Thymosin alpha-1
/THY-mo-sin AL-fa one/aka Thymalfasin, Zadaxin, Tα1
A 28-amino-acid thymic peptide that nudges T-cell maturation and innate immune signaling, approved outside the US mainly for chronic hepatitis B.
Thymosin alpha-1: educational research discussion. Not medical advice. Not a protocol for you.Details
- Reported range
- 1.6 mg
- Frequency
- 2× weekly (non-US label)
- Cycle length
- 6 months for hepatitis B
- Half-life
- ≈ 2 h
- Route
- SC
Reported figures describe what is published or discussed. None of them is a dose for you.
Mechanism
What it is proposed to do
Thymosin alpha-1 is an N-terminally acetylated 28-amino-acid peptide originally purified from calf thymus extract. In lab and mouse studies it acts on dendritic cells and other innate immune cells, with Toll-like receptor signaling (TLR9 in particular) proposed as the entry point. Downstream, it promotes T-cell maturation and Th1-type cytokines such as interferon-gamma and IL-2. Measured clinical effects in people are modest and specific to the disease studied.
- 01
Thymalfasin
28-aa acetylated peptide, SC injection
established - 02
Innate immune sensing
Toll-like receptor signaling on dendritic cells, from lab and mouse work
proposed - 03
T-cell maturation · Th1 cytokines
IFN-γ and IL-2 increases in lab studies
proposed - 04
Modest clinical effects
Some hepatitis B response in RCTs; no sepsis mortality benefit in TESTS
established
Half-life. About 2 hours after SC injection per the Zadaxin label, with peak serum levels at about 2 hours and no accumulation on repeat dosing.
Dosing & cycles
What is reported, and where it comes from
Numbers below are labeled FDA label, clinical trial range or commonly reported range. They describe the published and discussed landscape. They are not instructions and not a protocol for you.
Dosing studio
Every figure is labeled by where it comes from.
Scale view: Non-US label regimen (Zadaxin, hepatitis B)
Probe at 1.6 mg: at the listed figure Drag it to read the scale. It does not pick a number for you.
Typical cycle, as a calendar
Non-US label course: 2× weekly
26 weeks
Two injections a week, 3 to 4 days apart, for six months. This is the hepatitis B regimen outside the US, not an FDA-approved schedule.
Cycle patterns
Non-US label course: 2× weekly
Two injections a week, 3 to 4 days apart, for six months. This is the hepatitis B regimen outside the US, not an FDA-approved schedule.
TESTS sepsis course
Every 12 hours for a week in ICU patients with sepsis. Shown for context. It did not reduce 28-day mortality.
Stacks
What it gets combined with
Why people pair things, and what nobody has tested.
The "thymosin pair"
2 compounds- Why people combine these
- Sometimes discussed together because of the shared name, pairing immune modulation with tissue-repair claims.
- Caution
- Shared name, unrelated molecules and mechanisms. TB-500 is WADA-prohibited and approved nowhere. Thymosin alpha-1 is not on the WADA list and is approved in dozens of countries. Combining them mixes their evidence and anti-doping status.
- What is unknown
- No published human data on this combination: dosing, interactions and long-term effects are all unstudied.
Evidence
How much of this is known
Evidence grade
B
Human RCTs, narrow
Human RCTs exist but are narrow, early, or not for the popular use.
How we gradeGrade B: genuine RCTs, including a 1,106-patient double-blind sepsis trial, but in narrow indications with modest or null results. The popular "immune boost" use has no trial behind it.
Human trial status
Thymalfasin has decades of use outside the US and randomized trials in chronic hepatitis B and C, sepsis and some cancers, many of them small or open-label. The largest and most rigorous, TESTS (BMJ 2025), randomized 1,106 adults with sepsis to thymosin alpha-1 or placebo every 12 hours for 7 days. 28-day mortality was 23.4% vs 24.1% (hazard ratio 0.97). Older meta-analyses of smaller trials suggested benefit; higher-quality trials did not confirm it.
Limitations
- Many earlier trials were small, open-label or from a single country.
- No trial supports general "immune boost" or wellness use in healthy people.
- A 2025 meta-analysis found the apparent sepsis benefit was no longer significant when limited to higher-quality trials.
- Hepatitis B care now centers on newer oral antivirals, and much of the thymalfasin trial record predates them.
Sources to read
- Wu J et al. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial (BMJ 2025)
- Efficacy of thymosin α1 for sepsis: a systematic review and meta-analysis of randomized controlled trials (Front Cell Infect Microbiol 2025)
- Romani L et al. Thymosin α1 activates dendritic cells for antifungal Th1 resistance through Toll-like receptor signaling (Blood 2004)
- ZADAXIN (thymosin alfa 1) prescribing information, non-US label (Indonesian FDA, BPOM)
- FDA briefing document: Thymosin alpha-1 related bulk drug substances, Pharmacy Compounding Advisory Committee, December 4, 2024
Links open PubMed, DailyMed, FDA or trial-registry searches. We do not cite what we have not described accurately.
Safety
Side effects, cautions, status
Commonly reported effects
- Label-reported adverse effects are infrequent and mild: mainly local discomfort at the injection site, with rare redness, rash, joint pain or hand swelling.
- Label contraindication: hypersensitivity to thymosin alpha-1.
- The label advises against use during deliberate immunosuppression, such as after organ transplant, unless benefit clearly outweighs risk.
- Research-market vials are not Zadaxin. Identity, dose and sterility are unverified.
Who should be extra cautious
- Organ transplant or deliberate immunosuppression. The label advises against use unless the benefit clearly outweighs the risk.
- Autoimmune disease. Immune stimulation in autoimmune conditions is poorly studied. A theoretical concern, not a documented harm.
- Pregnancy or breastfeeding. No adequate data.
- Research-market product. Not the licensed product. Identity, dose and sterility are unverified.
Legal & regulatory status
Approved outside the US as Zadaxin, in dozens of countries, mainly for chronic hepatitis B. Never approved by the FDA. In December 2024 an FDA advisory committee voted against adding it to the 503A bulks list, in line with FDA staff advice.
2026 compounding context. Not part of the 2026 peptide review. FDA's compounding advisory committee had already voted against adding thymosin alpha-1 to the 503A bulks list in December 2024, after FDA staff cited poor characterization, immunogenicity questions and weak effectiveness evidence.
WADA. Not on the Prohibited List as far as we know. It is a different molecule from thymosin β4 (TB-500), which is prohibited under S2.3. Under strict liability, a mislabeled or contaminated vial is still the athlete's problem. Confirm with your federation.
Storage & handling, high level
The licensed product is a powder with diluent, stored at 2–8 °C and used immediately after mixing, per the Zadaxin label. Research-market vials come with no such guarantees.
Reconstitution and injection technique are clinical skills. The reconstitution calculator does arithmetic only.
FAQ
Questions people ask about Thymosin alpha-1
Is thymosin alpha-1 the same as TB-500?
Is it FDA-approved?
Does it help in sepsis?
Does it boost immunity in healthy people?
The monthly Evidence Brief
Keep up with the evidence
One email a month: new trials, grade changes and FDA or WADA updates across the whole atlas, Thymosin alpha-1 included. You get the free guide right away.
Free guide · PDF
10 Peptide Claims, Fact-Checked
13 pages · sent to your inbox right away
Email only. Unsubscribe from any email in one click. We do not sell or rent your address. Nothing we send is medical advice. Privacy