Thymosin alpha 1 (thymalfasin, TA1) is a 28-amino-acid immune peptide first isolated from the thymus in 1977.[5] Sold as Zadaxin, it was approved in more than 30 countries as of 2016, according to its maker SciClone, mainly for hepatitis B and immune support, with China its largest market.[4] FDA could not independently verify these approvals.[1] It has never been FDA-approved, and its largest recent trial, in sepsis, found no survival benefit.[10]
What is thymosin alpha 1?
Thymosin alpha 1 was isolated from a calf thymus extract by Allan Goldstein’s team.[5] It helps activate dendritic cells through toll-like receptors and supports T-cell responses.[17] Despite the similar name, it is unrelated to TB-500, a fragment of thymosin beta-4.
Where thymosin alpha 1 is approved
| Country | Approved use |
|---|---|
| China | Main market: 95% of Zadaxin sales in 2016, plus local generics[4] |
| Italy | Flu vaccine booster in people with weak immune systems[1] |
| India, Indonesia, Mexico | Chronic hepatitis B[1] |
| Singapore | Chronic hepatitis B and C[1] |
FDA says it cannot independently verify the full list of country approvals.[1]
Thymosin alpha 1 benefits: what trials show
Hepatitis B and C
- Taiwan trial: 40.6% complete response at 18 months versus 9.4% of controls.[6]
- US phase 3 trial: 14% vs 4%, which did not reach significance; the authors said it did not confirm benefit.[7]
In the Taiwan trial, 98 patients were split into three groups: 26 weeks of treatment, 52 weeks, or no treatment. Responses were similar at the end of treatment and grew over the following months. The 52-week group (26.5%) did not clearly beat controls, and no one cleared the hepatitis B surface antigen.[6]
The US trial was double-blind: 49 patients got 1.6 mg twice weekly for 6 months and 48 got placebo, with 6 months of follow-up.[7] A 2008 meta-analysis of 4 trials (199 patients) found thymosin alpha 1 worked less well than interferon at the end of treatment but better 6 months later. Most patients were HBeAg-negative.[23]
- Hepatitis C: adding it to interferon improved liver enzyme response at the end of treatment, but lasting response was low.[8] A meta-analysis compared it with interferon in hepatitis B.[23]
Sepsis
- ETASS (2013, China): 26.0% vs 35.0% mortality in 361 patients, a borderline result.[9]
- TESTS (2025, China): in 1,106 patients, mortality was 23.4% vs 24.1%: no benefit.[10]
- Meta-analysis: benefit overall, but none in high-quality or multicenter trials.[11]
ETASS ran in 6 Chinese hospitals and was single-blind, meaning not everyone involved was blinded to treatment. Its main result was borderline (P = 0.062 in one analysis and 0.049 in another), and no serious drug-related adverse events were recorded.[9]
TESTS was the stronger test. Adults aged 18 to 85 at 22 Chinese centers got injections every 12 hours for 7 days. Survival was essentially identical, and no secondary or safety outcome differed. Subgroups hinted at harm in people under 60 and benefit in people with diabetes, but those findings need confirmation. The drug’s maker helped fund the trial.[10]
COVID-19
A retrospective Wuhan study of 76 severe cases reported lower mortality (11.1% vs 30.0%),[12] but a 2023 meta-analysis of more than 5,000 patients found no mortality effect.[13] The Wuhan study looked back at records from 2 hospitals and was not randomized.[12] The meta-analysis pooled 9 studies with 1,152 treated patients. Older and more severely ill patients did better in subgroups, but the authors concluded the results do not support its use in hospitalized adults with COVID-19.[13]
Cancer
- Melanoma: median survival 9.4 vs 6.6 months with chemotherapy, not statistically significant.[14]
- Lung cancer: a meta-analysis of 10 small randomized trials (724 patients) reported better response when thymosin preparations were added to chemotherapy; it did not separate out thymosin alpha 1.[15]
Vaccines
In dialysis patients, Zadaxin improved the response to a flu vaccine.[18]
Thymosin alpha 1 vs thymosin beta 4 (TB-500)
| Thymosin alpha 1 | Thymosin beta 4 / TB-500 | |
|---|---|---|
| Size | 28 amino acids[5] | 43 amino acids; TB-500 is a short fragment[3] |
| Main role | Immune activation[17] | Tissue repair and cell movement |
| Approvals | Zadaxin, reported in more than 30 countries[4] | None |
| WADA 2026 | Not named[20] | Named as prohibited[20] |
Thymosin alpha 1 dosage
| Use (non-US labels) | Dose |
|---|---|
| Chronic hepatitis B | 1.6 mg under the skin twice weekly, 3 to 4 days apart, for 6 months[1] |
| Flu vaccine booster (Italy) | 1.6 mg twice weekly for 4 weeks, repeated from week 8 to 12[1] |
Blood levels peak about 2 hours after injection, and its half-life is about 2 hours.[16] In 9 healthy volunteers, it did not build up in the blood over 5 days of daily dosing.[16] Outside the US, a clinician sets the dose and length of treatment.
Thymosin alpha 1 side effects
- Common: discomfort at the injection site.[1]
- Rare: redness, joint pain with hand swelling, rash, and a temporary liver enzyme rise in hepatitis B.[1]
- Cautions: not for children, pregnancy, breastfeeding or people on deliberate immunosuppression such as transplant recipients; caution with autoimmune disease.[1]
- Exposure: more than 350,000 patients have used Zadaxin, according to authors affiliated with SciClone.[19][1]
Questions to ask your clinician
- Is there an approved treatment with stronger evidence for my condition, such as antiviral drugs for hepatitis B?
- Do I have an autoimmune disease, a transplant, or another reason my immune system should not be boosted?[1]
- What would tell us it is working, and when should we stop?
Is thymosin alpha 1 legal in the US?
- Not FDA-approved.[1]
- Compounding: in December 2024, FDA’s advisory committee voted 17 to 4 against adding it to the compounding list.[2] It is not on any category of FDA’s May 2026 list, so it cannot legally be compounded.[22][3]
- Sports: not named on WADA’s 2026 list; because it is approved in other countries, the non-approved substances category likely does not apply.[20]
Can you still get thymosin alpha 1?
After its only US compounding nomination was withdrawn, FDA reviewed thymosin alpha 1 itself for 13 possible uses and recommended against it; advisers agreed in December 2024.[1][2] It was not among the peptides FDA removed from its safety-risk list in April 2026, and online claims that it was restored for compounding are not supported by FDA documents.[24] Outside the US, Zadaxin and generic thymalfasin are sold as approved medicines in several countries.[4] For other immune and gut-focused peptides, see our BPC-157 and KPV profiles.
This is general information, not legal advice.
What users report
See also epitalon, another peptide studied for aging.
