Read This First
This page is not built on clinical evidence. It is built on the FDA's 2024 review of the compound, the regulatory record in the countries where it is approved, and peptide chemistry. What thymosin alpha-1 does in people is covered on the library entry; this page is about the difference between a finished drug product made to a national standard abroad and a vial of the same peptide sold in the United States.
Thymosin alpha-1 occupies an unusual position: a finished pharmaceutical product, Zadaxin, exists and is lawfully marketed in Italy, India, Hong Kong, Singapore and elsewhere, while in the United States the substance has been formally reviewed for compounding and rejected.12 Almost nothing sold as thymosin alpha-1 in the United States is Zadaxin, and the FDA's own review says the two should not be treated as interchangeable.
Legal Status
Where Does Thymosin Alpha-1 Stand?
Approved abroad, not at home. The FDA's review confirms marketing authorisations in Italy, India, Hong Kong and Singapore and records the manufacturer's claim of approvals across the Asia-Pacific, Latin America, Eastern Europe and the Middle East, mostly for hepatitis B and as a vaccine adjuvant. It adds that it "is unable to independently verify these claims of approval" and that the substance is not in the European or Japanese pharmacopoeias.1 It has never been approved by the FDA, the European Medicines Agency, the UK's MHRA or Health Canada.
In the United States it sat in the FDA's Category 2 of nominated compounding substances from September 2023, is now listed as nominated but withdrawn, and was rejected for the 503A bulks list by a 4 to 17 advisory vote on 4 December 2024.32 No US pharmacy has a lawful basis on which to compound it.
Why Did the FDA Reject It If It Is Approved Elsewhere?
Because the criteria are different, and the review says so in detail.
A foreign approval attaches to a finished product: a specific formulation, made by a specific manufacturer, to a specification the national regulator reviewed. A bulks list entry would attach to the raw substance, for any pharmacy to formulate. The FDA found that neither the free base nor the acetate was "well-characterized" for that purpose, because "certain critical characterization data specific to" the substance, "including impurities, aggregates, bioburden, and bacterial endotoxins, were not found in publicly available scientific literature and the nomination package lacked information to establish identity, purity, and impurity profiles."1
It also raised a practical objection that goes to what is being sold. The nomination proposed an injectable at 3 mg/mL. The FDA noted that "due to limited water solubility" of both forms, "it is unclear how it would be possible to formulate the proposed injectable dosage form" at that concentration, "and no information was provided to explain how this solubility could be achieved."1 Zadaxin is 1.6 mg/mL. A US product labelled at a higher concentration is either using a formulation the regulator could not identify, or is not what its label says.
Has It Been Compounded in the United States?
Yes, and the record is thin and unflattering. The FDA's review found no published study of a compounded thymosin alpha-1 product in humans and no outsourcing facility reporting that it made one. What it found was a 2018 Form 483, the inspection document issued when investigators observe conditions that may violate the law, showing a 503A pharmacy compounding products containing the substance, and internet searches showing it "being compounded as an injection and as a nasal spray" and marketed for hepatitis B, hepatitis C, HIV, chronic fatigue, inflammation, sepsis, COVID-19, Lyme disease, allergies, cancer, asthma, COPD and psoriatic arthritis.1
Product Identity
Zadaxin, Thymalfasin, Thymosin Alpha-1: Which Is Which?
| Name | What it is | What it guarantees |
|---|---|---|
| Thymosin alpha-1, Tα1 | The 28-residue acetylated peptide, as a substance | Nothing about any particular vial |
| Thymalfasin | The international nonproprietary name for the synthetic version; interchangeable with Tα1 as a substance1 | Nothing about any particular vial |
| Zadaxin | SciClone's finished product, 1.6 mg lyophilised powder, approved in specific countries | A national regulator has reviewed the manufacturing, specification and label |
| Chinese-market thymalfasin products | Generic finished products approved by China's regulator, used in the sepsis and pancreatitis trials | The Chinese regulator has reviewed them; they are not Zadaxin |
| "Thymosin alpha-1" from a US vendor or clinic | Powder of unverified origin, or a compounded preparation | Whatever the certificate of analysis can establish, which is less than it looks |
The FDA's own memo draws the line: thymalfasin and thymosin alpha-1 "can be used interchangeably," but Zadaxin "is a finished drug product available in other countries, which cannot be interchangeably used" with the substance.1 The 350,000-patient safety record belongs to the finished products. The vial does not inherit it by sharing a name.
What Makes This Peptide Hard to Verify?
Three things. It is 28 residues, long enough that a synthesis producing deletion or truncation impurities at each coupling step accumulates a meaningful fraction of near-identical sequences. It is N-acetylated, and a product missing the acetyl group has the same sequence, a mass 42 daltons lighter, and the FDA notes the acetylation is what makes the synthetic a "closer mimic of the native protein."1 And it is poorly water-soluble, so a product that dissolves easily at high concentration is a product with something else in it.
Purity Testing Versus Identity Testing
| Test | Question it answers | Question it cannot answer |
|---|---|---|
| HPLC purity | What proportion of the sample is one main peak | Whether the main peak is acetylated, full-length thymosin alpha-1 |
| Mass spectrometry identity | Whether the mass matches | How much of the sample it represents; whether deletion sequences co-elute |
| Endotoxin assay | Whether bacterial cell-wall fragments are present | Identity or content |
| Aggregate analysis | Whether the peptide has clumped | Almost never offered; this is the specific immunogenicity concern the FDA named1 |
Certificates of Analysis
What a Certificate Is
A laboratory report on one batch. Read it for the laboratory's independence from the seller, the lot number and whether it matches the vial, the date, the exact substance named including whether it is the free base or the acetate, and which tests were actually run rather than mentioned.
What It Cannot Establish
That the vial in hand came from the batch tested. Its value rests on a chain of custody the buyer cannot inspect. A certificate is a PDF, and nothing about the format prevents reuse or fabrication. The FDA has documented fabricated pharmacy names on gray-market peptide products.4
Importing the Approved Product
Can Zadaxin Be Brought Into the United States?
Not lawfully as a rule. Importing an unapproved drug for personal use is prohibited; the FDA exercises enforcement discretion in narrow circumstances involving serious conditions without an available US treatment, a small supply, and a US physician's involvement, and none of those describes buying an immune-support peptide online. ONPEPS does not advise on customs law and does not recommend the attempt. What can be said is that a product arriving in the United States labelled Zadaxin is either a genuine foreign product outside its lawful market, or a counterfeit of one, and the buyer cannot tell which.
"Research Use Only"
Does the Label Change Anything?
No. It is a liability position adopted by the seller, not a regulatory category. The FDA has held in warning letters that the disclaimer does not survive product copy establishing human use.5 For a molecule that is a licensed medicine in Italy, a page disclaiming human use while citing the Italian licence has supplied the agency's argument.
Storage and Handling
What Is Known
For the substance, the FDA's review reports the free base "is expected to be stable under storage conditions below -18°C" and the acetate below 20°C, while cautioning that peptide stability "is highly sensitive to the manufacturing process and quality attributes" of the finished product.1 That is a statement about raw material in a freezer, not about a reconstituted vial in a refrigerator door.
For Zadaxin, the foreign label governs, and it is written for that formulation. For a compounded or research product, no validated data exist; the ordinary rules apply, and the solubility problem adds one: a solution that turns cloudy on standing, or a powder that does not dissolve completely, is behaving the way the FDA said this peptide behaves at concentrations above the approved one.
Multi-Dose Vials
The approved dose is 1.6 mg twice weekly.1 Community protocols vary and often dose more frequently. The FDA's guidance for compounded injectables is to discard a multi-dose vial within 28 days of first use regardless of what remains.4
Signs to Stop Rather Than Proceed
Powder that does not dissolve completely, or a solution that is cloudy or develops particles on standing
Injection-site reactions that grow with successive doses, the pattern of an immune response and the specific risk the FDA named for aggregated peptide
Any symptom suggesting an autoimmune flare, given the foreign labels' contraindication in autoimmune disease1
A label or certificate that does not state free base or acetate, or a concentration above 1.6 mg/mL with no explanation of how it was achieved
Community Language
Tα1, TA1, Thymosin alpha
The compound. Not the same molecule as thymosin beta-4 or TB-500.
Thymalfasin
The nonproprietary name for synthetic thymosin alpha-1. Interchangeable with the substance; not with any finished product.1
Zadaxin
SciClone's finished product, 1.6 mg, approved in some countries and not in the United States or Europe. The FDA wrote in 2024 that it was unclear whether the company remained in business.1
Immune peptide, immune support
Marketing terms. The compound's immune effects are real and measurable on biomarkers; the FDA found no proposed use for which they translated into a clinical benefit over standard care.1
Category 2
The FDA's interim classification for nominated compounding substances that may present significant safety risks. Thymosin alpha-1 was placed there in 2023 and is now listed as nominated but withdrawn.3
Certificate of analysis
A laboratory report on one batch. It describes the batch tested, not necessarily the vial received.
Related ONPEPS Coverage
Thymosin alpha-1 library entry: the approvals, the trials and the December 2024 vote
Compounded, gray market and counterfeit semaglutide: what the regulators have documented
Sources
US Food and Drug Administration. Thymosin alpha-1-related bulk drug substances: FDA review for the December 4, 2024 meeting of the Pharmacy Compounding Advisory Committee. Docket FDA-2024-N-4777. fda.gov
US Food and Drug Administration. Final Summary Minutes of the Pharmacy Compounding Advisory Committee Meeting, December 4, 2024. Approved 21 February 2025. fda.gov
US Food and Drug Administration. Category 2 of the Bulk Substances Nominated Under Sections 503A or 503B of the Federal Food, Drug, and Cosmetic Act. Entry for thymosin-alpha 1. Content current as of 22 April 2026. fda.gov
US Food and Drug Administration. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. Drug Alerts and Statements. Cited for the agency's general guidance on multi-dose vials and fabricated labels. fda.gov
US Food and Drug Administration. Warning Letter: Gram Peptides. MARCS-CMS 721806, 31 March 2026. fda.gov
Not Medical Advice
Thymosin alpha-1 is a prescription medicine in some countries and an unapproved substance in the United States and Europe. If you are using a product obtained outside a pharmacy, the route back to oversight is a clinician who knows what you have taken.
Regulatory sources verified 16 September 2026. The FDA's compounding categories are under active revision and the record changes frequently. Re-verify before relying on any statement here.