Label, hepatitis B
Label- Dose
- 1.6 mg
- How often
- Twice weekly, 3–4 days apart
- How long
- 6 months (52 doses)
Zadaxin label outside the US. Each vial is mixed with the 1 mL of diluent supplied.
Draw 32 units5 mg + 1 mL
Approved drugImmune modulator
Thymosin Alpha-1
Thymalfasin (Zadaxin)
- Common dose
- 1.6 mg twice weekly
- Cycle
- Continuous
- Vial sizes
- 5, 10 mg
- Typical draw
- 32 units5 mg with 1 mL, 1.6 mg
A 28-amino-acid immune-modulating peptide first isolated from thymus tissue.
Evidence. Approved outside the US as Zadaxin: for chronic hepatitis B in countries such as Indonesia (and in some, hepatitis C), and in Italy only as an add-on to flu vaccination in people with weakened immune systems. Not FDA-approved.
Status
- Approval
- Approved outside the US as Zadaxin: for chronic hepatitis B in countries such as Indonesia, and in Italy as an add-on to flu vaccination in people with weakened immune systems. Not approved in the US.
- US compounding
- Nomination withdrawn, so no longer in FDA Category 2 (April 2026). In December 2024 FDA’s advisory committee voted 4 to 17 against adding it to the 503A bulks list. Not on the July 2026 agenda.
- Sport (WADA 2026)
- Not banned in sportNot named on the 2026 list. S2 names thymosin beta-4 and TB-500, not thymosin alpha-1.
Category 2 is FDA’s list of bulk substances that may present significant safety risks in compounding. The 503A bulks list adds substances that pharmacies may compound with. Advisory committee votes are advice; FDA makes the final decision. Sport status follows the WADA Prohibited List in force since January 1, 2026; athletes should confirm with their anti-doping organization. Checked October 5, 2026. Status of every compound.
How it works
Thymosin alpha-1 is a synthetic 28-amino-acid copy of a thymus hormone. The Italian label describes it only as a pleiotropic thymic hormone that acts as an immune modulator, naming no target. The published pharmacology adds detail from laboratory work: it activates dendritic cells, the cells that show threats to the immune system, partly through toll-like receptor 9, shifts helper T cells toward the antiviral type, and raises natural killer and cytotoxic T cell activity.
LabelBased on the product label.
Not known
The label states no mechanism beyond immune modulation. FDA’s 2024 review found most laboratory doses translate to human doses far above those used in trials, so how much of this applies at the dose people take is unclear.
Protocol
Label
Zadaxin, Indonesian label
1.6 mg under the skin twice weekly, 3–4 days apart. For hepatitis B the course is 6 months (52 doses). Some countries also approve it with interferon for hepatitis C.
- Frequency
- Twice weekly
- Route
- Subcutaneous injection
- Cycle
- Label: a continuous 6-month course for hepatitis B.
Continuous, shown over 26 weeks
Common protocols
The ways Thymosin Alpha-1 is most often run, each tagged with where it comes from.
Community immune support
Community- Dose
- 1.6 mg
- How often
- Twice weekly
- How long
- About 4–12 weeks (often 8), then a break
Copies the label dose for off-label uses. Not established for these uses.
Draw 32 units5 mg + 1 mL
Taking it
Where it goes
Under the skin (label). Guides rotate the belly, thigh and upper arm.
If you miss a dose
No label rule; Zadaxin doses are spaced 3–4 days apart. If you’re a day or two late, take it and resume the usual rhythm; don’t double up.
What to expect
- First weeks
Side effects are infrequent and mild, mostly injection-site discomfort.
Label - During treatment
In hepatitis B, a temporary liver-enzyme (ALT) flare to over twice baseline can occur.
Label - 12 months or more after finishing
The hepatitis B response can be delayed, showing a year or more after the 6-month course ends.
Label
Side effects and what to do
Discomfort at the injection site (label: the main effect; drug-related events under 1%)
Rotate sites; usually mild.
Rare: redness, rash, joint pain with hand swelling, brief muscle wasting (label)
Tell the prescriber; stop if a rash spreads or joints swell.
Liver-enzyme flare: ALT over twice baseline during hepatitis B treatment (label)
Usually temporary; the doctor tracks liver tests. Get care for yellow skin or dark urine.
Tiredness or mild flu-like feeling (community reports)
Usually passes within the first week.
Stop and get medical help
Rash, swelling or trouble breathing; yellow skin or eyes, or dark urine; or high fever or signs of a severe infection.
Who should avoid it
What its label rules out or warns about, most important first.
- Allergy to thymosin alpha-1 or other ingredients
- LabelRuled out by the Italian label.
- Pregnancy or breastfeeding
- LabelRuled out by the Italian label: there are too few data in pregnancy, and whether it passes into breast milk is unknown.
- Children
- LabelThe Italian label says it must not be used in children.
- Anyone deliberately immunosuppressed, such as transplant patients
- LabelThe Indonesian Zadaxin label treats this as a contraindication, since the drug works by enhancing the immune system, unless the benefits clearly outweigh the risks.
- Autoimmune disease
- LabelThe Italian label says use has to be judged case by case in people with autoimmune diseases.
- Allergy-prone people, or past allergic reactions
- LabelThe Italian label says to use it with caution in atopic people and in anyone who has had allergic reactions before.
Interactions
Only interactions stated on a product label or measured in a human study are listed.
- Other immune-modulating medicines
- LabelThymosin alpha-1 acts on lymphocyte function, so the Italian label advises caution when it is given with other immunomodulating medicines.
- Interferon alfa (such as peginterferon alfa-2a or alfa-2b)
- TrialIn a hepatitis C trial two people given the combination had a brief dip then a rise in thyroid-stimulating hormone, which FDA’s briefing notes may be more likely with the combination than with interferon alone.
Label: a product label rules it out, warns about it or lists the interaction. Trial: seen in human studies, or a group those studies left out. This isn’t a complete list: a pharmacist or doctor can check a specific medicine or condition.
Tracking and pairing
Worth tracking
- Liver tests (ALT) during treatment, per the label
- Hepatitis B markers (HBV DNA, HBeAg, HBsAg) at the end and 2, 4 and 6 months after (label)
- Blood count and metabolic panel at baseline (community guides)
Often paired with
- BPC-157
A common pairing in recovery protocols (immune plus tissue repair). Never studied together; Tα1 is often used alone.
Human studies
5 key published human studies, with how many people took part. Animal studies aren’t listed here.
-
In sepsis, 28-day deaths were 23.4% with thymosin alpha-1 vs 24.1% with placebo (HR 0.99, 95% CI 0.77 to 1.27).
- Design
- Randomized double-blind placebo-controlled phase 3 trial (TESTS)
- Dose
- Subcutaneous, every 12 hours
- Length
- 7 days
Wu et al., BMJ 2025 (TESTS), PMID 39814420
-
In predicted severe necrotizing pancreatitis, infected necrosis occurred in 15.7% vs 18.1% with placebo (P=0.48).
- Design
- Randomized double-blind placebo-controlled trial
- Dose
- 1.6 mg subcutaneous every 12 hours, then once daily
- Length
- 14 days
Ke et al., Intensive Care Med 2022, PMID 35713670
-
In metastatic melanoma, median survival was 9.4 months with thymosin alpha-1 vs 6.6 months without (HR 0.80, P=0.08).
- Design
- Randomized trial, 5 arms
- Dose
- 1.6 to 6.4 mg, with dacarbazine with or without interferon
- Length
- 12 months, follow-up to 24 months
Maio et al., J Clin Oncol 2010, PMID 20194853
-
In chronic hepatitis B, complete viral response at 18 months was 40.6% after a 26-week course vs 9.4% untreated (P=0.004).
- Design
- Randomized controlled trial
- Dose
- 1.6 mg subcutaneous twice weekly
- Length
- 26 or 52 weeks
Chien et al., Hepatology 1998, PMID 9581695
-
In chronic hepatitis B, complete response was 14% vs 4% with placebo (P=0.084), which did not confirm earlier positive trials.
- Design
- Randomized double-blind placebo-controlled phase 3 trial
- Dose
- 1.6 mg subcutaneous twice weekly
- Length
- 6 months, plus 6 months follow-up
Mutchnick et al., J Viral Hepat 1999, PMID 10607256
Limits of the evidence
The approvals sit outside the US and differ by country: the Italian label covers only use as an influenza vaccine adjuvant in immunocompromised people, the Indonesian one chronic hepatitis B. Large trials elsewhere found no benefit, among them 1,106 people with sepsis and 508 with severe pancreatitis. FDA’s 2024 review concluded evidence of effectiveness was lacking for every use it examined, including the uses sold online.
Mixing your vial
| Vial | Water | Common dose | Draw | Strength | Action |
|---|---|---|---|---|---|
| 5 mg | 1 mL | 1.6 mg | 32 units (0.32 mL) | 5 mg/mL | |
| 10 mg | 2 mL | 1.6 mg | 32 units (0.32 mL) | 5 mg/mL |
The label vial (1.6 mg) is mixed with 1 mL of sterile water and used right away. Multi-dose research vials are usually mixed with bacteriostatic water.
Dose chart
Syringe units for common doses at different water volumes. Select a cell to open it in the calculator.
| Dose | Water added | ||
|---|---|---|---|
| 1 mL | 2 mL | 3 mL | |
| 1.6 mg | |||
U-100 insulin syringe: 100 units is 1 mL. Draws over 100 units show in mL. Faded values are under 2 units, which is hard to measure. Check that your vial holds the larger volumes.
Datasheet
- Half-life
- about 2 hours LabelSerum half-life after subcutaneous injection, with peak levels at about 2 hours. The Italian label gives a later peak, at about 6 hours, with levels back to baseline within 24 hours.
- Type
- Peptide, 28 amino acids
- Molecular weight
- 3,108.3 g/mol
- Formula
- C129H215N33O55
- Sequence
- SDAAVDTSSEITTKDLKEKKEVVEEAEN
- Storage before use
- 2–8 °C (Zadaxin label).
- After mixing or opening
- Zadaxin label: use immediately after mixing (single-use 1.6 mg vial). Multi-dose research vials are commonly kept in the fridge and used within about 4 weeks.
- Half-life: Zadaxin package insert (BPOM Indonesia); Zadaxin label (RxList); Zadaxin SmPC, section 5.2 (AIFA, Italy, 2026)
- Molecule: PubChem CID 16130571 (thymalfasin, N-terminal acetyl)
- Storage: Label Zadaxin package insert (BPOM Indonesia); Zadaxin label (RxList)
Every compound side by side: the half-life chart and the storage chart.
Cautions
- Avoid with deliberate immunosuppression, such as after an organ transplant.
- Not FDA-approved.
Questions
What is thymosin alpha-1?
A 28-amino-acid immune-modulating peptide first isolated from thymus tissue.
How does thymosin alpha-1 work?
Thymosin alpha-1 is a synthetic 28-amino-acid copy of a thymus hormone. The Italian label describes it only as a pleiotropic thymic hormone that acts as an immune modulator, naming no target. The published pharmacology adds detail from laboratory work: it activates dendritic cells, the cells that show threats to the immune system, partly through toll-like receptor 9, shifts helper T cells toward the antiviral type, and raises natural killer and cytotoxic T cell activity. The label states no mechanism beyond immune modulation. FDA’s 2024 review found most laboratory doses translate to human doses far above those used in trials, so how much of this applies at the dose people take is unclear.
Is thymosin alpha-1 FDA-approved?
Approved outside the US as Zadaxin: for chronic hepatitis B in countries such as Indonesia, and in Italy as an add-on to flu vaccination in people with weakened immune systems. Not approved in the US. Nomination withdrawn, so no longer in FDA Category 2 (April 2026). In December 2024 FDA’s advisory committee voted 4 to 17 against adding it to the 503A bulks list. Not on the July 2026 agenda.
What is the usual thymosin alpha-1 dose?
From the label (Zadaxin, Indonesian): 1.6 mg under the skin twice weekly, 3–4 days apart. For hepatitis B the course is 6 months (52 doses). Some countries also approve it with interferon for hepatitis C. These are the amounts sources reference, not recommendations.
How much water do I mix thymosin alpha-1 with?
There’s no single right amount, because the water only sets the strength. With 1 mL of bacteriostatic water in a 5 mg vial, the strength is 5 mg/mL, so 1.6 mg is 32 units on a U-100 insulin syringe. More water makes small doses easier to measure. The dose chart above shows other volumes.
Who should avoid thymosin alpha-1?
Allergy to thymosin alpha-1 or other ingredients: Ruled out by the Italian label. Pregnancy or breastfeeding: Ruled out by the Italian label: there are too few data in pregnancy, and whether it passes into breast milk is unknown. Children: The Italian label says it must not be used in children. The “Who should avoid it” section lists 3 more groups and 2 interactions.
What is the half-life of thymosin alpha-1?
About 2 hours, according to the prescribing information. Serum half-life after subcutaneous injection, with peak levels at about 2 hours. The Italian label gives a later peak, at about 6 hours, with levels back to baseline within 24 hours.
How should thymosin alpha-1 be stored?
Before mixing: 2–8 °C (Zadaxin label). Zadaxin label: use immediately after mixing (single-use 1.6 mg vial). Multi-dose research vials are commonly kept in the fridge and used within about 4 weeks.
Is thymosin alpha-1 banned in sport?
No. Not named on the 2026 list. S2 names thymosin beta-4 and TB-500, not thymosin alpha-1.
Has thymosin alpha-1 been studied in people?
Yes. The human studies section lists 5 published studies. The largest, from 2025, included 1,106 people. In sepsis, 28-day deaths were 23.4% with thymosin alpha-1 vs 24.1% with placebo (HR 0.99, 95% CI 0.77 to 1.27).
Sources
- Zadaxin (thymalfasin) drug information (RxList)
- FDA advisory committee briefing: thymosin alpha-1 (2024)
For the protocol details
- Zadaxin (thymalfasin) package insert (Indonesian FDA, BPOM)
For how it works, who should avoid it and the limits of the evidence
- Zadaxin (timosina alfa-1) summary of product characteristics, sections 4.3 to 4.6, 5.1 and 5.2 (AIFA, Italy, document released February 2026)
- FDA advisory committee briefing document for thymosin alpha-1-related bulk drug substances (December 2024)
- Wu et al., thymosin alpha-1 for sepsis, TESTS trial (BMJ 2025) PMID 39814420
- Ke et al., thymosin alpha-1 in predicted severe necrotizing pancreatitis (Intensive Care Med 2022) PMID 35713670
- Chien et al., thymosin alpha-1 in chronic hepatitis B (Hepatology 1998) PMID 9581695