Thymosin Alpha-1What the studies show

Q: What kind of evidence exists for thymosin alpha-1 (thymalfasin)?

Large human trials. Randomized human trials of hundreds of patients each for hepatitis B, sepsis and melanoma, and marketing approval in some countries for hepatitis B; the sepsis and cancer results are suggestive but unsettled, and there is nothing in healthy people.

A 28-amino-acid thymus peptide that is an approved hepatitis B drug in China and Italy, tested in sepsis and cancer trials with unsettled results.

6 min read · Large human trials · Updated November 2026

In short

Name: thymosin alpha-1 (thymalfasin)
Evidence: Large human trials
Bottom line: Randomized human trials of hundreds of patients each for hepatitis B, sepsis and melanoma, and marketing approval in some countries for hepatitis B; the sepsis and cancer results are suggestive but unsettled, and there is nothing in healthy people.

Before you read this

This guide explains what published studies found. That is all it is.

It is not medical advice and it is not instructions for using anything. Where a dose appears, we are reporting what a named study gave the people or animals in it — a fact about the study, not an instruction to you.

The compounds here are not approved by the FDA for use in people and are sold for laboratory research only. A study tells you what happened to the subjects in it; it does not tell you what would happen to you. Talk to a doctor first.

What it is

Thymosin alpha-1 is a 28-amino-acid peptide first isolated in 1977 from thymosin fraction 5, a calf thymus extract that Allan Goldstein's laboratory had spent a decade separating into its active parts; it was the most immunologically active piece they found ¹. The thymus is where T cells mature, and thymosin alpha-1 is made there. The synthetic version is called thymalfasin. Its proposed job is to act on toll-like receptors on dendritic cells and other immune cells, pushing T cells to mature and steering the immune response toward a controlled, infection-fighting pattern rather than an exaggerated one ². Unlike most research peptides, this one is a marketed medicine: under the brand name Zadaxin it is approved in more than 30 countries, including China and Italy, for chronic hepatitis B and as an addition to vaccines in people with weak immune responses. It is not approved in the United States ².

The rest of this guide is free

Unlock the Thymosin Alpha-1 guide — and every other one

What the studies found, what they didn't, every source linked. Enter your email once; the link we send opens all of them on this device, plus one plain-language study breakdown a week.

Hepatitis B

The approvals rest on trials from the 1990s. In a 1998 randomized trial in Taiwan, 98 patients with chronic hepatitis B received thymosin alpha-1 at 1.6 mg by injection twice a week for six months, or no treatment. Eighteen months after the start, 40.6% of the treated group had cleared the virus's e-antigen and DNA from their blood with normal liver enzymes, against 9.4% of the controls ³. The response built slowly, often after the injections had stopped, which the authors took as a sign that the peptide was working through the immune system rather than on the virus directly ³. These trials predate today's antiviral tablets, which suppress the virus far more reliably, and that is why thymosin alpha-1 is now a minor player in hepatitis treatment even where it is approved.

Sepsis

The largest modern trial is in sepsis. The ETASS trial, run in Chinese intensive care units and published in 2013, randomized 361 patients with severe sepsis to thymosin alpha-1 injections or usual care. Twenty-eight-day mortality was 26.0% with the peptide and 35.0% without — a nine-point gap that just missed statistical significance (p = 0.062) ⁴. A difference that size would matter a great deal if real, but a trial of 361 people cannot tell it from chance, and the trial was single-blind, so a larger, fully blinded trial was needed to settle it.

Cancer

Thymosin alpha-1 was also tested as an immune booster alongside chemotherapy. In a 2010 trial, 488 patients with metastatic melanoma were randomized to dacarbazine with or without thymosin alpha-1, with or without interferon. The groups that received the peptide had more tumour responses, and the authors reported a trend toward longer survival that did not reach significance; they called for a phase 3 trial, which has not been reported ⁵.

COVID-19, and the limits of the data

During the pandemic a Chinese hospital group reported that among 76 patients with severe COVID-19, those given thymosin alpha-1 had lower mortality (11.1% versus 30.0%) and recovered their T-cell counts, especially patients whose counts had fallen very low ⁶. That study was retrospective: doctors chose who got the drug, so sicker or healthier patients may have been selected, and it cannot show cause. It is a fair example of the whole literature — real signals in real patients, with the decisive trials either small, single-blind or not yet done.

What does not exist is any trial of thymosin alpha-1 in healthy people for general immune support. Its approvals are for specific diseases in specific patients, and the evidence is about those patients.

What these studies are, and aren't

Large human trials. Randomized human trials of hundreds of patients each for hepatitis B, sepsis and melanoma, and marketing approval in some countries for hepatitis B; the sepsis and cancer results are suggestive but unsettled, and there is nothing in healthy people.