Thymosin Alpha-1: Benefits, Dosage, Side Effects and Legal Status

Dosing: what the trials used.

Thymosin alpha-1 has more clinical evidence behind it than anything else I cover in the research-chemical market: 864 papers and 64 randomised controlled trials. It also has an international non-proprietary name, thymalfasin, and marketing approval in a number of countries. Two similar names belong to different products: thymulin, a 9-amino-acid thymus hormone that needs zinc to work, and Thymalin, a Russian calf-thymus extract.

It is a real medicine, and a well-tolerated one. FDA’s review of the published trials found no adverse events clearly caused by it in most studies, and an earlier sepsis trial reported fewer deaths with it. What it hasn’t had is a trial of its main online use, general immune support in healthy adults. Its newest and largest sepsis trial also did not confirm a survival benefit.

Thymosin Alpha-1 benefits: what the research shows

• Sepsis, critical careincluding the 2025 BMJ trial
  2
• Severe acute pancreatitispredicted severe necrotising
  1
• Chronic hepatitiswith interferon alfa
  1
• Healthy adults, immune supportthe marketed use
  0

The clinical programme is in hospital medicine: sepsis, severe pancreatitis, chronic hepatitis. Nobody has run a randomised trial of thymosin alpha-1 for general immune support in healthy adults, which is what the online market sells it for.

Thymosin alpha-1 structure

Molecular formula

C₁₂₉H₂₁₅N₃₃O₅₅

Molecular weight

3108.3 g/mol

Length

28 amino acids

Sequence

SDAAVDTSSEITTKDLKEKKEVVEEAEN

Modifications

N-terminal acetyl.

PubChem CID

16130571

Identity checked against KEGG DRUG (thymalfasin).

The newest trial, and what it found

TESTS was a multicentre randomised controlled trial in adults with sepsis, published in the BMJ in 2025 Wu 2025 - 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., in the references below. Its main outcome was death from any cause within 28 days.

In its own words, the trial “found no clear evidence to suggest that thymosin α1 decreases 28 day all cause mortality in adults with sepsis.” No secondary or safety outcome differed significantly between the groups either.

The more encouraging result came earlier, from a severe-sepsis trial of 361 patients in six Chinese hospitals Wu 2013 - 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., in the references below:

• 28-day mortality was 26.0% with thymosin alpha-1 against 35.0% without it.
• That result was borderline: log-rank p = 0.049, but p = 0.062 in the unstratified analysis.
• An immune-function marker also recovered better.

That signal justified the larger trial, which did not reproduce it.

Other critical-care work includes a trial of immune enhancement in predicted severe acute necrotising pancreatitis Ke 2022 - 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., in the references below. The hepatitis research, where the medicine has its classic licensed use, includes work combining it with interferon alfa You 2006 - You J, Zhuang L, Cheng HY et al. Efficacy of thymosin alpha-1 and interferon alpha in treatment of chronic viral hepatitis B: a randomized controlled study. World J Gastroenterol. 2006;12(41):6715-21., in the references below.

Does thymosin alpha-1 work for general immune support?

Look at the settings again: sepsis, necrotising pancreatitis, chronic hepatitis. These are people in hospital, often in intensive care, whose immune systems are failing under severe acute stress. Thymosin alpha-1 is being studied as a way to modulate that.

Online, it is mostly sold to healthy adults for general immune support. That use hasn’t been tested in a controlled trial yet. The hospital trials don’t carry over in either direction: a sepsis result, positive or negative, says little about how a healthy immune system responds.

How it ranks against other peptides sold for inflammation: peptides for inflammation.

Thymosin Alpha-1 dosage

Thymosin alpha-1 has no FDA-approved US label. Community sources and foreign licensed-drug labels agree on 1.6 mg injected under the skin twice a week, 3-4 days apart, for roughly 3-12 months depending on use. That is common practice, not a dose any US regulator has set, though it matches what published trials and the foreign licensed label used. The full dosage page shows what each trial used by condition.

Thymosin Alpha-1 side effects

FDA reviewed the published clinical studies in December 2024, while considering whether to allow thymosin alpha-1 as a compounding ingredient. That review is the fullest safety picture anywhere for this molecule, and goes beyond what any single trial above reports.

Reported in studies

Across most studies, at 1 to 16 mg injected under the skin for up to 12 months, FDA found no adverse events clearly caused by thymosin alpha-1. The most common reaction was irritation, redness or discomfort at the injection site.

The trials above agree:

• The earlier severe-sepsis trial reports no serious drug-related adverse event Wu 2013 - 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., in the references below.
• In the 2025 BMJ trial, no safety outcome differed significantly from placebo Wu 2025 - 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., in the references below.
• The hepatitis B trial reports no side effects at all in the thymosin alpha-1 group, unlike its interferon comparator You 2006 - You J, Zhuang L, Cheng HY et al. Efficacy of thymosin alpha-1 and interferon alpha in treatment of chronic viral hepatitis B: a randomized controlled study. World J Gastroenterol. 2006;12(41):6715-21., in the references below.

Rare but serious

FDA’s review also found rarer, more serious reports in the published studies:

• liver-enzyme flares in hepatitis B patients
• thyroid-hormone abnormalities in hepatitis C patients
• nipple pain
• one fatal immune hemolytic anemia with engraftment failure in a stem-cell transplant recipient

FDA flagged that thymosin alpha-1 could worsen graft-versus-host disease or cause engraftment failure in stem-cell transplant patients. It also said there is too little data to judge its safety as a vaccine adjuvant.

Product quality is a separate concern. Thymosin alpha-1 is a 28-amino-acid peptide given by injection, and FDA’s chemistry review calls the compounded form “not well-characterized”. There is no public data on impurities or aggregation in the product sold, and FDA says both could raise the risk of an immune reaction to the peptide itself.

For the injection itself, injection-site reactions explains what is ordinary and what needs attention.

Is thymosin alpha-1 legal?

This compound shows most clearly why I record legal status country by country. “Thymosin alpha-1 is an approved medicine” and “thymosin alpha-1 is not approved” are both true, and which one applies to you depends on where you live. Sellers tend to quote only the first.

Regulatory changes for this compound and others, as they happen: US peptide regulation tracker.

How Thymosin alpha-1 compares

Immune and inflammation. Doses for unapproved compounds are common practice, not trial-established. All immune and inflammation peptides →

Compound	Main use	Evidence	Usual dose	US status
Thymosin alpha-1	Licensed in a number of countries, classically for chronic hepatitis B. Sold online for general immune support.	Strong human evidence	1.6 mg, twice weekly, subcutaneous injection, spaced 3-4 days apart	Research use only
LL-37	An antimicrobial peptide naturally present in human neutrophils, skin and mucosal surfaces. Sold as a research chemical for innate-immune and antimicrobial research; FDA has not approved it for any human use.	Limited human data	100-2000 mcg, 3-5 times weekly to once daily, subcutaneous, starting low and titrating up	Research use only
VIP	Sold to consumers as a research peptide marketed for immune modulation and inflammation. It is not an FDA-approved drug for any indication, and this site could not verify a completed human trial establishing benefit for that consumer use.	Limited human data	50 mcg, 4-8 sprays a day, split across the day, alternating nostrils	Research use only
ARA-290 (Cibinetide)	An investigational peptide developed by Araim Pharmaceuticals for neuropathic pain and inflammation-related conditions. Never reached FDA approval for any indication; now sold to consumers as a research chemical.	Moderate human evidence	1-8 mg, twice weekly to once daily, subcutaneous - a compounding pharmacy protocol titrates 2 mg toward 4 mg twice weekly, while the published phase 2b trial dosed 1, 4 or 8 mg once daily	Research use only

Where to buy Thymosin alpha-1 for research

Each vendor is checked against my vendor checklist.

1. 1.Spartan PeptidesFeatured

   5 mg vial

   • No FDA warning letter found
   • Names its testing lab

   $99.00as of 3 October 2026

   View product →My Spartan Peptides review

2. 2.BioEdge Research Labs

   10 mg vial

   • No FDA warning letter found
   • Names its testing lab
   • Batch COAs published

   $88.00as of 3 October 2026

   View product →My BioEdge Research Labs review

Compare every checked vendor for Thymosin alpha-1 →

See all vendors and how they compare →

Common questions

What is thymosin alpha-1?

A peptide that modulates immune function, with the international name thymalfasin. Unlike most compounds I cover, it is a licensed medicine in a number of countries. Its classic use is chronic hepatitis B, and it has been studied extensively in critical care.

Is thymosin alpha-1 FDA-approved?

No. It is approved in a number of other countries, which is why you will see it called an approved medicine without qualification. Both statements are true, and which one applies to you depends on where you are. That is why I record legal status per country rather than as a single fact.

What is the usual thymosin alpha-1 dose?

Community sources and foreign licensed-drug labels agree on 1.6 mg injected under the skin twice a week, 3-4 days apart, for roughly 3-12 months depending on use. That is common practice, not a dose any US regulator has set. The thymosin alpha-1 dosage page covers what published trials used.

Does thymosin alpha-1 work?

For general immune support in healthy people, the marketed use, it hasn't been tested in a controlled trial yet. In sepsis the picture is mixed. An earlier trial of 361 patients reported fewer deaths at 28 days (26% against 35%, a borderline result), but the larger 2025 BMJ trial found no clear evidence that it reduces 28-day mortality.

Who were the trials actually run in?

Patients with sepsis, predicted severe acute necrotising pancreatitis or chronic hepatitis: seriously ill people, in hospital, under supervision. Results in those groups say little either way about a healthy adult taking it for general immune support.

Is buying thymosin alpha-1 online the same product?

No. The licensed version sold in some countries is made to pharmaceutical standards. Research-chemical vendors sell the same molecule name with none of that assurance, and importing an unapproved medicine has its own legal consequences depending on where you live.

What are the side effects of thymosin alpha-1?

FDA's December 2024 review of the published studies found no adverse events clearly caused by thymosin alpha-1 in most of them; injection-site irritation and redness were the most common reaction. It also found rarer, serious reports: liver-enzyme flares in hepatitis B, thyroid abnormalities in hepatitis C, and a fatal immune reaction in a stem-cell transplant recipient. FDA flagged that it could worsen graft-versus-host disease in transplant patients.

What is the brand name of thymalfasin?

Thymalfasin has no brand on the US market. Outside the US it is sold as Zadaxin (SciClone Pharmaceuticals).

What is the generic name for Zadaxin?

The generic name for Zadaxin is thymalfasin. Zadaxin is SciClone Pharmaceuticals's brand, approved for depending on the country, chronic hepatitis B and C, and use as a vaccine or chemotherapy adjuvant. It is not approved in the US.

References (6 sources)

Show

Bibliographic detail is fetched from PubMed, not written by me - so a citation here cannot drift from the paper it names. Study design comes from PubMed's own tags rather than my judgement.

1. 1.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.Randomised trialPMID 39814420full text
2. 2.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.Randomised trialPMID 23327199full text
3. 3.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.Randomised trialPMID 35713670full text
4. 4.You J, Zhuang L, Cheng HY et al. Efficacy of thymosin alpha-1 and interferon alpha in treatment of chronic viral hepatitis B: a randomized controlled study. World J Gastroenterol. 2006;12(41):6715-21.Randomised trialPMID 17075991full text
5. 5.FDA Pharmacy Compounding Advisory Committee briefing slides, 'Thymosin Alpha-1 (Ta1) Related Bulk Drug Substances,' 4 December 2024. FDA's own chemistry, nonclinical and clinical-safety review of thymosin alpha-1 as a proposed 503A compounding substance, including its overall conclusion on clinical safety and immunogenicity concerns.- US Food and Drug Administrationregulatory
6. 6.Thymosin Alpha-1 (Ta1) Related Bulk Drug Substances - FDA briefing slides, 4 December 2024- U.S. Food and Drug Administration, Pharmacy Compounding Advisory Committeelabel

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