Living+Peptides

Peptide profile

Thymosin Alpha-1: FDA Status, Research and Uses

By Living+. Updated October 25, 2026.

Key takeaway

Thymosin alpha-1 is a 28 amino acid immune peptide first isolated from calf thymus in 1977. FDA has approved no product containing it, and an FDA advisory committee voted in 2024 against adding it to the 503A bulks list. It was studied in human trials for hepatitis B and C, melanoma and sepsis, with mixed results [1, 4, 8].

At a glance

Where it stands

Checked

  • FDA approvedNoNo approved product
  • Pharmacy compounding (503A)WithdrawnNot in category 1, 2 or 3 on FDA's 503A list updated May 14, 2026; FDA lists Ta1 as previously in category 2 with the nomination withdrawn. At the Dec 4, 2024 PCAC meeting FDA proposed NOT adding it and the committee voted 4 yes, 17 no for both the free base and the acetate.
  • Outsourcing facilities (503B)Category 3Listed in 503B category 3 (nominated without adequate support) on FDA's 503B list updated March 21, 2025.
  • WADA (sport)Not listedNot named on the 2026 Prohibited List
Same data as the Status Tracker. Checked against 10 sources

Key facts

Class
Immune-modulating thymic peptide. The synthetic form is called thymalfasin [1, 8]
Length
28 amino acids [1, 8]
Half-life
About 2 hours (under 3 hours) after injection under the skin in healthy adults [1, 9]
Route in studies
Subcutaneous injection in most clinical studies [1]
First studied
1977 [1, 8]

How it works

Thymosin alpha-1 was first isolated from a calf thymus extract called thymosin fraction 5. In 1977 researchers reported its sequence of 28 amino acids and described it as a signal that may help T cells develop and function [8]. The drug form, thymalfasin, is made by chemical synthesis and has the same amino acid sequence as the natural peptide [1].

FDA's 2024 review says thymosin alpha-1 acts on the immune system at least partly by binding Toll-like receptor 9 on dendritic cells and early immune cells [1]. In rodent studies it was linked to slower tumor growth, fewer viral infections, less sepsis, and stronger antibody responses to some vaccines. FDA noted that most of those animal doses were much higher than the doses used in people [1].

In healthy adults, thymosin alpha-1 given under the skin reached peak blood levels in about 1 to 2 hours and cleared with a half-life under 3 hours. Levels did not build up over 5 days of dosing [9]. EMA's orphan designation page says it is authorized in Italy to boost the immune response to flu vaccines [7]. FDA says it is not approved in the United States, Japan, or Europe except Italy, and that FDA could not verify claims of approval in other regions [1].

What the research shows

5 human trials1 lab study

  1. 2025Human trial

    TESTS sepsis trial (Wu et al.)

    Studied for sepsis survival

    1,106 adults with sepsis at 22 centers in China Phase 3, randomized, double-blind, placebo-controlled, 7 days of injections

    28 day death rates were 23.4% with thymosin alpha-1 and 24.1% with placebo, with no clear difference [13].

    Read the study
  2. 2012Human trial

    Hepatitis C retreatment trial (Ciancio et al.)

    552 adults with hepatitis C who did not respond to earlier therapy Randomized, placebo-controlled, added to peginterferon and ribavirin for 48 weeks

    Sustained response was 12.7% with thymosin alpha-1 vs 10.5% with placebo, not a significant difference [11].

    Read the study
  3. 2010Human trial

    Metastatic melanoma trial (Maio et al.)

    Studied as an add-on to chemotherapy

    488 adults with metastatic melanoma Randomized, 5 arms, added to dacarbazine with or without interferon alfa

    More tumor responses were seen in two thymosin arms. Survival difference was not significant (P = .08) [12].

    Read the study
  4. 1999Human trial

    Chronic hepatitis B phase 3 (Mutchnick et al.)

    97 adults with chronic hepatitis B Phase 3, multicenter, randomized, double-blind, placebo-controlled, 6 months plus 6 months follow-up

    Complete response was 14% vs 4% with placebo (P = 0.084). Authors said the results did not confirm earlier reports [10].

    Read the study
  5. 1999Human trial

    Pharmacokinetics in healthy volunteers (Rost et al.)

    9 healthy adults Randomized 3-way crossover, single and 5-day dosing of 3 formulations

    Peak levels came in 1 to 2 hours and half-life was under 3 hours, with no buildup [9].

    Read the study
  6. 1977Lab study

    Isolation and sequence (Goldstein et al.)

    Calf thymus tissue Peptide isolation and amino acid sequencing

    Identified a heat stable, highly acidic peptide of 28 amino acids from thymosin fraction 5 [8].

    Read the study

Doses used in studies

FDA's review found that clinical studies gave daily doses of about 1 to 16 mg, usually under the skin, for 1 day to 12 months. The most common study dose was 1.6 mg under the skin [1]. Hepatitis trials used 1.6 mg twice a week [10, 11]. A melanoma trial tested 1.6, 3.2 and 6.4 mg added to chemotherapy [12].

A pharmacokinetic study used 900 micrograms per square meter of body surface area in healthy adults [9]. FDA noted it knows of no human data for the 3 mg/mL strength proposed for compounding, and that the highest strength in clinical studies was 2 mg/mL [1]. No US dose exists because thymosin alpha-1 is not FDA approved [1].

Dosing is set by a licensed provider.

Side effects and safety

In most clinical studies, FDA found no significant adverse events clearly caused by thymosin alpha-1 at 1 to 16 mg under the skin for up to 12 months. The most common side effects were irritation, redness or discomfort at the injection site [1].

Studies also reported liver enzyme flares in people with hepatitis B, thyroid test changes in people with hepatitis C, and fatal immune hemolytic anemia and graft failure in stem cell transplant patients [1]. FDA warns that it could cause or worsen graft-versus-host disease in transplant patients, and that its safety as a flu vaccine add-on is not known [1].

Labels outside the US warn about use in children, pregnant or nursing women, people with autoimmune disease, and people with weakened immune systems [1]. FDA also flags a risk of immune reactions to the peptide itself, which may rise with impurities or clumping [1, 2]. There is no FDA label or boxed warning.

Common questions

Is thymosin alpha-1 FDA approved?

No. FDA says it has approved no drug products containing thymosin alpha-1 [1].

Is thymosin alpha-1 approved in any country?

EMA's orphan designation page says thymalfasin is authorized in Italy to boost the response to flu vaccines [7]. FDA says a maker claims approvals in parts of Asia, Latin America, Eastern Europe and the Middle East, which FDA could not verify [1].

What is Zadaxin?

FDA describes Zadaxin as a SciClone Pharmaceuticals product of thymosin alpha-1, 1.6 mg solution for injection. It is not approved in the US [1].

Can a US pharmacy compound thymosin alpha-1?

Not under section 503A as of October 25, 2026. It is not on the 503A bulks list, and an FDA advisory committee voted 17 to 4 against adding it in 2024 [4, 3].

Is thymosin alpha-1 the same as thymalfasin?

Yes. Thymalfasin is the chemically made form, with the same amino acid sequence as natural thymosin alpha-1 [1].

Did thymosin alpha-1 help in sepsis?

A 2025 trial of 1,106 adults found no clear reduction in 28 day deaths compared with placebo [13].

Is thymosin alpha-1 banned by WADA?

It is not named on the WADA 2026 Prohibited List [6]. Athletes should check with their anti-doping agency before use of any unlisted substance.

What are the side effects of thymosin alpha-1?

In studies, the most common were injection site irritation, redness and discomfort. Serious events, including a fatal one, were reported in stem cell transplant patients [1].

What did FDA conclude about thymosin alpha-1 effectiveness?

FDA found a lack of evidence that it is effective for the 12 uses it reviewed, including hepatitis B, hepatitis C, HIV and COVID-19 [1].

Sources

  1. FDA: Thymosin alpha-1 (Ta1) related bulk drug substances, PCAC presentation, December 4, 2024
  2. FDA: Certain bulk drug substances for use in compounding that may present significant safety risks
  3. FDA: Bulk drug substances nominated for use in compounding under section 503A (updated May 14, 2026)
  4. FDA: Final summary minutes, Pharmacy Compounding Advisory Committee, December 4, 2024
  5. FDA: Bulk drug substances nominated for use in compounding under section 503B (updated March 21, 2025)
  6. WADA: 2026 Prohibited List
  7. EMA: Orphan designation EU/3/02/110, thymalfasin
  8. Goldstein et al. 1977, PNAS (PubMed)
  9. Rost et al. 1999, Int J Clin Pharmacol Ther (PubMed)
  10. Mutchnick et al. 1999, J Viral Hepat (PubMed)
  11. Ciancio et al. 2012, J Viral Hepat (PubMed)
  12. Maio et al. 2010, J Clin Oncol (PubMed)
  13. Wu et al. 2025, BMJ, TESTS trial (PubMed)

Updated October 25, 2026. Educational information, not medical advice.