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Thymosin Alpha-1
Approved in more than 30 countries — not including the US — as an immune-modulating drug for chronic hepatitis B. Its largest, most recent test in a different use, sepsis, found no benefit at all.
In brief
Should you care? Yes, especially if you've heard it discussed for sepsis or general immune support — its single largest, most rigorous recent trial found no mortality benefit at all.The short version
- Genuinely approved in 35+ countries for chronic hepatitis B, an older and different indication.
- Not approved in the US for any use.
- A 1,106-patient sepsis trial found no benefit — worth as much weight as any positive finding.
What it is
Thymosin alpha-1 (Tα1) is a 28-amino-acid thymic peptide, marketed abroad as Zadaxin, that modulates immune signaling through toll-like receptor pathways.
Approved abroad, for a specific use
Thymosin alpha-1 is approved in more than 30 countries — not including the United States — for chronic hepatitis B, typically as an adjunct to other antiviral treatment. It is not FDA-approved for any indication in the US.
The sepsis trial evidence
An earlier, smaller Chinese trial (ETASS) reported improved outcomes in severe sepsis. But the far larger and more rigorous TESTS trial — 1,106 adults across 22 centres in China, randomized, double-blind, placebo-controlled, published in 2025 — found no significant reduction in 28-day mortality: 23.4% in the thymosin alpha-1 group versus 24.1% on placebo (hazard ratio 0.99, p=0.93) (PMID 39814420). That negative result, from the larger and more recent trial, deserves at least as much weight as the earlier positive signal, not less.
A bigger, later, negative trial should carry more weight than a smaller, earlier positive one
This is a general pattern worth watching for in any supplement or peptide claim: an early small trial reporting benefit, followed by a larger, better-controlled trial that doesn't replicate it.
US legal status
Thymosin alpha-1 sits on FDA's Category 2 restricted-compounding list in the US, reviewed by the same Pharmacy Compounding Advisory Committee process covered on BPC-157's legal status page. Selling it in the US outside that process is the same unapproved-drug situation as BPC-157 or TB-500, regardless of its real approval status elsewhere.
References
- The TESTS Trial Investigators, "The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial," BMJ 2025, PMID 39814420.
- ETASS Study Group, "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.