Thymosin Alpha-1 Peptide: Status, Evidence and Sellers

Thymosin alpha-1 is a peptide of 28 amino acids first isolated in 1977 from calf thymus, the gland where immune T cells mature [1]. A synthetic version, also called thymalfasin, was developed by a company that sold it as Zadaxin, and reports cited by FDA say it is approved in parts of Asia, Latin America, Eastern Europe and the Middle East, mostly for hepatitis B and as a vaccine booster. FDA could not independently verify those approvals [1].

In the United States it has no approval and is sold through wellness clinics and peptide sites for immune support. Its research record is far larger than most peptides on those menus: by one count FDA cites, more than 4,400 people have enrolled in its trials, in hepatitis, cancer, sepsis and other infections [1]. When FDA reviewed it in 2024 for 13 uses, it found insufficient evidence that it works for any of them [1], and its advisory committee voted 17 to 4 against adding either form to the list pharmacies may compound from [2].

It is not one of the six peptides in the October 28, 2026 HHS policy, and FDA's safety-risk page now lists it among withdrawn nominations [3].

Thymosin alpha-1 has been used abroad as an immune modulator, alone or with interferon, and sellers describe it as strengthening T cells and other immune cells, according to FDA's review [1]. That is the basis for pairing it with hepatitis treatment and vaccines in the countries where it is sold, and for testing it in sepsis, where the immune system can be overactive and exhausted at once.

The same action is the main safety question. In people whose immune systems are deliberately suppressed, such as stem cell transplant recipients, FDA noted it could in theory worsen graft-versus-host disease or contribute to graft failure [1].

The best trial is recent and clear. In 2025 a phase 3 trial at 22 hospitals in China randomized 1,106 adults with sepsis to thymosin alpha-1 or placebo, injected every 12 hours for seven days. Deaths at 28 days were 23.4% on the peptide and 24.1% on placebo, a hazard ratio of 0.99, and no secondary or safety outcome differed [4].

For chronic hepatitis B, a 2026 Cochrane review pooled 10 randomized trials with 1,349 participants. The pooled numbers leaned toward fewer deaths and fewer serious adverse events, but the reviewers rated the certainty very low or low for every outcome and concluded they were not sure it reduces deaths or does anything else [5]. FDA reached a similar view across all 13 uses it reviewed, pointing to small trials, design flaws and comparisons against treatments that are now out of date [1].

None of those trials tested what US buyers are usually sold it for: general immune support in healthy adults, long COVID or Lyme disease. Grade C: a large literature, a null result from its largest trial, and no trial of the uses it is marketed for. Our evidence review goes through the hepatitis, sepsis and COVID-19 studies.

No dose of thymosin alpha-1 is approved in the United States. The 2025 sepsis trial gave it under the skin every 12 hours for seven days [4], and the hepatitis B trials in the Cochrane review used a range of doses and schedules [5]. None of these regimens was tested for general immune support, and this page does not repeat clinic protocols.

FDA also questioned whether pharmacies could make a stable injectable at the strength nominators proposed, because of the peptide's limited solubility in water [1].

Trials abroad generally describe it as well tolerated, with injection-site discomfort among the reported complaints, and FDA's adverse-event database held one applicable US report through June 10, 2024: a man on peginterferon and thymosin alpha-1 in a hepatitis C trial who was hospitalized with atrial fibrillation and anxiety, which FDA noted could also be explained by peginterferon [1]. FDA's overall judgment is that the safety information is inadequate to understand the extent of any safety issues [3].

People with autoimmune disease, transplant recipients and anyone on immune-suppressing drugs should treat an immune stimulant with particular caution and raise it with the doctor managing that condition.

Thymosin alpha-1 is not named on the World Anti-Doping Agency's 2026 Prohibited List. Thymosin beta-4 and its fragment TB-500 are a different substance and are banned [6].