- For
- Immune support and modulation
- Form
- SubQ injection (1.6 mg)
- Dose
- 1.6 mg, 2x / week
- Cycle
- 6-12 weeks
- Safety
- Avoid if on transplant immunosuppressants
- Legal
- Research peptide in the US (approved abroad)
Bottom line: An immune-modulating peptide approved abroad (Zadaxin) for hepatitis and cancer support. Real clinical track record, but not FDA-approved here.
Safety at a glance
- Status: Approved abroad (Zadaxin); not FDA-approved in the US.
- Evidence: Real international clinical trials; approved abroad, not FDA.
- Do not use if: you have an active autoimmune condition or take immunosuppressants - discuss first.
- Not medical advice - decide this with a licensed physician.
What it does - plain English
Thymosin Alpha-1 is a 28-amino-acid peptide (a short chain of protein building blocks) your thymus gland makes naturally. The thymus is a small gland behind your sternum - the flat bone in the center of your chest. It plays a central role in training your immune system, especially when you're young. The synthetic version, sold internationally as Zadaxin, does what the natural peptide does: it activates and matures T cells, the white blood cells your body uses to recognize and respond to specific threats like viruses and abnormal cells.
It is approved in 35+ countries for three main uses: chronic hepatitis B and C (a viral infection of the liver that can become long-term and damaging if untreated), as a vaccine adjuvant for adults over 65 to improve the immune response to flu and pneumonia vaccines, and as supportive care alongside chemotherapy for certain cancers. An adjuvant, in this context, means something that helps a vaccine work better - not the vaccine itself.
It is not FDA-approved in the US, but functional-medicine doctors and longevity-focused providers use it for immune dysregulation, including post-COVID recovery cases where the immune system remains stuck in an abnormal state after the infection clears.
What people use it for
An A-tier immune peptide. Where people use it:
Status
A-tier internationally. B-tier for US-specific use cases. The approval in 35+ countries is not a technicality - it reflects real clinical trial data. Phase-3 hepatitis trials and chemotherapy-adjunct trials produced evidence that meets FDA-quality standards. The US absence is a regulatory gap, not a scientific one. No pharma company has funded the US approval process, which is expensive and often not worth it for a generic peptide where patents have expired.
Functional-medicine and post-COVID providers use it for immune dysregulation where conventional options are limited. Long-term safety data exists from decades of clinical use in countries where it is an approved drug.
Legal status
Approved as Zadaxin - manufactured by SciClone Pharmaceuticals - in Italy, China, India, Mexico, and 30+ other countries. Not FDA-approved in the US. That means it is not available at a US pharmacy as a standard prescription drug.
Two legal paths exist in the US:
- Compounding pharmacy via prescription - some states allow compounding pharmacies to prepare patient-specific formulations of bulk drug substances under FDA 503A compounding rules, which require physician oversight and a patient-specific prescription. This is the most common route in US functional medicine.
- International Zadaxin import for personal use - legal in some states under the FDA's personal importation policy, which allows individuals to import up to a 90-day supply of a foreign drug for personal use. Gray zone in others, and the rules are not uniformly enforced.
The compounded-prescription route is more common and better vetted in the US. Providers who use it regularly have established relationships with compounding pharmacies that carry it.
Where to source
Always ask for a COA - a Certificate of Analysis, which is the lab report a pharmacy or vendor provides to confirm the compound contains what the label says, at the stated concentration, without contaminants. No COA, no buy.
Two main paths in the US:
- Compounding pharmacy via telehealth - the most legal route. A licensed provider writes the prescription, a 503A-compliant pharmacy prepares it, and you get a batch-specific COA. Functional-medicine and longevity-focused telehealth services often have established pharmacy relationships for this.
- International Zadaxin - the brand-name approved drug, sourced from a country where it is legal. Higher quality assurance than research peptides, but the import rules vary by state and are not uniformly enforced. Personal-use quantities only.
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Two studies worth reading
Hepatitis C trial
Sherman, K. et al., Hepatology
Multicenter randomized trial in chronic hepatitis C patients. Combination therapy with thymosin alpha-1 plus interferon (a standard antiviral drug) outperformed interferon alone on viral response rates. This is the trial that established the international approval basis for hepatitis C treatment.
Vaccine response in older adults
Adults over 65 receiving thymosin alpha-1 alongside a flu vaccine showed improved antibody response compared with vaccine alone. This is the trial that supports its use as a vaccine adjuvant in older adults, whose immune systems tend to respond less robustly to vaccines than younger people.
Watch: Huberman on peptides (June 2026)
Thymosin alpha-1 is tied, in the June 2026 episode, to thymus involution - the immune-training gland that shrinks with age - and paired with actually measuring thymic health through bloodwork and immune-cell counts. One of the peptides with more real clinical precedent behind it.
Source: Huberman Lab - Peptides: The Science, Uses & Safety (Dr. Abud Bakri). See our full decode of what he says about every peptide.
Last reviewed · 2026·06·04 · Status reviewed weekly