What isThymosin Alpha-1FDA Status, Evidence and Risks (2026)
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Off FDA's Category 2 list since Sept 2024 (nomination withdrawn), voted down 4 to 17 by FDA's advisory panel for the 503A bulks list in Dec 2024, and not approved by FDA for any use. Source: FDA, Category 2 page current as of Apr 22, 2026 1, and Dec 4, 2024 voting results 2.
Part of the Sommar Peptide Guide. Thymosin alpha-1 has a real drug history abroad. It also has a clean no from FDA's panel. Both are true. Here is how.
What thymosin alpha-1 is
Thymosin alpha-1 (Tα1) is a 28-amino-acid peptide first isolated from calf thymus in 1977 3. Tα1 is studied for how it adjusts immune activity 4.
The synthetic version is called thymalfasin 3. Some databases also list it as Zadaxin. FDA notes that name belongs to a finished product made abroad and is not interchangeable with the substance itself 3.
FDA reviewed two forms for compounding, the free base and the acetate salt, as separate substances 3.
Where FDA stands
- Sept 2024: FDA took thymosin alpha-1 off Category 2 after the nomination was withdrawn 1,5.
- Dec 4, 2024: FDA reviewed it anyway, on its own initiative, for a long list of uses, including hepatitis B and C, HIV, COVID-19 and sepsis 3. Staff said the evaluation criteria "weigh against" both forms 3.
- Same day: the advisory committee voted 4 yes, 17 no for the free base, and 4 yes, 17 no for the acetate 2.
- Today: not on Category 2, not on the 503A bulks list 6, not approved by FDA for any use. FDA says it "has approved no drug products containing Ta1" 3.
Leaving Category 2 approved nothing. It happened because the nominator withdrew. Are peptides legal? has the full explanation.
Not in the HHS policy. On Oct 28, 2026, HHS filed an interim enforcement discretion policy for compounding six peptides: BPC-157, TB-500, MOTS-c, epitalon, semax and KPV 7. Thymosin alpha-1 is not one of them, so its status above stands, as of Oct 29, 2026. The FDA Peptide Tracker logs every change.
Approved abroad, not here
A 2023 review says thymosin alpha-1 "is now approved in over 35 countries for the treatment of hepatitis B and C" 4. FDA's briefing lists labels it could check 3:
| Country | Use FDA lists |
|---|---|
| India | Chronic hepatitis B, adults with compensated liver disease |
| Indonesia | Chronic hepatitis B |
| Italy | Add-on to flu vaccination in people with weak immune systems |
| Mexico | Chronic hepatitis B, flu vaccine add-on |
| Singapore | Chronic hepatitis B |
Singapore and Mexico come from drug monographs, not labels. FDA also says the broader list of approving countries comes from a 2014 company annual report it could not verify 3.
In the US, the record is thinner. FDA notes four US orphan drug designations 3. An orphan designation is not an approval. In Europe, it has an orphan designation for liver cancer, which FDA also notes is not an approval 3.
What the research shows
Human trials exist, which puts it ahead of most peptides on this shelf. FDA cites a count of over 4,400 people enrolled in Tα1 trials across several conditions 3. The results are mixed.
Hepatitis B. The 2023 review calls efficacy "confirmed by many studies," but its own table is mixed 4. Paired with entecavir, it "tends to inhibit" liver cancer development. Added short term to peginterferon, it was not better. Paired with lamivudine, no clear benefit 4.
COVID-19. In one study of 771 patients, deaths were lower in the Tα1 group before adjustment, but after adjusting for other factors there was no significant link to 28-day survival 4. The review calls safety and efficacy "uncertain" and points to a lack of large randomized trials 4.
Sepsis and HIV, lab side. In a mouse sepsis study FDA cites, a trend toward lower bacterial load and better survival did not reach statistical significance 3. In HIV lab work, the concentrations that worked were about 2,000 times higher than blood levels from the usual 1.6 mg dose 3.
Side effects and FDA's concerns
FDA's stated concerns, from its Category 2 page 1:
- Immune reactions and impurities. Possible immunogenicity for certain routes, plus complexities with peptide-related impurities and characterization.
- Thin data. "The safety-related information is inadequate."
From FDA's 2024 briefing 3:
- Common: irritation, redness and discomfort at the injection site.
- In trials: liver enzyme flares in a hepatitis B trial of 316 people, 22 flares across both dose groups. Shifts in thyroid-stimulating hormone (TSH) in 2 people in a hepatitis C trial. Nipple pain in more than 10% of treated people in a liver cancer trial.
- After stem cell transplant: in one small study, 1 of 6 patients developed fatal immune hemolytic anemia after donor cell infusion plus Tα1.
- Vaccines: FDA says lack of effect with flu vaccine "is viewed as a safety issue," and adding Tα1 to any vaccine could raise significant safety concerns.
- Quality: neither form is well characterized. The certificates FDA saw had no impurity, endotoxin or bioburden testing. The free base may not even dissolve at the proposed 3 mg/mL.
- Animal data: "too limited to inform safety considerations." No cancer studies found.
Some foreign labels say not to use it in people whose immune systems are deliberately suppressed 3. FDA concluded that compounding with it "may raise safety concerns" 3.
Why it's sold online anyway
Thymosin alpha-1 is pitched online for immune support. A real overseas drug name makes that pitch sound settled. It is not settled for compounding in the US, and FDA's panel said so 4 to 17.
A "research use only" label is the seller saying the product is not for people. No prescriber, no pharmacy compounding for one patient, no quality check for human use. FDA found a 2018 inspection note of a 503A pharmacy compounding Tα1, in an unclear form 3. More: Prescription vs Research Peptides.
Other Shelf 5 pages: kisspeptin-10, the last peptide on the pharmacy Category 2 list, and melanotan II, awaiting review.
Asked a lot.
The sources.
- FDA. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks (content current as of Apr 22, 2026). Read it
- FDA. Pharmacy Compounding Advisory Committee, Dec 4, 2024, voting results. Read it
- FDA. Pharmacy Compounding Advisory Committee, Dec 4, 2024, briefing document: thymosin alpha-1 related bulk drug substances. Read it
- Tao N, Xu X, Ying Y, et al. Thymosin α1 and Its Role in Viral Infectious Diseases: The Mechanism and Clinical Application. Molecules. 2023;28(8):3539. Read it
- Reed Smith. FDA removes certain peptide bulk drug substances from Category 2 (2024). Read it
- FDA. Bulk Drug Substances Used in Compounding Under Section 503A of the FD&C Act (content current as of May 14, 2026). Read it
- HHS. Enforcement Discretion Policy Regarding 503A Compounding Using Certain Peptide Bulk Drug Substances. Docket HHS-OASH-2026-0496, FR Doc 2026-22192 (filed for public inspection Oct 28, 2026; scheduled to publish in the Federal Register Oct 30, 2026). Read it
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