Thymosin Alpha-1 Dosage Calculator and Label Dosing Chart
A 28-amino-acid thymic peptide. Not FDA-approved, but approved abroad as ZADAXIN — for chronic hepatitis B in Indonesia and as an add-on to flu vaccination in Italy.
Written by Kashif Khan, who builds and maintains this site and is not a clinician. Every figure links to its primary source.
Not FDA-approved; approved abroad as ZADAXIN
ZADAXIN is 1.6 mg of synthetic thymosin alpha-1 per vial. Indonesia's regulator approves it for chronic hepatitis B in adults with compensated liver disease and active viral replication. Italy's approves it only as an adjuvant to influenza vaccination in immunocompromised adults. In the US, the FDA proposed at its December 2024 compounding advisory committee meeting that thymosin alpha-1 (free base and acetate) not be placed on the 503A Bulks List, and it does not appear in any category of the 503A nominations list updated 14 May 2026.
Thymosin alpha-1 has no FDA approval. It is approved outside the US as ZADAXIN, but for different uses in different countries, so there is no single approved dose to quote — each figure below names the label it comes from. The FDA has stated that labelling a product “research use only” or “not for human consumption” does not change its legal status when the seller's own marketing shows it is intended for people.
Key facts
- Molecular weight
- 3,108 DaAcetylated 28-residue peptide, pI 3.8, as stated on the Indonesian ZADAXIN label.
- Half-life
- About 2 hours in serum after a subcutaneous dose, back to baseline within 24 hours
- Also sold as
- Tα1, TA1, thymalfasin, Zadaxin
Doses used in published studies
These are reproduced for reference. They are not recommendations, and an animal dose in mg/kg does not convert to a human dose.
How good is the evidence?
Two regulators have approved it, which is more than any other research-track compound on this site can say, but for narrow uses. The hepatitis B evidence on the Indonesian label is three small randomised trials pooled to 223 patients, with a 36% versus 19% response a year after treatment. The Italian approval is for boosting flu-vaccine response in immunocompromised adults. Neither label covers the uses it is sold for online, including autoimmune disease, general immune support or recovery.
Thymosin alpha-1 dosage: the two labels that exist
The 1.6 mg twice-weekly figure quoted everywhere online comes from the hepatitis B label. The Indonesian ZADAXIN label sets it out exactly: 1.6 mg subcutaneously twice a week, doses 3 or 4 days apart, for six months without interruption — 52 doses — and not by intramuscular or intravenous injection.
The Italian label is for something else entirely and doses differently. It is an adjuvant to flu vaccination in immunocompromised adults, given intramuscularly or subcutaneously twice weekly for four weeks from the first vaccination and again from week 8 to week 12, with the dose calculated from body surface area. Two approvals, two regimens, one molecule. Neither is a general-purpose schedule, and neither applies outside the condition it was approved for.
Why there is no autoimmune dosing chart
Neither label lists any autoimmune disease. Both approved uses aim to strengthen an immune response — clearing hepatitis B, or improving a vaccine response in people whose immunity is weak — and the Indonesian label treats deliberate immunosuppression, as in organ transplant patients, as a reason not to use it unless the benefit clearly outweighs the risk. A chart of autoimmune doses would have to come from somewhere other than either label.
When the FDA took thymosin alpha-1 to its compounding advisory committee in December 2024, it proposed not placing the substance on the 503A Bulks List. It is not on that list, and it no longer appears in any category of the FDA's 503A nominations list as updated in May 2026.
Reconstitution and how long it keeps
Each ZADAXIN vial is 1.6 mg of powder reconstituted with the 1 mL of diluent supplied, giving 1.6 mg/mL — the whole vial is one dose, which is 100 units on a U-100 syringe. Unopened vials are stored at 2–8 °C with a three-year shelf life.
After mixing, the Indonesian label says to use it immediately. The Italian label allows single use within 24 hours and says to discard what is left. Both are single-dose presentations. A multi-dose retail vial of thymosin alpha-1 has no published in-use stability at all, so how long one keeps after reconstitution is unknown rather than 28 days.
Common questions
Sources
Every number on this page comes from one of the documents below. Where no reliable source exists, the page says so instead of filling the gap.
- FDA labelBPOM (Indonesian FDA)· 2024-08-02ZADAXIN (thymosin alpha 1) for injection — approved label, chronic hepatitis B
- FDA labelAIFA-authorised SmPC (document made available by AIFA 26 Feb 2026)· 2026-02-26ZADAXIN 1,6 mg/ml — Riassunto delle Caratteristiche del Prodotto (Italian SmPC)
- RegulatorFDA· 2024-12-04PCAC briefing document — substance descriptions and 503A statutory conditions
- RegulatorFDA· 2026-05-14Bulk Drug Substances Nominated for Use in Compounding Under Section 503A