Thymosin Alpha-1: What It Is, Where to Get It Prescribed, and What It Costs

Thymosin alpha-1 is a 28-amino-acid peptide with an acetyl group on its N-terminus. The body cuts it out of a larger precursor called prothymosin alpha, and it was first isolated in 1977 from a calf thymus preparation known as thymosin fraction 5. The thymus is where T cells mature, and that origin is the whole story: everything studied since has been about what it does to immune cells.

It is not a hormone, not a steroid and not a growth factor. It is a signalling peptide that changes how certain immune cells respond, not a drug that forces a single outcome.

The international non-proprietary name is thymalfasin. The synthetic version has been sold since the 1990s as Zadaxin, a 1.6 mg subcutaneous injection developed by SciClone Pharmaceuticals. That is unusual: most compounded peptides have never been through a drug approval process anywhere, whereas thymalfasin is approved in a number of countries, principally in the Asia-Pacific region, Latin America, Eastern Europe and the Middle East, and in Italy, mainly for hepatitis B and as a vaccine adjuvant. The FDA's 2024 briefing notes it is not approved in the United States, Japan, or Europe other than Italy, and that the agency could not independently verify every approval the manufacturer lists. A compounded thymosin alpha-1 prescribed in the US is not Zadaxin and carries none of those approvals.

It is not an antibiotic or a direct antiviral; every mechanism described runs through the immune system. It is not an immunosuppressant. And it is not thymosin beta-4 or its fragment TB-500, despite the shared name; the two are confused constantly and are not interchangeable.

The most-cited mechanism is action on Toll-like receptors, particularly TLR9 and TLR2, on dendritic cells, the cells that decide what the rest of the immune system responds to. In mouse and cell work this is described as increasing cytokine production and antigen presentation, which is why the compound has been studied as a vaccine adjuvant and in chronic viral infection.

The original thymic-factor work describes thymosin alpha-1 promoting maturation of immature T cells into functional CD4 and CD8 populations, with increased interleukin-2 and IL-2 receptor expression. Some of this has been observed in people: a 2020 report on severe COVID-19 measured a rise in circulating CD4 and CD8 T cells in treated patients.

Cell and animal studies describe increased natural killer cell activity and increased MHC class I expression on tumour cells, which makes them easier for the immune system to recognise. This is the rationale for the oncology-adjunct trials.

A 2010 review in the Annals of the New York Academy of Sciences is titled "the regulator of regulators" for a reason. In mouse work the compound is described as inducing indoleamine 2,3-dioxygenase in dendritic cells, which pushes the response toward regulatory T cells and tolerance rather than inflammation. The picture is of a peptide that sharpens immune recognition where it is deficient while restraining runaway inflammation.

This is the indication behind most of the international approvals. A 1998 randomised controlled trial in Hepatology allocated 98 patients to a 26-week course, a 52-week course, or no treatment. Complete virological response at 18 months was reported in 40.6 percent, 26.5 percent and 9.4 percent respectively, with responses accumulating after the course finished rather than during it.

The most current synthesis is a 2026 Cochrane review of ten randomised trials and 1,349 participants. It reported that thymosin alpha-1 may reduce all-cause mortality and serious adverse events compared with control, but graded the certainty as very low for almost every outcome. That is the honest state of the hepatitis B evidence: consistent signals across small trials, not settled proof, in a disease where oral antivirals have since changed the standard of care.

A 2012 trial in the Journal of Viral Hepatitis randomised 552 non-responders to peginterferon alfa-2a plus ribavirin with either thymosin alpha-1 or placebo for 48 weeks. Sustained viral response was not different between arms (12.7 percent against 10.5 percent), and direct-acting antivirals have since made the question largely historical.

Two randomised trials, both in China, tell a two-part story. The 2013 ETASS trial in Critical Care randomised 361 patients with severe sepsis and reported 28-day mortality of 26.0 percent with thymosin alpha-1 against 35.0 percent with conventional care, a difference of marginal significance. The follow-up, TESTS, published in the BMJ in 2025, was a multicentre, double-blind, placebo-controlled phase 3 trial of 1,106 adults with sepsis: 28-day mortality was 23.4 percent against 24.1 percent, and the trial found no clear evidence that thymosin alpha-1 reduces mortality. It is the largest and best-designed trial of the compound in any indication.

A 2010 trial in the Journal of Clinical Oncology randomised 488 patients with metastatic melanoma to dacarbazine chemotherapy with or without thymosin alpha-1. Tumour responses were reported in 10 and 12 patients in the thymosin alpha-1 groups against four in the control group, and median survival was 9.4 months against 6.6 months, a difference that did not reach statistical significance (P = 0.08). The authors described a signal warranting further evaluation.

Trials from the 1980s and 1990s studied thymosin alpha-1 alongside influenza vaccination in older adults and haemodialysis patients, reporting higher antibody responses in some; the FDA's review noted those vaccines are not the ones used in the US today and found the evidence insufficient. The 2020 Clinical Infectious Diseases paper was a retrospective review of 76 severe COVID-19 patients in Wuhan reporting lower mortality in the treated group (11.1 percent against 30.0 percent); later randomised work did not confirm a mortality benefit, and a retrospective series is the weakest kind of comparative evidence.

Randomised controlled trials exist for hepatitis B, hepatitis C, sepsis, melanoma and vaccine response, spanning more than thirty years and, in the 2025 sepsis trial, over a thousand patients. That is far more human evidence than exists for almost any other compounded peptide: a compound that is consistently well tolerated, that measurably changes immune-cell counts in people, and that has produced positive signals in hepatitis B and melanoma but a clear negative primary result in its largest trial.

None of it was generated in the population that typically asks about compounded thymosin alpha-1. There are no randomised controlled trials of thymosin alpha-1 for immune support in otherwise healthy adults, for frequent infections, for long COVID or for chronic fatigue. That use rests on clinical judgement, not on the trials.

• It does not establish a benefit in healthy adults or in general immune support. That trial has not been run.
• It does not establish a mortality benefit in sepsis, or efficacy as a cancer treatment; the largest trial found no mortality benefit and the oncology results were signals, not significant differences.
• It does not establish a dose or duration for any off-label use. The branded product's schedule (1.6 mg twice weekly) is not the compounded direction; a physician sets that.
• It does not establish anything as an FDA-approved treatment. It is not approved for any use in the United States.

Thymosin alpha-1 does not produce a sensation. Noticing nothing in the first weeks is normal rather than a sign of a problem.

This is the span of a single vial at the Monday-to-Friday schedule, and roughly the window in which immune-cell changes were measured in trials. Whether those changes translate into anything a person can perceive depends on why it was prescribed, and often they will not be perceptible at all.

In the hepatitis B trials the response accumulated after the course finished, and courses ran 26 weeks. Nothing is expected to happen quickly, and a physician may propose a longer course; have that conversation before starting.

No withdrawal, rebound or dependence is described in the published work, which spans decades of use of the branded product.

The directions are written in millilitres and insulin-syringe units because that is what you can read off the barrel. 0.25 mL is 25 units on a U-100 syringe; at 2 mg/mL that is 0.5 mg of thymosin alpha-1. The number is printed on your medication, and your physician may set a different dose.

Subcutaneous means into the fat layer, not the muscle. Abdomen and thigh are the usual sites, rotated. Five days a week with weekends off is a common prescriber pattern; the branded product's labelled schedule is twice weekly, so do not be surprised that physicians write it differently. Your directions will say.

20 doses at five per week is four weeks, which is why refills run on a 28-day cycle. One vial per fill, always. The 3 mL Pen cartridge contains 15 mg, dosed in clicks.

Refrigerated, and it ships that way in insulated packaging with ice packs. Beyond-use dating runs from first puncture and is on the label.

Not related beyond the name. TB-500 is a fragment of thymosin beta-4, studied around actin regulation and soft-tissue repair, with no human randomised trials. Thymosin alpha-1 is studied around immune signalling and has decades of them.

Thymalin is a thymic peptide preparation with a largely Russian and Eastern European, mostly older, literature; thymosin alpha-1 is a single defined sequence with a branded product and international trials. Neither is FDA approved.

The FORTIFY blend combines thymosin alpha-1 with BPC-157, KPV and LL-37 in one vial. The single product is for a question that is specifically immune modulation; the blend is for when a physician wants the immune component alongside gut, inflammation and antimicrobial-peptide coverage.

Identical medication. The pepti Pen is a pre-filled cartridge in a reusable click-dial injector, dosed in clicks rather than drawn from a vial with a syringe. It costs more and it removes the draw step. Neither is clinically better.