Thymosin-Alpha 1 (A1)

Last Updated: August 2026

Thymosin Alpha-1 (Tα1) is a synthetic 28-amino-acid thymic peptide approved in dozens of countries as Zadaxin for the treatment of hepatitis B and hepatitis C, with additional approved indications in some markets for cancer chemotherapy support and severe immunodeficiency. It is not currently FDA-approved in the United States, though it has approval pathways in most major non-US markets. Research-grade Thymosin Alpha-1 is also sold by third-party laboratories for laboratory use only.

What is Thymosin Alpha-1 and how does the immune-modulation mechanism work?

Thymosin Alpha-1 is a synthetic 28-amino-acid peptide originally isolated from thymus tissue, functioning as a key immune modulator through Toll-like receptor 9 (TLR9) activation, dendritic cell maturation, and T-cell differentiation toward Th1 responses. Unlike the Khavinson Thymalin extract (which contains multiple peptides), Thymosin Alpha-1 is a defined single-sequence peptide with characterized receptor-binding mechanisms. It enhances cell-mediated immune responses, particularly against viral infections and tumors. Evidence level: Human RCT (Phase 3 for hepatitis B/C); approved in dozens of countries.

What does the clinical evidence show for Thymosin Alpha-1?

In Phase 3 trials for chronic hepatitis B, Thymosin Alpha-1 produced sustained virological response rates of 25–40% as monotherapy and higher rates in combination with interferon — leading to approval as Zadaxin in dozens of countries. Thymosin alpha-1 has also been studied as an immunomodulator in sepsis, where the evidence is mixed. A 2025 systematic review and meta-analysis of 11 randomized controlled trials (1,927 patients) found a pooled reduction in 28-day mortality (odds ratio 0.73, 95% CI 0.59–0.90), but that signal disappeared in the higher-quality, multicenter trials.1 source The largest and most rigorous trial to date — the 1,106-patient TESTS phase 3 trial (2025), multicenter, double-blind and placebo-controlled — found no clear reduction in mortality or clinical improvement, and trial-sequential analysis concluded the overall evidence base is still too small to confirm efficacy.1 source Evidence level: Human RCTs (multiple, sepsis); pooled benefit not confirmed in the largest trial. Additional clinical research has explored cancer chemotherapy support (improving immune recovery), severe combined immunodeficiency adjunctive therapy, and septic shock immune modulation. The clinical evidence base is among the strongest for any thymic peptide. Evidence level: Human RCT (Phase 3, multiple indications); regulatory approval in dozens of countries.

How does Thymosin Alpha-1 compare to other immune peptides?

Thymosin Alpha-1 is the most clinically-developed immune-modulating peptide, with Phase 3 evidence and broad international approval distinguishing it from research-stage compounds in the same therapeutic space.

What are the documented side effects of Thymosin Alpha-1?

Thymosin Alpha-1's safety profile in Zadaxin clinical use is favorable — the most common side effects are mild injection-site reactions, occasional transient lymphocyte changes, and mild headache. Serious adverse events are rare. Long-term safety data from approved clinical use in dozens of countries provides one of the more robust safety bases for any immune-modulating peptide. Evidence level: Phase 3 clinical safety; post-market surveillance in dozens of countries.

What is the regulatory status of Thymosin Alpha-1?

Thymosin Alpha-1 (Zadaxin) is approved in dozens of countries for hepatitis B, hepatitis C, and related immune indications, but is not currently FDA-approved in the United States. In the US, thymosin alpha-1 is not FDA-approved for any use. On the FDA's bulk-substances page (updated April 2026) it is not on the active Category 2 "significant safety risks" list; instead it appears in the FDA's separate list of bulk substances whose compounding nomination was withdrawn by the nominator. A withdrawn nomination is not a safety clearance and does not authorize compounding — the FDA still notes potential immunogenicity and API-characterization concerns for compounded thymosin alpha-1 (as of August 2026).1 source Current US regulatory status in FDA peptide regulation 2025–2026 timeline.

Why does Thymosin Alpha-1 vendor verification matter even with international approval?

Because the international approvals apply to Zadaxin specifically — not to research-grade Thymosin Alpha-1 sold by third-party laboratories. Research-grade material is subject to wider purity variance, occasional underdosing, and inconsistent endotoxin control. Independent HPLC purity testing and mass spectrometry identity confirmation (expected MW 3,108.3 Da for the 28-amino-acid peptide) are essential.1 source Guidance in how-to-test-peptides hub.

How does Peptigrity verify Thymosin Alpha-1 vendors?

Peptigrity collects independent third-party Certificates of Analysis for Thymosin Alpha-1 from shops' official channels and verifies each for authenticity before publishing. Because shops choose which COAs to publish, results reflect the batches shops have made public rather than every production batch — a selection bias we state openly. A community-funded Testing Grant to purchase and test vials anonymously is in development. Tests in the database come from independent labs such as Janoshik Analytical, Freedom Diagnostics, and Chromate. The lab test table above this section shows the cross-shop dataset. Methodology in how we calculate trust scores.

Frequently Asked Questions

Is Thymosin Alpha-1 FDA-approved?

No, not currently in the United States, though it is approved in dozens of countries as Zadaxin. In the US it is not FDA-approved; the FDA lists thymosin alpha-1 among bulk substances whose compounding nomination was withdrawn by the nominator (FDA bulk-substances page, updated April 2026), which does not authorize compounding.1 source

Is Thymosin Alpha-1 legal to purchase?

Thymosin Alpha-1 is legal to purchase as a research chemical for laboratory use in most jurisdictions and is available by prescription as Zadaxin in countries where approved. The withdrawal of thymosin alpha-1's compounding nomination does not open a compounding pathway — it means the nominator pulled the request, and thymosin alpha-1 remains unauthorized for pharmacy compounding under 503A. Country breakdown in peptide legal status guide.

How is Thymosin Alpha-1 different from Thymalin?

Thymosin Alpha-1 is a defined single 28-amino-acid synthetic peptide with characterized receptor binding (TLR9, T-cell differentiation pathways); Thymalin is a multi-component polypeptide extract from bovine thymus tissue. Thymosin Alpha-1 has Phase 3 clinical evidence and international approval; Thymalin operates within the Khavinson bioregulator framework with Russian/CIS approval.

How can I verify a Thymosin Alpha-1 vendor is selling real product?

Independent third-party HPLC certificate of analysis from a lab the vendor does not own or pay, with mass spectrometry identity confirmation (expected MW 3,108.3 Da) and endotoxin testing. COA interpretation in red flags in peptide certificates of analysis.

Does Peptigrity recommend a Thymosin Alpha-1 dose?

No. Peptigrity is an independent review platform, not a medical authority. The approved Zadaxin dose for hepatitis B is 1.6 mg subcutaneously twice weekly under physician supervision. Anyone considering Thymosin Alpha-1 should consult a licensed physician.

Does Thymosin Alpha-1 show up on a drug test?

Thymosin Alpha-1 is not specifically prohibited by WADA but related immune-modulating peptides may face scrutiny in competitive sport. For drug-testing context across the peptide class, see do peptides show up on drug tests.

This section is for educational and informational purposes only and does not constitute medical advice. Thymosin Alpha-1 is approved in dozens of countries as Zadaxin but is not currently FDA-approved in the United States. Always consult a qualified healthcare provider before using any peptide or research compound. Peptigrity is an independent review platform and does not sell, endorse, or recommend specific products or vendors.