Thymosin alpha-1 is a 28-amino-acid immune peptide sequenced from calf thymus in 1977; its synthetic drug form, thymalfasin, is sold as Zadaxin.
Goldstein and colleagues reported its sequence in 1977 and identified it as one of several peptides in thymosin fraction 5, a calf-thymus extract (PNAS, 1977). A 2020 review in the World Journal of Virology describes thymosin alpha-1 as a Toll-like receptor 2 and 9 agonist on myeloid and dendritic cells that promotes the maturation of T-cell progenitors (Dominari et al., 2020). That immune-signaling role is why it has been tested in hepatitis, cancer and severe infection.
The largest sepsis trial is negative. TESTS, a double-blind phase 3 trial at 22 centers in China, randomized 1,106 adults with sepsis between 2016 and 2020. Twenty-eight-day mortality was 23.4% with thymosin alpha-1 and 24.1% with placebo, with no significant difference (BMJ, 2025). An earlier 361-patient trial, ETASS, had reported a borderline result (Critical Care, 2013) that the larger trial did not confirm.
No. Thymalfasin has no FDA approval, although a 2020 review reports it is approved in more than 35 countries, including for hepatitis B and C.
FDA placed thymosin alpha-1 in Category 2 of its interim policy for 503A pharmacy compounding, the group of nominated substances with significant safety risks. FDA’s summary cites possible immunogenicity for some routes, peptide-related impurities and difficulty characterizing the active ingredient, and calls the safety information inadequate (FDA). The nominators later withdrew the nomination, and FDA’s May 2026 list does not place thymosin alpha-1 in any of its three categories (FDA 503A categories).
Testing matters here because FDA’s stated concerns are about the molecule as manufactured: impurities from peptide synthesis and an active ingredient that is hard to characterize. A research-market vial is not covered by any approval granted to Zadaxin, and its label is the seller’s claim rather than a measurement. A third-party identity and purity result is the only independent check on a specific batch.
Before trusting any thymosin alpha-1 label, check that batch’s published result in our latest tests or in Janoshik's public results database. No results are published for thymosin alpha-1 yet in our index. When the first published result arrives, a vendor leaderboard appears above.