Thymosin Alpha-1

Thymosin Alpha-1 (also called thymalfasin, and sold overseas as Zadaxin) is one of the more genuinely studied peptides you’ll find for sale. But being well-studied doesn’t mean it does everything wellness clinics say it does.

What it is

Thymosin Alpha-1 is a lab-made copy of a small peptide — a short chain of 28 amino acids (the building blocks of proteins) — that was first found in the thymus, the gland that helps train your immune system’s T cells. It works as an immune modulator (something that fine-tunes the immune system rather than just cranking it up across the board). In lab studies it gently encourages T cells to mature and helps with signaling between immune cells. It’s given as a subcutaneous (under-the-skin) injection. And unlike a lot of so-called “research peptides,” it has actually been through real, large human trials over several decades.

The claims

Clinics and sellers pitch it for all sorts of things: “immune support,” long-running infections, faster recovery, as an add-on to cancer treatment, Lyme disease, autoimmune conditions, and general anti-aging. The honest answer is a lot narrower than that wish list.

What the evidence actually shows

The strongest human evidence is for chronic hepatitis B (a long-term liver infection). Several randomized controlled trials (studies where patients are randomly assigned to treatments, the gold standard for fairness) and meta-analyses (studies that pool the results of many trials) found it lowers the amount of virus in the body and brings liver enzymes back to normal about as well as older interferon therapy, while being easier to tolerate. The catch: these trials were small (one representative meta-analysis combined just four trials and about 200 patients), mostly enrolled a particular group of patients (HBeAg-negative), came largely from China, and were compared against interferon rather than the antiviral drugs doctors actually use today. So it’s approved abroad based on this older evidence, not on modern head-to-head comparisons.

In sepsis (a life-threatening, body-wide reaction to infection), the story is more of a warning. Early meta-analyses hinted at fewer deaths, but they pooled together small, lower-quality trials. For example, the 2013 ETASS trial was only single-blind (meaning patients didn’t know their treatment, but the researchers did, which can bias results) and its benefit just barely cleared the bar for significance. The biggest and best-built study so far, the TESTS trial (published in BMJ in 2025, with 1,106 patients enrolled and 1,089 in the main analysis, double-blind and placebo-controlled — the strongest study design), found no drop in deaths at 28 days (hazard ratio 0.99, p=0.93). A look at smaller subgroups even hinted at possible harm in younger and non-diabetic patients — though subgroup findings are just exploratory clues, not firm conclusions.

For COVID-19, one study seemed to show fewer deaths, but that benefit vanished once researchers accounted for how sick the patients already were. And for cancer add-on use, Lyme, autoimmune disease, and general “immune support” in healthy people, there’s no solid randomized human evidence at all — the marketing is way out ahead of the science.

Legal and regulatory status

Thymosin Alpha-1 is not approved by the FDA for any use, and never has been, even though US hepatitis trials were run decades ago. It does hold orphan-drug designations (a special status for treatments aimed at rare diseases, including liver cancer), but that is not the same as approval. It is approved in roughly 35 other countries as Zadaxin for hepatitis B.

In the US, you can only get it through compounding pharmacies (which custom-make medications), and even that has been a moving target. The FDA put it on, then took it off, its problematic “Category 2” bulk-substances list, and a December 2024 advisory-committee (PCAC) review is now feeding into rules still being written. As of mid-2026, whether it can be compounded at all is still up in the air — so check the current FDA 503A bulk drug substances list yourself rather than trusting a seller’s claim. A note for athletes: Thymosin Alpha-1 is not on the WADA Prohibited List — unlike the unrelated Thymosin Beta-4 / TB-500, which is banned.

Safety

Across decades of trials in seriously ill patients, its track record is clean: the main reported effects are mild, short-lived reactions at the injection site, plus the occasional headache or flu-like feeling. The big caveat is where that safety data comes from — almost entirely from supervised use in people with serious illness, not from healthy adults injecting themselves for “longevity.” In that healthy group, the long-term effects of nudging your immune signaling simply haven’t been studied. None of this is medical advice.

Bottom line

This is a real drug with real but uneven human evidence: reasonably backed up in hepatitis B, a flat negative result in the best sepsis trial, and unproven for the immune-boosting and anti-aging uses it’s marketed for.

Evidence grade: 8/10 · Good.

Sources

• Liu et al. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. BMJ, 2025.
• Liu et al. The efficacy of thymosin α1 as immunomodulatory treatment for sepsis: a systematic review of randomized controlled trials. BMC Infectious Diseases, 2016.
• Wu et al. The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial. Critical Care, 2013.
• Yang et al. Comparison of the efficacy of thymosin alpha-1 and interferon alpha in the treatment of chronic hepatitis B: a meta-analysis. Antiviral Research, 2008.
• Liaw. Thymalfasin (thymosin-alpha 1) therapy in patients with chronic hepatitis B. Journal of Gastroenterology and Hepatology, 2004.
• The effect of thymosin α1 on mortality of critical COVID-19 patients: a multicenter retrospective study. International Immunopharmacology, 2020.
• FDA. December 4, 2024 Meeting of the Pharmacy Compounding Advisory Committee (thymosin alpha-1 bulk drug substances).
• World Anti-Doping Agency. The Prohibited List (2026) — Thymosin-β4 and derivatives listed under S2; Thymosin Alpha-1 not listed.