Thymosin Alpha 1Dosage, Side Effects & Legal Status
Unlike most research peptides sold in the US, thymosin alpha 1 has a real approval track record abroad: sold as Zadaxin (thymalfasin), it has been used clinically for chronic hepatitis B, hepatitis C, and as a cancer-treatment adjunct, backed by completed human randomized trials. It has never been approved by the FDA, and the general "immune longevity" claims driving most of its US biohacking demand haven't been tested in any of those trials.
Reviewed by the PepGuard team · Last reviewed Jul 28, 2026
Naturally occurring 28-amino-acid peptide, N-acetylated, derived from the thymic hormone prothymosin alpha · also sold as Ta1, Thymalfasin, Zadaxin
Photo: National Cancer Institute / UnsplashThe studies behind the headline.
Results are reported as published. A result in one population does not establish a benefit in another.
| Study | Population | Observed result | Limit |
|---|---|---|---|
| Thymosin alpha 1 literature review | Historical trials across hepatitis, sepsis, cancer, and immunocompromised populations | The review reports approval of thymalfasin in more than 35 countries for selected indications, not a US FDA approval. | A review cannot establish that results from one disease apply to healthy people seeking immune support. |
| ETASS randomized sepsis trial | Patients with severe sepsis receiving standard care | This multicenter trial tested thymosin alpha 1 as an adjunct, rather than as a wellness treatment. | Sepsis results cannot be used to infer prevention of ordinary infections or longevity benefits. |
Thymosin alpha 1 is not a generic immune supplement
Its human studies address disease-specific questions such as hepatitis and sepsis. Those results cannot be repurposed as proof that a healthy person needs an injectable immune protocol.
What the literature and self-reports actually say.
| What's cited | Range |
|---|---|
| Clinical dose (Zadaxin, hepatitis B, approved abroad) | 1.6 mg, subcutaneous, twice weekly, 6-12 months |
| Sepsis trial dose (ETASS RCT) | 1.6 mg, twice weekly, alongside standard care |
| Commonly cited "immune support" protocol (unofficial, US biohacking) | 300-500 mcg/day |
Two very different numbers circulate, and it matters which one you're looking at. The clinical dose used in the Zadaxin hepatitis B trials abroad is a real, label-derived figure, not a biohacker guess. The lower daily doses circulating in US wellness and longevity content are anecdotal, vendor-reported protocols for general "immune support," a use case none of the trials below actually tested.
Signal strength, not just a list.
How often each effect shows up in the available sources, and how solid that evidence actually is.
| Signal | Frequency | Context |
|---|---|---|
| Injection site pain | commonly reported | Described in trials as mild, resolving within about 30 minutes. |
| Fever or flu-like symptoms | occasionally reported | Reported across randomized trials; generally mild to moderate. |
| Nausea | rarely reported | Occurs in a minority of trial participants. |
| Effects specific to the 'immune longevity' use case | unknown / not studied | No trial has evaluated thymosin alpha 1 for general immune support or longevity in healthy adults; all completed RCTs targeted specific diseases (hepatitis, sepsis, cancer). |
Not FDA-approved in the US. Approved abroad (as Zadaxin/thymalfasin) for specific indications.
Thymosin alpha 1 has never been approved by the FDA for any indication. Outside the US it is marketed as Zadaxin (thymalfasin) for selected clinical uses, including chronic viral hepatitis in some jurisdictions. International approvals, a compounding nomination, and a vendor's legal claim are not interchangeable with FDA approval. Check the current FDA record directly before relying on a claim about US availability or compounding status.
Every claim above, traced back.
Primary studies, reviews, regulatory pages, and trial registries. Click through and check them yourself.
Thymosin Alpha 1 questions, answered.
No, not in the US. As Zadaxin (thymalfasin), it's approved in a number of other countries for chronic hepatitis B, hepatitis C, and as a cancer-treatment adjunct, but it has never cleared FDA approval for any indication here.
Two different numbers circulate. The dose actually used in the hepatitis B trials that got Zadaxin approved abroad is 1.6 mg subcutaneous, twice weekly. The lower 300-500 mcg/day figure that shows up in US wellness content for general "immune support" is an unofficial biohacking protocol, not the clinical dose, and hasn't been tested in a trial.
Its human safety record is the strongest of the research peptides typically discussed alongside it, given decades of real-world use abroad and multiple completed randomized trials (hepatitis B, sepsis, cancer adjunct). That track record covers its approved indications, not the general immune-longevity use most US vendors market it for, which hasn't been separately studied.
FDA approval and international approval are separate processes. A product can be used or approved in another jurisdiction without becoming FDA approved in the US. Do not treat a past FDA compounding review, a vendor statement, or an international product name as proof of a current US approval.
Keep researching.
This is a preview of the Thymosin Alpha 1 report.
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