The short answer. Thymosin alpha-1 is not FDA-approved and, as of September 26, 2026, has no compounding pathway. FDA’s advisory committee voted 4 to 17 against listing both the free base and the acetate on December 4, 2024, in line with FDA’s recommendation.
| FDA approved | No. No USP monograph, and not a component of any approved drug. |
|---|---|
| 503A pharmacy | No pathway. Not on 21 CFR 216.23 and in no category since Sep 27, 2024. |
| 503B outsourcing facility | No pathway. In 503B Category 3, nominated without adequate support. |
| Latest FDA action | Advisory committee, Dec 4, 2024: 4-17 against the free base and 4-17 against the acetate. |
| MDside position | Our providers do not prescribe it. |
| Last verified | 2026-09-26 |
What happened, in order
| Date | What happened |
|---|---|
| Feb 2021 | Wells Pharmacy Network nominated thymosin alpha-1 for the 503A list. |
| Sep 29, 2023 | FDA placed it in 503A Category 2. |
| Sep 18, 2024 | The nominator withdrew. FDA removed it from Category 2 by the Sep 27, 2024 update. |
| Dec 4, 2024 | FDA took both forms to its advisory committee anyway. The committee voted 4-17 against each. |
Approvals abroad do not carry over
Thymosin alpha-1, also called thymalfasin, is sold outside the United States under the name Zadaxin. FDA’s briefing notes those foreign approvals and says FDA cannot independently verify them. Either way, no foreign approval creates a US compounding pathway. A 503A pharmacy needs a USP monograph, an FDA-approved drug containing the substance, or a listing at 21 CFR 216.23, and thymosin alpha-1 has none of the three.
Thymosin alpha-1 and thymosin beta-4 are separate questions
The names are close and vendors often sell them together. They are different molecules with different records. The TB-500 page covers the thymosin beta-4 fragment. Both lack a pathway.
What FDA told the committee
FDA’s briefing found that its evaluation criteria weighed against listing both forms. Its safety-risk page lists thymosin alpha-1 among substances nominated and then withdrawn, citing a possible immunogenicity risk, impurities and inadequate safety information. The committee took separate votes on the free base and the acetate, and both failed 4 to 17. No rulemaking has followed.
What this means for you
Take thymosin alpha-1 off your menu, including immune support bundles marketed during cold and flu season. If a supplier points to Zadaxin or to research in hepatitis or cancer, none of it creates a US pathway. For what an immune or wellness offering can lawfully include, talk to us about peptide programs.
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Frequently asked questions
Is thymosin alpha-1 legal to compound?
No. As of September 26, 2026 thymosin alpha-1 is not on the 503A bulks list, sits in no 503A category and is in 503B Category 3. FDA’s advisory committee voted 4 to 17 against listing both forms in December 2024, and no rule has followed.
Is thymosin alpha-1 the same as Zadaxin?
Zadaxin is the brand name for thymalfasin, which is thymosin alpha-1, sold outside the United States. It is not FDA-approved. FDA’s briefing notes the foreign approvals but says FDA cannot verify them independently, and a foreign approval creates no US compounding pathway in any case.
Why did FDA review thymosin alpha-1 after the nomination was withdrawn?
FDA chose to take both forms to its advisory committee on December 4, 2024 even though the nominator had withdrawn in September. The committee voted 4 to 17 against the free base and 4 to 17 against the acetate, matching FDA’s recommendation, and no rule has followed since.
Primary sources. Last verified September 26, 2026. Corrections are welcome: contact us.
- FDA, 503A bulk drug substance categories (updated May 14, 2026)
- FDA, Certain bulk drug substances that may present significant safety risks (updated Apr 22, 2026)
- FDA, Interim Policy on Compounding Using Bulk Drug Substances Under Section 503A (Jan 2025)
- 21 CFR 216.23, the 503A bulks list
- FDA, 503B bulk drug substance categories (updated Mar 21, 2025)
- FDA briefing document, thymosin alpha-1 (Dec 2024 PCAC)
- FDA, PCAC minutes, Dec 4, 2024
- FDA, PCAC transcript, Dec 4, 2024 (afternoon)
- Regulations.gov, withdrawal of thymosin alpha-1 nomination, FDA-2015-N-3534-0470 (Sep 2024)
This is general information, not legal advice. Rules vary by state and change. Confirm your own facts with counsel.