As of September 2026, the FDA and the EMA have not approved thymosin alpha-1. The record says it is approved as Zadaxin in a number of countries, mainly in Asia and the Middle East. In 2024-2025 the FDA reviewed it for compounding and recommended against, citing insufficient evidence. Compounded products are still marketed online in the US, and you cannot tell from the outside what one holds.
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What is thymosin alpha-1?
Thymosin alpha-1 is a peptide fragment found in nature, first isolated from thymus gland tissue in the 1970s. It was later made in the lab as an immune-modulating agent. Outside the US it has been marketed for decades as Zadaxin, and compounded versions are marketed online in the US.
Does thymosin alpha-1 work?
Nobody can say from this record. It lists one randomized trial, in 361 patients with severe sepsis, published in 2013. The record has not been searched for others yet. The FDA found the evidence of effectiveness insufficient in its 2024-2025 review, and a number of countries have approved it as Zadaxin.
What we found. 1 completed randomized trial listed: 361 patients with severe sepsis, published in 2013. The record has not been searched for others yet. The FDA review and the approvals abroad are in the legal section below.
| Study | Tested in | Status | What it shows |
|---|---|---|---|
| ETASS trial of thymosin alpha-1 in severe sepsis | People | Published in 2013. Multicenter, single-blind, randomized | 361 patients in six hospitals in China were assigned to thymosin alpha-1 or to a control group, and deaths were compared |
Is thymosin alpha-1 safe?
The record names no side effects and holds no safety trial. The FDA's 2024-2025 review found the safety evidence insufficient across the conditions it looked at. It also found the substance not well characterized, with unresolved impurity issues. Those findings are about the substance put forward for compounding. The impurity point carries over to the vial, in the section after the legal status.
Is thymosin alpha-1 legal?
- United States
- Not approved by the FDA. The record says it has never received FDA approval.
- Compounding
- In 2024-2025 the FDA reviewed a request to allow it as a compounding ingredient and recommended against. It found the substance not well characterized, with unresolved impurity issues and insufficient evidence of safety and effectiveness across 12 conditions. The record says compounded products are still marketed online in the US without FDA authorization.
- European Union
- No EMA marketing authorisation.
- Elsewhere
- The record says it is approved as Zadaxin in a number of countries, mainly in Asia and the Middle East. It has been marketed there for chronic hepatitis B and as an immune-support add-on. The record names no country.
- Sport
- Not named on WADA's list. That is not clearance.
Not medical advice. Approval and sport status as of . Medical disclaimer
What is in a vial sold as thymosin alpha-1?
The record says compounded thymosin alpha-1 products are marketed online in the US without FDA authorization. The FDA's review found the substance not well characterized, with unresolved impurity issues. A "research use only" label guarantees nothing about purity. A certificate of analysis counts only if it names your lot number, a lab with no stake in the result, and the method used. Purity and identity are separate tests.
Three questions to ask the seller:
- Which lot number is this certificate for?
- Who ran the test, and are they independent of you?
- Does the report confirm identity by mass spectrometry, or purity alone?
What don't we know?
- Whether it helps in the conditions people use it for. No FDA-reviewed, US-based trial program has shown that.
- What trials stand behind the approvals outside the US. This record lists one trial, in sepsis.
- What the impurity issues mean for people. The FDA's review left them unresolved.
- What is in any given vial.
The other side
Thymosin alpha-1 has been marketed for decades as Zadaxin in a number of countries. A personal result is real to the person who had it. It cannot separate the compound from the vial, or from the passing of time.
What would change this record?
- A rigorous randomized trial in one specific condition, reviewed by the FDA.
- Resolved questions about how it is made and how pure it is. The record says the two together would answer the FDA's stated concerns.
Change log
- Count corrected from 0 to 1 and grade from C to B. A randomized trial in 361 patients with severe sepsis, published in 2013, is now listed.
- Record checked. No change logged yet.
Three questions to ask next
- What does 'research use only' actually mean?
- How can you tell if a peptide vendor is legitimate?
- How do you read a certificate of analysis?
Ask better questions.
More questions about thymosin alpha-1
Is thymosin alpha-1 approved by the FDA?
- No. The record says it has never received FDA approval, and it has no EMA marketing authorisation.
Is thymosin alpha-1 the same as Zadaxin?
- Zadaxin is the brand name it has been marketed under outside the US. The record says that brand holds approvals in a number of countries across Asia and parts of the Middle East.
Did the FDA review thymosin alpha-1 for compounding?
- Yes, in 2024-2025. It recommended against adding it to the compounding bulks list, citing insufficient evidence of safety and effectiveness.
Is thymosin alpha-1 the same as Thymalin?
- No. Thymosin alpha-1 is a single peptide fragment, first isolated from thymus tissue and later made in the lab. Thymalin is a mixture of peptides extracted from calf thymus.
Is thymosin alpha-1 banned in sport?
- It is not named on WADA's list. That is not clearance.
Record checked on . Each source shows the date it was checked.