An immune-modulating peptide, sold as Zadaxin in some countries for chronic hepatitis B and studied in sepsis, COVID-19, vaccine response, and cancer. It has never been approved in the United States, and a 2025 trial of 1106 sepsis patients found no survival benefit. The FDA has placed compounded thymosin alpha-1 among substances with significant safety concerns.
Thymosin alpha-1 is a synthetic copy of a peptide that the thymus gland produces. It is believed to act on the immune system by strengthening T-cell function, pushing immature thymocytes to differentiate or converting them into active T cells. Because it shifts broad immune signaling rather than hitting one target, it has been tried wherever immune function is depressed, from viral hepatitis to sepsis to cancer. That breadth is also why its results vary so much from one disease to the next.
Injected under skin (1/7)
A copy of a thymus peptide, thymosin alpha-1 is injected under the skin.
It is absorbed quickly after injection, peaking within 2 hours, and is cleared with a half-life of about 2 hours, so blood levels return to baseline within a day. In severe COVID-19 patients it raised T-cell counts in people with severe lymphocytopenia and reduced markers of T-cell exhaustion on CD8 cells, in parallel with a rise in markers of thymus output. In sepsis it improved monocyte HLA-DR expression, a marker of immune competence, by 3.9% at day 3 and 5.8% at day 7 relative to controls. Moving those immune markers did not translate into fewer deaths in the large sepsis trial.
Thymosin alpha-1 is sold online and by compounders as an immune booster for infections, long COVID, autoimmune disease, and cancer support. As the drug thymalfasin, it is approved in Singapore and some other countries for chronic hepatitis B, where a 26-week course cleared hepatitis B virus DNA and e antigen in 40.6% of patients against 9.4% of untreated controls when assessed 18 months after entry. It has also been tested as an add-on in severe sepsis, severe COVID-19, influenza vaccination in older adults, and several cancers. None of those uses is approved in the United States.
In trials it is injected under the skin, typically 1.6 mg twice a week for hepatitis B or every 12 hours for a week in sepsis. The sepsis story is cautionary: a 2013 trial of 361 patients hinted at lower 28-day mortality, 26.0% versus 35.0%, but the definitive 2025 TESTS trial of 1106 patients found 23.4% mortality with thymosin alpha-1 and 24.1% with placebo, no difference at all. Products sold in the United States are compounded or imported and are not the Zadaxin product studied in trials. The FDA reviewed thymosin alpha-1 for compounding in 2024 and lists it among substances that may present significant safety risks.
Human evidence is real but uneven across more than 30 trials and thousands of patients. For chronic hepatitis B, a randomized trial of 98 patients found delayed virological responses and histological improvement with a 26-week course. For sepsis, the 2013 ETASS trial of 361 patients was followed by the 2025 TESTS trial of 1106 patients, which showed no mortality benefit. A retrospective series of 76 severe COVID-19 patients reported lower mortality with treatment, 11.11% versus 30.00%, and older trials examined influenza vaccine responses in the elderly and cancer combinations.
The biggest question is which findings survive modern standards: the hepatitis trials predate current antivirals, the COVID data are retrospective, and the one large blinded trial was negative. The 2024 comprehensive review that calls the FDA's restriction unfounded is a narrative review rather than a meta-analysis. Compounded and imported products have not been shown to match the material used in trials. Whether any immune-marker change produces a clinical benefit in a given patient remains unproven outside hepatitis B.
In the hepatitis B trial no significant side effects were observed, and a 2024 review of more than 30 trials describes it as well tolerated. In the TESTS sepsis trial no safety outcome differed between thymosin alpha-1 and placebo, though a prespecified subgroup analysis found higher mortality in patients under 60, a signal the authors flagged for interpretation. A transient rise in liver enzymes, called a flare, can occur during hepatitis B treatment according to the Singapore label. The FDA's compounding review raises immunogenicity and impurity concerns for compounded products that are separate from the trial drug.
No regulator has approved this compound, and no company is seeking approval.
Thymosin alpha-1 has never been approved by the FDA. As thymalfasin, sold as Zadaxin, it is a prescription-only medicine in Singapore for chronic hepatitis B. The FDA's Pharmacy Compounding Advisory Committee reviewed thymosin alpha-1 related bulk substances in 2024, and the agency lists compounded thymosin alpha-1 among substances that may present significant safety risks because of immunogenicity, impurity, and characterization concerns. Products sold in the United States are therefore compounded or imported without approval.