Thymosin Alpha-1: Approval Status, Trials, and Safety

Thymosin alpha-1 is a peptide first isolated from thymic tissue, where researchers identified it as the compound that restored immune function in mice whose thymus had been removed. It is studied as an immune modulator.

Despite the similar name, it is a different substance from thymosin beta-4 and from TB-500, a fragment of thymosin beta-4. FDA and WADA list them separately.

A 2016 review describes thymosin alpha-1 acting through Toll-like receptors in dendritic cells, immune cells that help coordinate immune responses. This signaling can influence immune-cell activity and cytokine production. Both of the review's authors were affiliated with SciClone Pharmaceuticals, so it is not an independent review.

These biological effects explain research interest, but laboratory and immune-marker findings do not establish a clinical benefit.

Sepsis: the TESTS phase 3 trial randomized 1,106 adults with sepsis at 22 centres in China to thymosin alpha-1 or placebo. It found no clear difference in 28-day all-cause mortality, and no secondary or safety outcome differed significantly between groups. The trial received support from SciClone Pharmaceuticals, according to its competing-interests statement. A prespecified subgroup analysis suggested effects differed by age and diabetes, with higher mortality on thymosin alpha-1 in participants under 60; subgroup results like these are exploratory. A 2025 meta-analysis of 11 sepsis trials, by a team that overlaps with the TESTS investigators, reported a pooled mortality signal, but its higher-quality and multicentre subgroups did not show a mortality benefit, and its authors judged the total sample size inadequate.

Chronic hepatitis B: a September 2026 Cochrane review examined 10 randomized trials with 1,349 participants, published from 1991 to 2018. Certainty of evidence was low for serious adverse events and very low for every other outcome. Pooled estimates pointed toward fewer deaths and serious adverse events, but at this certainty the authors were not sure of effects on mortality, quality of life and liver-related outcomes. Four trials were industry funded, and sixteen more studies awaited assessment because reporting was incomplete.

These findings come from hospital sepsis and hepatitis B populations. They cannot establish benefits for general wellness or other conditions.

In the TESTS sepsis trial, no safety outcome differed significantly between the thymosin alpha-1 and placebo groups. In the Cochrane hepatitis B review, the certainty of evidence for serious adverse events was low, and for non-serious adverse events very low, so the safety picture is uncertain.

For compounded products, FDA says thymosin alpha-1 may pose a significant risk of immune reactions against the peptide (immunogenicity) for some routes, may raise problems with peptide-related impurities, and has safety information too limited to understand the extent of any risks.

Thymosin alpha-1 is not currently approved by the FDA, and it is not approved as a therapeutic by Health Canada. It appears on FDA's list of bulk drug substances that may present significant safety risks. WADA does not name thymosin alpha-1 on its Prohibited List, although the list does name thymosin beta-4 and TB-500 under S2. Several sections of the list say they include substances that are not named.