Not approved by the FDA in the United States or by the EMA in the European Union. Marketed as an approved prescription medicine under the brand name Zadaxin in a number of other countries, principally as an adjunct treatment for chronic hepatitis B and C.

What it is

Thymosin alpha-1 is a synthetic 28-amino-acid peptide, originally identified in extracts of thymus gland tissue and now manufactured chemically. It's studied mainly as an immune modulator — meant to help the immune system respond to infection, cancer, or vaccination, rather than to directly kill a pathogen or tumor cell itself.

Its regulatory status is genuinely split by geography, more than any other compound in this library. It is not approved by the FDA in the United States and has no approval from EU regulators. In a number of other countries it is an approved prescription medicine, marketed under the brand name Zadaxin, chiefly as an add-on treatment for chronic hepatitis B and C.

What the studies actually found

Thymosin alpha-1 has by far the largest human trial record of any peptide in this library — 64 registered studies at the time of writing, most run in China, spanning chronic hepatitis, sepsis, cancer, HIV, and more recently COVID-19.

Its evidence is genuinely mixed rather than uniformly positive or negative. In chronic hepatitis C, a large Phase 3 trial run across Europe in patients who had not responded to standard interferon-based therapy found that adding thymosin alpha-1 did not improve the rate of sustained virologic response — the trial's primary goal — but did significantly reduce the relapse rate in patients who completed the full course of treatment (Ciancio & Rizzetto, 2010, Annals of the New York Academy of Sciences).

In sepsis, the picture flipped over time. An earlier, smaller, single-blind trial in 361 patients with severe sepsis found a borderline reduction in 28-day mortality with thymosin alpha-1 (26.0% vs. 35.0%, log-rank P=0.049) and improved recovery of a marker of immune function (Wu et al., 2013, Critical Care). That result helped justify a much larger, more rigorous follow-up: a multicenter, double-blind, placebo-controlled Phase 3 trial in 1,106 adults with sepsis across 22 centers in China. That trial — the largest ever run on this compound — found no difference in 28-day all-cause mortality between thymosin alpha-1 and placebo (23.4% vs. 24.1%, hazard ratio 0.99, P=0.93), a clear null result on the primary outcome (Wu et al., 2025, BMJ).

Trials in various cancers have generally tested thymosin alpha-1 as an add-on to chemotherapy, radiotherapy, or immune checkpoint inhibitors, aiming to counter treatment-related immune suppression rather than to treat the cancer directly. Many of these are small, single-center Chinese studies, and several remain incomplete or have unreported outcomes.

What exists in humans

Sixty-four trials are registered on ClinicalTrials.gov under thymosin alpha-1 or its trade names (Zadaxin, thymalfasin). Completed trials with posted results or full publications exist for chronic hepatitis B and C, severe sepsis, prevention of infection after acute pancreatitis, and HIV-related immune reconstitution, among others. A large share of the cancer-adjunct trials are still recruiting, listed as status unknown, or have not posted results.

What doses have been studied in humans

Where thymosin alpha-1 is an approved medicine, its dose is set by that country's approved product label and administered under a prescriber's care — nothing here substitutes for that.

Outside that approved-use context, clinical trials have tested it by subcutaneous injection at various regimens, typically several times per week for periods ranging from about one week (in the sepsis trials) to many months (in the hepatitis trials), always alongside another standard treatment such as interferon, an antiviral, or a checkpoint inhibitor rather than alone.

None of this trial dosing was designed for, or validates, the general immune-support, anti-aging, or wellness use the peptide is sold for in the unregulated US research-chemical market. No trial has tested thymosin alpha-1 in healthy people seeking those effects.

What remains unknown

  • Why an earlier sepsis trial suggested benefit while the larger, more rigorous Phase 3 trial found

none — possible explanations include chance, changes in standard sepsis care over the decade between trials, or the smaller trial's less rigorous single-blind design

  • Whether the modest hepatitis C relapse-rate benefit translates into any meaningful long-term liver

outcome

  • Whether the many ongoing cancer-adjunct trials will show a consistent benefit once completed
  • What accounts for the regulatory split — approved in a number of countries, never approved by the

FDA — given the underlying trial evidence has always been mixed

  • Whether product sold in the US research-chemical market matches the purity and formulation of the

approved product used in trials

Registry and study records

The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial
PubMed · Wu et al., 2025, BMJ. n=1,106; 28-day mortality 23.4% vs 24.1%, HR 0.99, P=0.93. DOI: 10.1136/bmj-2024-082583
The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial
PubMed · Wu et al., 2013, Critical Care. n=361; 28-day mortality 26.0% vs 35.0%, log-rank P=0.049. DOI: 10.1186/cc11932
Thymalfasin in the treatment of hepatitis B and C
PubMed · Ciancio & Rizzetto, 2010, Annals of the NY Academy of Sciences. Reviews the European Phase 3 HCV trial: no improvement in sustained virologic response, but reduced relapse rate. DOI: 10.1111/j.1749-6632.2010.05487.x
Efficacy and safety of thymosin alpha 1 for sepsis (TESTS)
ClinicalTrials.gov NCT02867267 · Phase 3, n=1,106, completed 2021, results published in BMJ 2025
Efficacy of thymosin alpha 1 for severe sepsis (ETASS)
ClinicalTrials.gov NCT00711620 · n=361, completed 2010, results posted
Phase III trial of thymosin alpha 1 + peginterferon + ribavirin in chronic hepatitis C nonresponders
ClinicalTrials.gov NCT01178996 · sigma-tau i.f.r. S.p.A., n=552, completed 2008
See the full Evidence Index →

Related research

No dedicated research articles yet.

Evidence last reviewed: September 2026. Peptide Proof reports what studies found. We do not give medical advice, dosing guidance, or treatment recommendations.