Thymosin alpha-1

Also called thymalfasin, Tα1, TA1, Zadaxin^{1, 9}

Thymosin alpha-1 is a 28-amino-acid peptide first found in an extract of the thymus, the gland where immune T cells mature. As thymalfasin (Zadaxin), its maker says it is approved in more than 30 countries, mainly for hepatitis B and as a vaccine add-on, but never in the US.^{1, 12, 13} The human trials are mixed. Two US hepatitis C trials failed, and the largest trial, of 1,106 adults with sepsis, found no change in deaths at 28 days.^{14, 20} COVID-19 studies were mostly observational and disagree.^{17, 18} In 2024 FDA's advisers voted against letting pharmacies compound it, and it was left out of the 2026 HHS policy.^{2, 5}

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At a glance

Where to get it

MoveWell sells no peptides and links only to legal options. All options

Molecular structure

A straight chain of 28 amino acids with an acetyl group on the first one, serine. Too large to draw atom by atom, so this is a sequence diagram.

The acetyl group on serine 1

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Sequence

Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN-OH

Length

28 amino acids

Formula

C129H215N33O55

Weight

3,108.3 g/mol

Forms

Free base and acetate

Drug name

Thymalfasin

Sequence, formula and weight from FDA's 2024 review, for the free base.¹ Drawn from that sequence; the formula we computed matches FDA's.

What it is

The thymus is a gland behind the breastbone where T cells, a type of white blood cell, mature. In the 1970s researchers made a crude thymus extract called thymosin fraction 5, and thymosin alpha-1 is one of its parts.⁹ It was isolated in 1977, and the US National Cancer Institute began early trials around 1980.¹

SciClone Pharmaceuticals developed a synthetic version, thymalfasin, sold as Zadaxin. US hepatitis B trials in 1992 were inconclusive.¹² SciClone says Zadaxin is approved in more than 30 countries, for uses that include hepatitis B and C, some cancers and as an immune enhancer, depending on each country's approval.¹³ Italy approves it as an add-on to flu vaccines in people with weak immune systems.^{1, 11} FDA "has approved no drug products containing Ta1."¹

FDA says it has four US orphan drug designations, a status that supports research into rare diseases but is not approval.¹ We confirmed two: liver cancer (2000) and stage IIb to IV melanoma (2006).^{10, 25} A National Cancer Institute drug database also lists chronic active hepatitis B and DiGeorge anomaly with immune defects.⁹ Europe also gave it an orphan designation for liver cancer in 2002.¹¹

Despite the name, it is unrelated to thymosin beta-4, the protein that TB-500 is cut from.

How it works

It is described as an immune modulator. Most of the detail comes from cells and animals.¹

• Immune activity in animals. FDA reviewed animal studies that showed immune effects, but said "it is difficult to define the clinical relevance" of them.¹
• T cells in COVID-19. A Chinese hospital study linked it to recovering lymphocyte counts in severe COVID-19, but it was not randomized.¹⁶
• Doses in the dish. In HIV lab studies, FDA noted the concentration used was nearly 2,000 times higher than peak blood levels after clinical doses.¹

Evidence map

Each claimed use is graded separately. A high grade can mean strong evidence that something doesn't work.

Scale: 5 FDA-approved, 4 strong human, 3 limited human, 2 animal or lab only, 1 anecdotal

Key studies

Many of the trials ran in China.^{14, 15, 16} FDA's 2024 advisers said the evidence did not show convincing effectiveness for the uses reviewed.² Even a provider that covers it says there is no evidence for general wellness or longevity use.²⁴

By goal

Pick your goal to see what the evidence supports.

Sources: TESTS,¹⁴ 2023 meta-analyses,^{17, 18} Chien 1998,¹⁹ SciClone,²⁰ Maio 2010.²¹

How it's used

We don't give a dose. There is no US label, and no US-approved product sets one.

In trials it was injected under the skin, usually over weeks to months.^{1, 19} FDA noted that the 3 mg/mL injection it was asked to review is nearly twice the 1.6 mg/mL strength used in the clinical studies.¹ FDA also found it sold online in a "Quad Immune" injection package and as nasal sprays.¹ We found no human study of the nasal route. We report these as information, not instruction.

Side effects and who should avoid it

• Usually well tolerated in trials. FDA's review noted that in many studies no side effects attributable to it were reported.¹ In the large sepsis trial, side effects were as common as on placebo, 66.4% vs 67.6%.¹⁴
• Immune reactions. FDA "cannot rule out the potential for immunogenicity," especially from impurities and clumping in injected products.¹ Its compounding list notes it "may pose significant risk for immunogenicity."⁴
• Liver flares. FDA's review describes liver enzyme flares in people with chronic hepatitis B.¹
• Thyroid. Thyroid test changes were reported when it was given with interferon.¹
• Transplants. FDA notes it could bring on or worsen graft-versus-host disease, or affect the graft, in stem cell transplant patients. One stem cell transplant patient died of immune red cell destruction after it was given with donor immune cells.¹
• Who should avoid it. Anyone with a transplant or on immune-suppressing drugs, given FDA's concerns. There is no US label to set other limits.¹
• Unapproved products. Research vials and sprays have no FDA check on content, strength or sterility.²³

FDA noted that approved drugs treat many of the conditions it reviewed.¹

Compared with

Sources: FDA briefing,¹ PCAC minutes,² HHS Appendix B,⁵ WADA 2026 list.²² See our TB-500 guide.

Stacks

Thymosin alpha-1 is sold online in "immune" bundles with other peptides. FDA found it in a "Quad Immune" injection package.¹ We found no study of any such combination. In trials it was added to standard treatments such as interferon or chemotherapy, not to other peptides.^{20, 21}

Myths vs evidence

All options

MoveWell sells no peptides. Checked November 1, 2026

Licensed compounding, with a prescription

None. It has no USP monograph and isn't in an FDA-approved drug, so a 503A pharmacy could use it only if FDA added it to the bulks list.^{1, 8} FDA's advisers voted 4–17 against that in 2024, it remains on FDA's "nominated but withdrawn" list, and it isn't one of the six peptides in the HHS policy.^{2, 4, 5} Superpower, which offers other peptides, says it does not sell it.²⁴ Telehealth pages offering "thymosin alpha-1 prescriptions" have no lawful US product to fill them with, so we don't link them.

Outside the US

Zadaxin is sold in some other countries under their own approvals.¹³ We don't link foreign pharmacies.

Research use only products

We don't list sellers. In August 2026 FDA warned five peptide sellers that a "research use only" label doesn't excuse products clearly meant for people.²³

Legal status history

Newest first.

1. Current statusNov 1, 2026Not FDA-approved. No lawful 503A compounding pathway. Not in the HHS peptide policy. Approved abroad as Zadaxin.^{1, 2, 5}
2. Next · no dateNo FDA review or HHS action scheduled that we could find.
3. Oct 28, 2026HHS enforcement discretion policy covers BPC-157, TB-500, KPV, MOTS-c, semax and epitalon. Thymosin alpha-1 is not included.^{5, 6}
4. Jun 4, 2026A pharmacist commentary on the Secretary's February remarks lists it among peptides that "may return to Category 1," the interim group pharmacies may compound from. That did not happen.⁷
5. Apr 22, 2026FDA's list still shows it as "nominated but withdrawn," with a note on immunogenicity risk.⁴
6. Jan 2025The TESTS sepsis trial reports no effect on survival.¹⁴
7. Dec 4, 2024FDA's compounding advisory committee votes 4–17 against adding the free base and the acetate to the 503A bulks list. FDA staff had recommended against both.^{1, 2}
8. Sep 27, 2024Removed from Category 2 after the nominator withdrew. That did not make it legal to compound; FDA lists it as "nominated but withdrawn."^{3, 4}
9. Sep 2023Placed in Category 2 with other peptides, over safety concerns.³
10. May 25, 2006SciClone reports its second US hepatitis C phase 3 trial showed no significant benefit.²⁰
11. Mar 20, 2006FDA orphan drug designation for stage IIb to IV melanoma.¹⁰
12. Mar 6, 2000FDA orphan drug designation for liver cancer (hepatocellular carcinoma).²⁵
13. Jul 30, 2002European orphan designation for liver cancer.¹¹
14. 1977Isolated from thymus extract.¹

FAQ

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References