Thymosin Alpha-1

What it is

Thymosin alpha-1 (Tα1), international nonproprietary name thymalfasin, is a 28-amino-acid immunomodulatory peptide, marketed outside the United States under the brand Zadaxin.S3

Marketed as

To consumers, thymosin alpha-1 is marketed as an "immune booster" — for immune support, recovery, and chronic infections. It has a real clinical trial record (below), but consumer immune-boost marketing outruns what is established for healthy users.S3

Regulatory status (US)

Tα1 is not FDA-approved in the United States, but is approved as a pharmaceutical in numerous other countries (reported in 30+ markets) for indications such as hepatitis B/C and as an immune adjuvant.S3 On FDA's compounding framework, Thymosin-alpha 1 appears in the "nominated but withdrawn" portion of Category 2; it is not among the seven peptides on the July 2026 PCAC agenda.S1

Around the world

This is the sharpest US-vs-world contrast on the site: thymosin alpha-1 is an approved medicine (Zadaxin / thymalfasin) in 35+ countries — including China, Italy, India, Argentina, the Philippines, and much of Asia and Latin America — for hepatitis B/C and as a cancer immune-adjuvant, yet it is not FDA-approved in the US.S3

Evidence

Among thymic peptides, Tα1 has the most extensive human clinical literature: numerous randomized trials across hepatitis, sepsis, cancer-immunotherapy adjuvant use, and vaccine response,S3 and it was studied in COVID-19 (NCT04487444).S2 The evidence is mixed and indication-dependent — some sepsis meta-analyses suggested benefit, while a large placebo-controlled sepsis trial did not show a clear mortality benefit — so any efficacy statement should be made cautiously and per indication.S3

Anti-doping

Tα1 is an immune-signaling peptide, not a growth factor, and based on available information is not currently listed as a named prohibited substance on the WADA List. This should be confirmed directly before it is stated without qualification.

Safety

Tα1 is generally reported as well tolerated in the trial literature. US compounded material still carries FDA's general Category 2 concerns — immunogenicity for some routes, impurity/characterization complexity, and limited safety data.S1S3

What's changing

No PCAC review of thymosin alpha-1 has been verified as scheduled; its US status remains "not approved, nomination withdrawn" pending any future FDA action.S1