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Thymosin Alpha-1 Compounding Pharmacy Quality Red Flags to Avoid

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9 linked public sourcesEditorial, sourcing, and corrections policy

At a glance

  • US approval status / Not FDA-approved; orphan designations only [1]
  • FDA committee vote, December 2024 / 4 yes, 17 no on the bulks list [2]
  • FDA list, state-licensed pharmacies / In none of the three categories [4]
  • FDA list, outsourcing facilities / Category 3, nominated without adequate support [5]
  • HHS October 2026 peptide policy / Not one of the six peptides covered [6]
  • WADA Prohibited List / Not named on the 2026 or 2027 list [7]

Red flag 1: 'FDA-approved' or 'FDA-authorized'

Thymosin alpha-1 (thymalfasin) is a 28-amino-acid peptide with an acetylated N-terminal end, originally isolated from thymosin fraction 5, a thymus extract [1]. A search of FDA's drug approval database found no US application for it [1]. Its four US orphan drug designations are a separate process, not an approval [1].

It is authorized as a medicine in some other countries. FDA's December 2024 review lists Italy, where Zadaxin is authorized as an adjuvant to influenza vaccination in immunocompromised people, India, for chronic hepatitis B in adults 18 or older with compensated liver disease and virus replication, and Hong Kong [1]. The European Medicines Agency holds only an orphan designation for hepatocellular carcinoma, which is not a marketing approval [1].

Red flag 2: 'FDA lets pharmacies compound it'

On FDA's list of bulk substances for state-licensed pharmacies and physicians, thymosin alpha-1 is in none of the three categories [4]. On the separate list for outsourcing facilities it is category 3, 'nominated without adequate support' [5], and the category 1 policy under which FDA generally does not act against certain compounders does not apply to it [4][5]. HHS's October 2026 draft enforcement-discretion policy names six other peptides, not this one [6].

Red flag 3: bulk powder sold as if it were Zadaxin

Zadaxin is the finished 1.6 mg injection sold outside the US, and it is not interchangeable with bulk thymosin alpha-1 powder [1]. The Italian label says to give 1.6 mg subcutaneously or intramuscularly twice weekly for 4 weeks from the first vaccination, repeated from week 8 to week 12 at the second vaccination [1]. No US-approved dose exists [1].

Red flag 4: safety and efficacy claims with no trial behind them

FDA's posted concerns remain even though the substance sits in the 'nominated but withdrawn' table: 'significant risk for immunogenicity for certain routes of administration', peptide-impurity and characterization problems, and safety information 'inadequate for the agency to sufficiently understand the extent of any safety issues' [3].

A 2026 Cochrane review of 10 randomised trials in 1,349 adults with chronic hepatitis B rated the evidence very low certainty, and the reviewers said they are 'not sure' thymosin alpha-1 helps [8]. In a double-blind trial of 98 adults, HBeAg loss at 72 weeks was 45.8% with thymosin alpha-1 plus interferon versus 28.0% with interferon plus placebo, a difference of 17.8% (95% CI -1.2 to 35.3, P=0.067) that did not reach statistical significance [9]. A claim that it cures hepatitis B is not supported by these trials.

Red flag 5: mixing it up with TB-500

Thymosin alpha-1 and thymosin beta-4 / TB-500 are different peptides. WADA names thymosin-beta-4 and derivatives such as TB-500 in section S2.3, but does not name thymosin alpha-1 [7]. A listing that treats the two as interchangeable is a warning sign.

Frequently asked questions

Is thymosin alpha-1 FDA-approved?
No. FDA's approval database has no US application for it, and its four US orphan drug designations are not an approval [1]. It is authorized in some other countries, including Italy, India and Hong Kong [1].
Does FDA allow compounding of thymosin alpha-1?
It appears in none of the three categories on FDA's list for state-licensed pharmacies and physicians [4], and in category 3, nominated without adequate support, on the outsourcing-facility list [5]. FDA's advisory committee voted 4 yes to 17 no in December 2024 [2].
Is thymosin alpha-1 on the WADA Prohibited List?
It is not named on WADA's 2026 or 2027 list. WADA names thymosin-beta-4 and derivatives such as TB-500 in section S2.3, which is a different peptide [7].

References

  1. U.S. Food and Drug Administration. U.S. Food and Drug Administration
  2. U.S. Food and Drug Administration. U.S. Food and Drug Administration
  3. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks | FDA. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks | FDA
  4. U.S. Food and Drug Administration. U.S. Food and Drug Administration
  5. U.S. Food and Drug Administration. U.S. Food and Drug Administration
  6. Official web page. Official web page
  7. World Anti-Doping Agency. World Anti-Doping Agency
  8. Naing C, Ni H, Aung HH, Aye SN, Nikolova D, Poovorawan Y, et al. Thymosin-ɑ1 for people with chronic hepatitis B. Cochrane Database Syst Rev. 2026. Thymosin-ɑ1 for people with chronic hepatitis B
  9. Lim SG, Wai CT, Lee YM, Dan YY, Sutedja DS, Wee A, et al. A randomized, placebo-controlled trial of thymosin-alpha1 and lymphoblastoid interferon for HBeAg-positive chronic hepatitis B. Antivir Ther. 2006. A randomized, placebo-controlled trial of thymosin-alpha1 and lymphoblastoid interferon for HBeAg-positive chronic hepatitis B
Evidence overview for Thymosin Alpha-1 Compounding Pharmacy Quality Red Flags to Avoid