Evidence
BLimited human data
Approved in 37+ Countries — Key Fact
Zadaxin (thymalfasin) is approved and commercially distributed in more than 37 countries including China, Italy, Singapore, Philippines, Peru, Colombia, and others. Approved indications by market: chronic hepatitis B; chronic hepatitis C (adjunct); cancer immunotherapy adjuvant (China, Italy — including melanoma adjuvant in Italy); immunodeficiency states. NOT FDA-approved in the United States. Available via compounding pharmacies in the US with shifting regulatory status (see Section 8 for the full 2023-2026 US regulatory timeline).
Not an Immunostimulant — An Immunomodulator
The most important mechanistic distinction. Tα1 does not uniformly stimulate all immune activity — it calibrates immune activity bidirectionally. It promotes Th1-type immunity and DC maturation (pro-immune) while simultaneously activating IDO (indoleamine 2,3-dioxygenase) in dendritic cells, which drives tryptophan catabolism and Treg generation (tolerogenic). The net effect: enhanced protective immunity against pathogens and tumors, with calibrated suppression of excessive inflammatory or autoimmune responses. This bidirectional profile is why Tα1 has been studied for conditions involving immune exhaustion (viral hepatitis, cancer) AND for sepsis (where the problem is often immune dysregulation, not simple suppression).
The Hepatitis B Evidence — The Strongest Indication
The most extensive and highest-quality clinical evidence for any thymic peptide in history: Meta-analysis of 5 RCTs (n=425): HBeAg seroconversion rate ratio 2.31 (95% CI 1.52-3.51, p<0.001). Meta-analysis of 8 RCTs (n=898): sustained virological response OR 2.67 (95% CI 1.76-4.05). Combination with interferon-alpha (Chien et al.): 50% sustained response vs 25% IFN alone vs 13% placebo. Standard of care for HBV in China and several Asian markets. Grade A evidence in the HBV indication.
The Sepsis Story — Mixed then Negative
ETASS Trial 2013 (n=361): Tα1 reduced 28-day mortality 26.0% vs 35.0% (p=0.049) — a positive signal. Community and clinical enthusiasm followed. TESTS Trial 2025 (Wu et al., BMJ, n=1,106): the definitive Phase 3 sepsis trial. Result: 28-day mortality 23.4% (Tα1) vs 24.1% (placebo), HR 0.99 (p=0.93). Primary endpoint not met. The sepsis indication must be considered unestablished by Phase 3 evidence as of 2025. Subgroup analyses suggest possible benefit in immunosuppressed subpopulations — but that requires confirmation.
US Regulatory Timeline — 2023-2026
Sept 2023: FDA places Tα1 on Category 2 (restricted compounding list) alongside 18 other peptides. Sept 27, 2024: Nomination withdrawn by nominator → Tα1 removed from Category 2 → referred to PCAC for formal review. Feb 27, 2026: HHS Secretary Kennedy announces ~14 of 19 restricted peptides returning to Category 1, including Tα1. April 23, 2026: Formal removal from Category 2 effective. July 23-24, 2026: PCAC meeting scheduled to review Tα1 for Category 1 inclusion. Current status (May 2026): removed from Category 2; pending formal Category 1 confirmation at July PCAC meeting.
WADA Status
Not listed on the WADA Prohibited List. Tα1 is an endogenous peptide with immunomodulatory function — it does not provide ergogenic benefit in the performance enhancement sense. Not banned for competitive athletes. No detection concern for tested athletes.