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FDA-approvedVendors pendingFacts verified · 2026-05-25

Thymosin Alpha-1

Also known as thymalfasin, tα1, zadaxin · Wikipedia

Thymosin alpha-1 (thymalfasin, brand name Zadaxin from SciClone) is a synthetic 28-amino-acid N-acetylated peptide originally isolated from calf thymus fraction 5. It is approved in more than 35 countries (including across Europe, Asia, and South America) for chronic hepatitis B and C and for immune reconstitution in immunocompromised patients, and is used adjunctively in sepsis and oncology protocols. It is NOT FDA-approved in the United States, where it is available only via compounding pharmacies or as a research chemical. It is one of the better-characterized immunomodulatory peptides in clinical use, with multiple randomized trials. Also known as thymalfasin, Talpha1, Zadaxin.

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Mechanism of action

Thymosin alpha-1 is an N-acetylated 28-amino-acid peptide originally isolated from thymosin fraction 5 of bovine thymus. Its principal molecular activity is agonism at Toll-like receptor 9 (TLR9), with additional activity at TLR2, on plasmacytoid dendritic cells and monocytes (https://pmc.

Thymosin alpha-1 is an N-acetylated 28-amino-acid peptide originally isolated from thymosin fraction 5 of bovine thymus. Its principal molecular activity is agonism at Toll-like receptor 9 (TLR9), with additional activity at TLR2, on plasmacytoid dendritic cells and monocytes (https://pmc.ncbi.nlm.nih.gov/articles/PMC7747025/). This biases adaptive responses toward Th1 by increasing IL-12 production, enhances cytotoxic T-cell, natural killer cell, and dendritic-cell maturation, and modulates indoleamine 2,3-dioxygenase (IDO) and tolerogenic dendritic-cell pathways depending on context. The dual activating and balancing effects underlie use in chronic viral hepatitis, sepsis (https://pmc.ncbi.nlm.nih.gov/articles/PMC5025565/), HIV, post-chemotherapy immune reconstitution, and as adjunctive cancer therapy in approved jurisdictions.

Pharmacokinetic properties

Half-life

approximately 2 hours plasma after SC dosing

Routes

subcutaneous · intramuscular

Bioavailability

Approximately 80% SC bioavailability. Not orally active. Stable lyophilized; reconstituted with sterile water.

Amino-acid sequence

Ac-Ser-Asp-Ala-Ala-Val-Asp-Thr-Ser-Ser-Glu-Ile-Thr-Thr-Lys-Asp-Leu-Lys-Glu-Lys-Lys-Glu-Val-Val-Glu-Glu-Ala-Glu-Asn-OH

Use & research dosing

RecoveryImmune Support

The approved Zadaxin regimen for chronic hepatitis B/C is 1.6 mg subcutaneously twice weekly for 6-12 months. The same 1.6 mg twice-weekly schedule is used for immune reconstitution and vaccine adjuvant indications. In severe sepsis trials (ETASS) the dose was 1.6 mg subcutaneously every 12 hours for the first week. Research and compounded protocols typically use 1.6 mg subcutaneously twice weekly in 4-12 week cycles. Research framing only in the US.

Research-use framing only. SavePeptides sells nothing for human consumption. Doses above reflect reported research / self-experimentation ranges, not clinical recommendations.

Editorial perspective

One of the best-characterized thymic peptides with decades of clinical use abroad. The US-vs-rest-of-world regulatory split matters: outside the US it is a prescription drug with defined labeling; inside the US it is dispensed only via compounding (and even that status has been narrowed by recent FDA bulk-substances list decisions) or sold as a research chemical. Evidence is strongest for chronic hepatitis B/C, severe sepsis adjunct, and post-chemotherapy immune support, with weaker but suggestive data in some malignancies and COVID-19 adjunctive use.

— SavePeptides editorial desk · last updated 2026-05-25

Cautions & contraindications

Before researching this compound, note:

  • Not FDA-approved in the United States; status is investigational or compounded
  • Caution in solid-organ transplant recipients (theoretical risk of allograft rejection)
  • Caution in active autoimmune disease (e.g., lupus, rheumatoid arthritis) due to immune-activating effects
  • Pregnancy and lactation - insufficient data; avoid
  • Drug interactions expected with immunosuppressants (corticosteroids, calcineurin inhibitors)
  • Injection-site reactions are the most common adverse event in clinical trials
  • Discontinue if febrile illness or rash develops
  • Quality and identity of compounded or research-chemical supply varies; prefer pharmaceutical-grade product

Research foundation

The papers behind the page.

  1. [01]https://pmc.ncbi.nlm.nih.gov/articles/PMC7747025/
  2. [02]https://pmc.ncbi.nlm.nih.gov/articles/PMC5025565/
  3. [03]https://pmc.ncbi.nlm.nih.gov/articles/PMC4056079/

Facts verified

2026-05-25

Confidence

high

What this means

  • approved outside US only - not FDA approved

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