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Thymosin alpha-1 is a 28-amino-acid peptide first isolated from the thymus, the gland where T cells mature. Its synthetic form, thymalfasin, is sold under the brand name Zadaxin and is approved in China and dozens of other countries for chronic hepatitis B and as an immune booster in some settings. It has never been approved by the FDA for any condition.
That split makes the TA1 peptide confusing. It has far more human data than most peptides sold as “research chemicals,” but many of those trials were small, and the largest placebo-controlled one, a 1,106-patient sepsis trial published in 2025, found no survival benefit. In the US, the FDA flagged compounded versions for possible safety risks in 2023, and its advisers voted against allowing it in compounding in 2024.
This guide covers what it is, how it works, what the trials found, the doses used in studies, side effects, and where it stands with the FDA and anti-doping rules.
Key Takeaways
- Thymosin alpha-1 (thymalfasin, Zadaxin) is a 28-amino-acid immune peptide. It is approved in China and more than 30 other countries, but never by the FDA.
- It appears to act on dendritic cells and T cells through Toll-like receptor signaling, nudging the immune system toward a stronger antiviral response.
- Evidence is mixed. A 2026 Cochrane review of 10 hepatitis B trials rated almost all of its benefits as very low certainty, and the 2025 TESTS sepsis trial found no reduction in 28-day mortality.
- The labeled dose abroad for hepatitis B is 1.6 mg injected under the skin twice a week for 6 to 12 months. That is a medical dose for a specific disease, not a wellness protocol.
- Side effects in trials were mostly mild injection site reactions. The FDA placed it in compounding Category 2 in 2023 over immunogenicity and impurity concerns, and its advisers later voted against adding it to the compounding list.
What Is Thymosin Alpha-1?
Thymosin alpha-1 is a short protein made of 28 amino acids. It was first isolated in 1977 from thymosin fraction 5, an extract of calf thymus glands, according to a 2020 review in the World Journal of Virology. It has been studied ever since as a way to support immune function in people weakened by infection, cancer, age or kidney failure.
In the body, it comes from a larger precursor protein called prothymosin alpha. A 1984 PNAS study isolated this roughly 112-amino-acid protein from rat thymus and found the thymosin alpha-1 sequence at its starting end.
Thymalfasin and Zadaxin
Thymalfasin is the synthetic version, developed commercially by SciClone Pharmaceuticals as Zadaxin. NPR reported in 2020 that regulators in China and more than 30 other countries have approved it for some conditions, and the 2020 review puts the figure at more than 35 countries.
Approved uses vary. A 2002 drug profile lists hepatitis B approvals in markets including China, India, Singapore and the Philippines, and approval in South Korea as a flu vaccine adjuvant. The European Medicines Agency’s orphan designation record notes that in the EU it was authorized in Italy to enhance the immune response to flu vaccines.
Quick Facts
| Property | Detail |
|---|---|
| Also known as | Thymalfasin, Zadaxin, TA1, Tα1 |
| Size | 28 amino acids |
| Natural source | Thymus; cut from prothymosin alpha |
| Studied route | Subcutaneous injection |
| Approved | China and 30+ other countries (hepatitis B, some immune uses) |
| FDA status | Not approved for any condition; orphan designations only |
| WADA | Not named on the 2026 Prohibited List |
For a one-page summary, see our thymosin alpha-1 compound profile.
How Thymosin Alpha-1 Works
Thymosin alpha-1 is an immune modulator. It does not kill viruses or tumor cells directly. Instead, studies suggest it changes how certain immune cells behave.
- Dendritic cells. These cells sample germs and present them to T cells. A 2004 study in Blood found that thymosin alpha-1 triggered dendritic cell maturation and interleukin-12 production through Toll-like receptor signaling, the system the body uses to detect microbes. In mice given bone marrow transplants, it protected against a dangerous fungal infection.
- T cells and NK cells. A 2004 review of thymalfasin in hepatitis B summarizes lab work showing it can influence T-cell production and maturation, raise Th1 cytokines such as interferon-gamma and interleukin-2, and activate natural killer cells.
- T-cell exhaustion. In a 2020 study of severe COVID-19 patients, treated patients had lower levels of the exhaustion markers PD-1 and Tim-3 on their CD8+ T cells.
The common thread is a push toward Th1 immunity, the arm of the immune system that handles viruses and abnormal cells. That is also why the Zadaxin label warns against using it in people who are deliberately immunosuppressed, such as organ transplant recipients.
Thymosin Alpha-1 Benefits: What the Clinical Evidence Shows
Most claims about thymosin alpha 1 benefits trace back to the conditions below. Here is what the trials found, including the results that usually get left out.
Chronic Hepatitis B
Hepatitis B is the main approved use abroad, and the evidence is real but weaker than it looks.
- In a 1998 randomized trial in Hepatology, 98 patients received 1.6 mg twice weekly for 26 weeks, the same dose for 52 weeks, or no treatment. At 18 months, the virus had cleared in 40.6% of the 26-week group and 26.5% of the 52-week group, versus 9.4% of controls. Responses built up after treatment ended.
- A 2001 meta-analysis of five trials (353 patients) found no virological benefit at the end of treatment but a significant one 12 months after stopping. Liver enzymes did not improve compared with placebo.
- The 2026 Cochrane review pooled 10 randomized trials with 1,349 participants. Thymosin alpha-1 may lower all-cause mortality and serious adverse events, but the evidence was very low certainty for every outcome except serious adverse events (low), and the authors said they are “not sure” it helps.
The trials in the review were published between 1991 and 2018, and some tested it as an add-on to antivirals such as entecavir or tenofovir rather than on its own.
Hepatitis C
A 1998 trial of 109 patients found that adding thymosin alpha-1 to interferon raised end-of-treatment response to 37.1% versus 16.2% with interferon alone, but sustained responses were only 14.2% versus 8.1%. A larger European trial of 552 patients who had failed earlier treatment found no difference in sustained viral response (12.7% versus 10.5%). Hepatitis C is now treated with direct-acting antivirals, so this use is largely historical.
Sepsis
Sepsis is where the most rigorous test has been done.
- The 2013 ETASS trial randomized 361 patients with severe sepsis in China. Mortality at 28 days was 26.0% with thymosin alpha-1 and 35.0% in controls, a borderline result (log-rank P = 0.049) from a single-blind trial.
- The 2025 TESTS trial in The BMJ was designed to confirm it. This double-blind, placebo-controlled phase 3 study enrolled 1,106 adults across 22 centers. Mortality at 28 days was 23.4% with thymosin alpha-1 and 24.1% with placebo (hazard ratio 0.99), and no secondary or safety outcome differed significantly.
Subgroup analyses hinted at possible benefit in patients 60 and older or with diabetes, and possible harm in those under 60. The authors treat these as hypotheses, not findings.
COVID-19
Thymosin alpha-1 was widely used in China early in the pandemic, and the data are almost entirely observational.
- A 2020 retrospective study of 76 severe cases reported 11.1% mortality with treatment versus 30.0% without.
- A 2021 retrospective study of 275 patients found it did not restore CD4+ or CD8+ T-cell counts, and treated patients took longer to clear the virus.
- A 2023 meta-analysis of 9 studies and 5,352 patients found no significant effect on mortality (risk ratio 1.03) and said the results do not support its use in hospitalized adults.
Cancer (Add-On Use)
The most cited cancer trial is a 2010 randomized study in the Journal of Clinical Oncology of 488 patients with metastatic melanoma. Adding thymosin alpha-1 to chemotherapy produced more tumor responses in two arms. Median survival was 9.4 months versus 6.6 months, but that difference was not statistically significant (P = 0.08), and no toxicity was added. This is early-phase data, not proof that it treats cancer.
Vaccine Response in Older Adults and Dialysis Patients
Vaccines often work less well in older people and people on dialysis. Two small trials tested thymosin alpha-1 as a booster:
- A 1989 double-blind trial in 90 men aged 65 to 99 gave it twice weekly with the flu vaccine. The authors reported a stronger antibody response and no toxicity.
- A 2012 pilot study in hemodialysis patients found that adding 3.2 mg or 6.4 mg to a pandemic H1N1 vaccine improved antibody levels at day 21, with no adverse events attributed to the peptide.
Neither shows that it improves immunity in healthy adults.
Claims Without Good Evidence
Several claims that circulate online are not supported by human data:
- “Boosts the immune system” in healthy people. No trial has tested this in healthy adults or athletes.
- Rheumatoid arthritis. We found no clinical trial showing it changes disease severity.
- Antioxidant effects. The cited study was in rats with diet-induced fatty liver.
- Lower liver cancer risk. The Cochrane review called the effect on hepatitis B-related illness “very uncertain.”
Research vs Online Claims
| Common claim | What the research shows |
|---|---|
| Clears hepatitis B | Some trials showed delayed viral clearance; Cochrane rates the evidence very low certainty |
| Saves lives in sepsis | Largest placebo-controlled trial (1,106 patients) found no mortality benefit |
| Treats COVID-19 | Mixed observational data; a 5,352-patient meta-analysis found no mortality effect |
| Fights cancer | Possible benefit as a chemo add-on in melanoma; survival difference not significant |
| Improves vaccine response | Small trials in older men and dialysis patients; no data in healthy adults |
| No side effects at any dose | Mostly mild in trials, but long-term use in healthy people is unstudied |
Thymosin Alpha-1 Dosage: Studied and Labeled Doses
The doses below come from foreign labeling and published trials. They show what has been studied. They are not recommendations, and none were tested in healthy people.
Labeled Dose (Zadaxin, Outside the US)
The Zadaxin prescribing information lists 1.6 mg (900 µg/m²) injected under the skin twice a week for 6 to 12 months for chronic hepatitis B, with 40 µg/kg for patients under 40 kg. Each vial contains 1.6 mg of freeze-dried peptide.
Doses Used in Clinical Trials
| Study | Condition | Studied dose |
|---|---|---|
| Chien 1998 | Hepatitis B | 1.6 mg twice weekly for 26 or 52 weeks |
| Sherman 1998 | Hepatitis C | 1.6 mg twice weekly with interferon |
| Ciancio 2012 | Hepatitis C | 1.6 mg twice weekly for 48 weeks with peginterferon and ribavirin |
| TESTS 2025 | Sepsis | 1.6 mg every 12 hours for 7 days |
| Maio 2010 | Metastatic melanoma | 1.6, 3.2 or 6.4 mg alongside chemotherapy |
| Gravenstein 1989 | Flu vaccine, older men | 900 µg/m² twice weekly for 8 doses |
According to the label, blood levels peak about 2 hours after an injection and the half-life is about 2 hours, with no build-up after repeated doses.
What People Report Online
Online protocols commonly borrow the 1.6 mg twice-weekly label dose and apply it to healthy people for “immune support.” That use is commonly reported online, not studied. There is no trial evidence for any dose, schedule or cycle in healthy adults. The trials above used pharmaceutical-grade product under medical supervision. For background on how subcutaneous injections work, see our guide to peptide injection sites.
Stacking
There are no studies of thymosin alpha-1 combined with other peptides. In trials, it was paired only with standard treatments such as interferon, antivirals, chemotherapy or vaccines.
Thymosin Alpha-1 Side Effects and Safety
Thymosin alpha-1 has more human safety data than most peptides sold online, but the data have limits.
What Trials and Labeling Show
- Mostly mild. The Zadaxin label describes adverse effects as infrequent and mild, mainly discomfort at the injection site, with rare reports of redness, temporary muscle wasting, joint pain with hand swelling, and rash.
- Similar to placebo in sepsis. In TESTS, adverse events occurred in 66.4% of the treated group and 67.6% on placebo, and serious adverse events in 26.8% versus 29.3%. Those high rates reflect how sick sepsis patients are.
- No signal in hepatitis B. The Cochrane review found fewer serious adverse events with treatment than control, based on low-certainty evidence.
- Combination effects. With interferon, fever, fatigue and muscle aches were reported, which the 2020 review links to the combination.
Who Should Not Use It
The label lists a known allergy to thymosin alpha-1 as a contraindication and says it should be considered contraindicated in people who are deliberately immunosuppressed, such as organ transplant patients, unless benefits clearly outweigh risks. People with autoimmune disease, cancer or a transplant should not use it outside medical care.
What Is Unknown
- Long-term use in healthy people. Most trials lasted 6 to 12 months in patients with a specific disease.
- Immunogenicity. The FDA said compounded versions “may pose significant risk for immunogenicity,” meaning the body could form antibodies against the peptide or its impurities.
- Product quality. Vials sold as “research use only” are not checked by any regulator. See our guide on how to read a peptide COA for what a certificate of analysis can and cannot tell you.
Thymosin Alpha-1 vs Thymosin Beta-4
The names are similar, and both were first studied from thymus extracts, but they are different molecules with different jobs.
| Thymosin alpha-1 | Thymosin beta-4 | |
|---|---|---|
| Main role | Immune modulation (T cells, dendritic cells) | Tissue repair and cell movement |
| Approved drug | Yes, abroad (Zadaxin) | No |
| WADA 2026 | Not named | Prohibited at all times (S2), including TB-500 |
For the repair peptide, see our thymosin beta-4 guide and our page on TB-500, a synthetic fragment of thymosin beta-4, not of thymosin alpha-1. Other thymus peptides, such as thymogen and thymalin, are separate compounds as well.
Is Thymosin Alpha-1 Legal? FDA, Compounding and WADA
FDA Approval Status
Thymosin alpha-1 has never been approved by the FDA for any condition. It has received US orphan drug designation, which gives sponsors research incentives for rare diseases but is not an approval. The EMA record notes US orphan status for hepatocellular carcinoma, and the National Cancer Institute’s SEER*Rx drug database lists designations for chronic active hepatitis B, hepatocellular carcinoma and DiGeorge anomaly with immune defects. As NPR reported, most drugs with orphan designation never reach the market, and thymosin alpha-1 did not.
Compounding Status
Without FDA approval, US patients have mainly obtained it through compounding pharmacies. That route has narrowed:
- September 29, 2023. The FDA’s 503A categories update added it to Category 2, bulk substances that raise significant safety risks, along with BPC-157 and other peptides.
- Nomination withdrawn. It was later moved to a list of substances “previously in category 2” whose nominations were withdrawn. The FDA’s safety risk page still lists its concerns: possible immunogenicity, peptide-related impurities, and too little safety information to understand the risks.
- December 4, 2024. The FDA’s Pharmacy Compounding Advisory Committee voted 17 to 4 against placing thymosin alpha-1 or thymosin alpha-1 acetate on the 503A bulks list. Several members said there was no compelling evidence of effectiveness and safety for the uses reviewed.
- May 14, 2026. The FDA’s 503A categories update from that date does not list it in Category 1, 2 or 3, and it is not on the 503A bulks list.
Some clinic and vendor websites claim 2026 policy changes restored access to thymosin alpha-1. We could not confirm that in any FDA document. For a broader overview, see our guide to whether peptides are legal.
WADA and Drug Testing
Thymosin alpha-1 is not named on the WADA 2026 Prohibited List. The S0 class bans substances with “no current approval by any governmental regulatory health authority for human therapeutic use,” and thymalfasin holds approvals in a number of countries. Thymosin beta-4 and TB-500, by contrast, are banned by name under S2.
That does not make it risk-free for tested athletes. Products sold online can be mislabeled or contaminated, so athletes should confirm with their national anti-doping organization before using any peptide.
The Bottom Line
Thymosin alpha-1 is a real drug with decades of human research, but the best recent evidence is more modest than online claims suggest, and it is not FDA-approved.
- It is approved abroad as Zadaxin, mainly for chronic hepatitis B, where a 2026 Cochrane review rated the evidence very low certainty.
- The largest placebo-controlled trial, in sepsis, found no survival benefit, and COVID-19 data are mixed.
- Studied doses, usually 1.6 mg twice a week, were for specific diseases under medical care. There is no evidence for use in healthy people.
- The FDA has raised safety concerns about compounded versions, and its advisers voted against allowing it in compounding.
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