Thymosin Alpha-1 is the immune peptide. It is sold as an “immune support” staple and shows up in long COVID and recovery stacks. Unlike most gray-market peptides, it is a real approved medicine in dozens of countries, under the brand Zadaxin. It has never been approved in the US, and its biggest recent trial came back empty.
Mixed, and weaker than the reputation. There are human trials, which is more than many peptides can say. The hepatitis B data shows a modest, delayed benefit in small, older trials. The largest and best trial, a 2025 sepsis study of over 1,100 patients, found no effect on survival. The COVID data is mostly weak observational work. There is no trial evidence for long COVID or general immune support.
Thymosin Alpha-1, also called thymalfasin or Ta1, is a chain of 28 amino acids. It was first isolated in 1977 by Allan Goldstein’s group from a calf thymus extract. The thymus is the gland where T cells, a key part of your immune system, mature. Today it is made synthetically.
Do not confuse it with Thymosin Beta-4, the protein behind TB-500. Same “thymosin” family name, completely different molecule.
After early US research, a company called SciClone developed it as Zadaxin. China approved it in 1995, and it is now approved across parts of Asia, Latin America, Eastern Europe and the Middle East, mostly for hepatitis B and as a vaccine booster. In Italy it is approved only as a flu vaccine booster for people with weak immune systems, not for hepatitis.
In the US, SciClone ran Phase 3 trials for hepatitis C. They missed their goals, and it was never approved.
It appears to act on toll-like receptors, the immune system’s early warning sensors, on dendritic cells. Dendritic cells are the immune system’s scouts. Activating them helps push immature cells to become working T cells and B cells. Supporters also argue it calms overactive immune responses, making it a modulator rather than just a stimulant. The FDA noted there is no clear dose-response relationship, and much of the mechanism work comes from researchers connected to the manufacturer.
Hepatitis B. A 2001 meta-analysis of five small trials found no benefit at the end of treatment but a delayed benefit a year later. Adding it to an older antiviral boosted response in another analysis. These trials are small, mostly Asian, and predate today’s standard antivirals.
Hepatitis C. Negative. Modern direct-acting antivirals have made it irrelevant there.
Sepsis. This is the big one. The TESTS trial, published in the BMJ in 2025, randomized 1,106 patients across 22 Chinese hospitals. Death at 28 days was 23.4% on Thymosin Alpha-1 versus 24.1% on placebo. No difference on any secondary outcome either. Earlier smaller studies had looked positive. The best trial did not.
COVID-19. The positive signals come from early 2020 Chinese studies that looked back at records, which are easily skewed. Later studies found no benefit. We found no completed randomized trial showing benefit.
Cancer. A 2010 melanoma trial found a survival difference that did not reach statistical significance.
General immune support, long COVID, chronic fatigue, Lyme disease, frequent infections, and “immune stacks” alongside other peptides. We found no randomized trial evidence for any of these.
In trials it is generally well tolerated, with injection-site discomfort as the main effect and rare rash or joint pain. The FDA noted some signals worth knowing: liver enzyme flares in hepatitis B patients, thyroid hormone shifts when combined with interferon, and one fatal immune reaction in a stem cell transplant patient where the cause was unclear.
Because it acts on the immune system, people with autoimmune disease, transplant recipients on immune-suppressing drugs, and pregnant women have real reasons for caution. Labels abroad reflect that. Gray-market vials add the usual purity and sterility risks.
From labels abroad, not a recommendation. Zadaxin is 1.6 mg injected under the skin twice a week, three to four days apart. Community use usually copies that, or uses daily dosing in short cycles. We do not give mixing or injection steps.
Not FDA-approved. The FDA placed it in Category 2 in 2023, and it was removed in September 2024 when the nomination was withdrawn. The FDA then reviewed it anyway. At a December 4, 2024 Pharmacy Compounding Advisory Committee meeting, the panel voted 17 to 4 against adding it to the compounding list, citing no compelling evidence it works. So it cannot lawfully be compounded by 503A pharmacies.
It was not part of the July 2026 review, where six other peptides were recommended. You can read about that vote in our post on what happens after the FDA panel vote.
Its WADA status is not clear. It is not named on the list, and it may not fall under the non-approved category since it is approved abroad. Athletes should check with their anti-doping agency.
Is Thymosin Alpha-1 FDA-approved? No. It is approved in other countries, but not the US.
Is it the same as TB-500? No. TB-500 comes from Thymosin Beta-4, a different peptide.
Does it help with COVID or long COVID? Not shown in randomized trials.
Did it work for sepsis? The largest trial, published in 2025, found no survival benefit.
Can a US pharmacy compound it? Not under 503A. An FDA advisory panel voted 17 to 4 against it in 2024.
Is it safe? Generally well tolerated in trials, but it is an immune drug. Autoimmune disease, transplants and pregnancy call for caution.
Why is it approved elsewhere but not here? Different regulatory standards, and its US trials failed.
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