An immune-modulating peptide your body already makes, and a registered prescription drug in 35+ countries. Just not in the U.S. Sold internationally as Zadaxin for hepatitis and immune support.
The 30-second read
Thymosin Alpha-1 is a 28-amino-acid peptide originally isolated from the thymus gland, the small organ in your chest that trains your immune system. The body produces it naturally; the synthetic version (sold as Zadaxin) has been a registered pharmaceutical product since the 1990s. It's approved in roughly 35 countries. Italy, China, Mexico, Japan, the Philippines, and many others, for hepatitis B, hepatitis C, vaccine boosting, and as an immune adjuvant in cancer care. The FDA has not approved it in the U.S. The clinical evidence for the approved indications is real and decades-long; the broader "boost your immune system" use case popular in research-peptide circles has much thinner evidence.
Why this peptide is on people's radar
Thymosin Alpha-1 has the most unusual regulatory profile of any peptide in this database. It's been a registered prescription drug for over thirty years, with hundreds of clinical studies behind it, used routinely in some of the world's largest healthcare systems for chronic hepatitis and as an adjuvant in cancer treatment, but it's never received FDA approval. SciClone Pharmaceuticals, which holds the original commercial rights, sought U.S. approval years ago and didn't reach the bar; the molecule has continued in international clinical practice ever since.
For people in the U.S. who pay attention to peptides, that creates an unusual dynamic: a drug that's clinically established elsewhere and almost completely off the radar at home. Researchers, clinicians, and longevity-focused readers tend to know about it. The general public usually doesn't. Renner's recovery-protocol post in 2023 mentioned Thymosin Alpha-1, which pulled it briefly into the Wolverine-stack conversation, though the use case there (recovery from massive multi-system trauma) is very different from the clinical applications the peptide is approved for elsewhere.
The peptide is also a recurring topic in long-COVID, post-viral, and persistent-infection contexts, partly because of its known role in T-cell maturation and immune balancing. The clinical evidence for those specific uses is much thinner than for the approved indications.
What people are usually trying to do with it
People exploring Thymosin Alpha-1 are usually focused on:
Supporting immune function during chronic infection (hepatitis, persistent viral conditions)
Recovering from major surgery, trauma, or illness
Boosting response to vaccination (used as a vaccine adjuvant in some countries)
Adjunct support in cancer treatment (the most-studied off-label use case internationally)
Long-COVID or post-viral fatigue contexts (less rigorously studied)
General "immune support" in longevity protocols (the thinnest-evidenced use case)
What the science actually shows
Thymosin Alpha-1 has more clinical evidence than most non-FDA-approved peptides on this site, with the strongest signals concentrated in specific indications. Plain-English summary:
Chronic hepatitis B and C (approved indications)
Multiple clinical studies, including randomized trials, have reported antiviral and immune-modulating effects of Thymosin Alpha-1 in chronic hepatitis B and C. Effect sizes are modest as monotherapy and larger when combined with interferon-based regimens.12
Vaccine response in immunocompromised patients
Trials in dialysis patients and elderly adults show enhanced antibody response to influenza and hepatitis B vaccines when Thymosin Alpha-1 is given as an adjuvant. This is one of the better-evidenced use cases.3
Cancer adjuvant use
Trials in melanoma, hepatocellular carcinoma, and non-small-cell lung cancer have studied Thymosin Alpha-1 alongside conventional treatments, with positive signals in some studies and inconsistent results in others. Not a primary cancer therapy.4
Sepsis and severe infection
Some Chinese clinical research has investigated Thymosin Alpha-1 in sepsis and severe pneumonia (including in COVID-19 contexts), with reports of mortality benefit in defined patient populations. Independent replication outside China is limited.5
What hasn't been demonstrated
FDA approval for any indication. Strong evidence for general "immune boosting" in healthy adults. Conclusive evidence in long-COVID or persistent post-viral conditions. That the broad immune-support claims popular in longevity protocols translate into measurable health outcomes.
The honest read
What's solid:
For its approved international indications, chronic hepatitis B and C, vaccine adjuvant in immunocompromised populations, certain cancer-adjuvant uses, the evidence is real and the regulatory legitimacy in 35+ countries is meaningful. The mechanism (T-cell maturation, dendritic cell modulation) is well-characterized. Three decades of post-marketing safety data exist.
What's still being worked out:
Why the FDA has not approved it. The U.S. regulatory process is more demanding in specific ways than the systems that have approved Thymosin Alpha-1 elsewhere. The healthy-adult immune-support use case isn't well-evidenced. Long-COVID and persistent post-viral applications are being studied but haven't produced FDA-approval-quality data.
What's hyped beyond the evidence:
Treating Thymosin Alpha-1 as a general "immune boost" supplement. The clinical evidence is concentrated in specific patient populations with measurable immune dysfunction (chronic viral hepatitis, immunocompromised, cancer). Healthy adults using it as preventive support have far less evidence behind that decision. Also: the COVID-era enthusiasm sometimes outran the data, some studies reported benefit, others didn't, and the picture remains mixed.
Things to know if you're looking into it
How it's used: a subcutaneous injection, typically twice weekly. International clinical doses are 1.6 mg per dose for hepatitis indications.
Brand name internationally: Zadaxin (SciClone Pharmaceuticals), available in roughly 35 countries.
Regulatory status in the U.S.: not FDA-approved. Not on the FDA Category 2 list as of 2026.
Most legitimate use cases are clinical: hepatitis B/C, immunocompromised vaccine response, cancer-adjuvant. The healthy-adult "immune support" use case has much thinner evidence.
Reported tolerability: generally favorable across decades of clinical use. Mild injection-site reactions are most common.
Healthcare provider involvement: essential. This is a real prescription drug in much of the world, not an over-the-counter supplement, and the indications it's used for are serious clinical conditions.
Specific dosing protocols, mechanism, and the full reference list: all in the "Want to go deeper?" section below.
Reconstitution & dose calculator
Approved in ~35 countries, not FDA-approved in the U.S. The standard clinical dose internationally (Zadaxin®) is 1.6 mg per injection, twice weekly, for hepatitis and immune-adjuvant use. This is an educational reference for that established dose math, not a recommendation. A clinician's involvement is essential.
Suggested start
1.6 mg/inj
Standard Zadaxin clinical dose
Common range
1.6 mg/inj
2× weekly is the standard rhythm
Max per injection
3.2 mg/inj
Research upper bound; doubles the standard
Cycle
8–12 wks
Longer in chronic indications
mL
Defaults to a 5 mg/mL dilution — standard 1.6 mg dose lands at a clean 32 units. Adjust to taste.
mg
Subcutaneous injection, twice weekly. Some chronic protocols increase frequency to daily.
Above the typical research range. Per-injection doses above 3.2 mg exit common protocols. The international clinical evidence base sits at 1.6 mg per injection.
Concentration
5.0 mg/mL
Per dose
0.32 mL
32 units on insulin syringe
Doses per vial
~6
~6 injections (~3.0 weeks) at 2× weekly
When to stay put vs. adjust
Stay put at 1.6 mg twice weekly for the duration of the cycle if you're tolerating it. This is the dose that drove the international approvals and where the clinical evidence lives. There's no titration ladder built into the established protocols — the standard dose is just the dose.
Consider stepping frequency up to 3× weekly or daily only in serious chronic indications and with clinician guidance. Some hepatitis-adjuvant and severe-immune-dysregulation protocols use more frequent dosing; the per-injection amount usually stays at 1.6 mg. Going to 3.2 mg per injection is uncommon outside specific research contexts.
Cycle off at the 8–12 week mark for general immune-support contexts. Long-term continuous use exists in chronic-disease indications under medical supervision but isn't the default pattern. Long-term safety data is favorable but most informative in the populations the drug is approved for, not in healthy adults.
The honest read on dosing. Healthy-adult immune-support use is the weakest-evidenced application. The 1.6 mg twice-weekly protocol comes from clinical trials in hepatitis and immunocompromised patients. Whether that dose is "right" for a healthy adult looking for immune optimization is genuinely an open question.
For educational and research purposes only. This is not medical advice. Thymosin Alpha-1 is a prescription drug in 30+ countries (sold as Zadaxin®) but is not FDA-approved in the U.S. Consult a licensed healthcare provider before any health decision.
What people often ask
Why is it approved in 35 countries but not the U.S.?
Honest answer: nobody who isn't deep inside SciClone really knows. Different regulatory systems weigh evidence differently. The FDA has historically asked for stronger or different forms of evidence than some of the systems that approved Thymosin Alpha-1. The drug is real and clinically used, it just hasn't cleared the FDA's particular bar.
Is it a "natural" peptide?
The body produces it naturally, it's secreted by the thymus gland, which trains your immune system. The synthetic version used clinically and in research is the same sequence as the natural one.
Will it help me fight off colds and flu?
The evidence for healthy adults using it preventively for common viral illness is thin. The clinical use cases are concentrated in people with measurable immune dysfunction or chronic viral conditions.
Did it help with COVID-19?
Some Chinese trials reported mortality benefit in severe COVID-19 cases. Independent replication outside China was limited and the global picture was mixed. Not approved by Western regulators for COVID-19.
Can it help with long-COVID or persistent fatigue?
Studied informally and in some smaller trials. The evidence isn't yet strong enough to support confident claims in either direction. Long-COVID is heterogeneous; what helps one patient may not help another.
What's the safety profile like?
Generally favorable across three decades of clinical use. Mild injection-site reactions are the most common adverse event. Long-term safety data exists from the international clinical experience.
How does it differ from Thymosin Beta-4?
Different peptide, different function. Thymosin Alpha-1 (28 amino acids) is involved in T-cell maturation and immune signaling. Thymosin Beta-4 (43 amino acids, the parent of TB-500) is involved in cell migration and tissue repair. Both come from the thymus gland but they do different things.
FDA and regulatory status
Status as of May 5, 2026: Not FDA-approved in the United States. Approved as a prescription medication in approximately 35 countries (sold as Zadaxin by SciClone Pharmaceuticals), including in Italy, China, Mexico, Japan, the Philippines, and across much of Asia and Latin America. Indications vary by country but commonly include chronic hepatitis B, chronic hepatitis C, vaccine adjuvant use in immunocompromised patients, and adjuvant support in certain cancers. Not currently on the FDA Category 2 list. Status updates land here when they happen.
Notable commentary
Public reference that has raised Thymosin Alpha-1's profile in the U.S. peptide conversation.
"Everyday, countless hours of physical therapy, peptide injections, iv drips and pushes, stem cell and exosomes, red light / IR therapy."
Jeremy Renner, in a 2023 Instagram post detailing his recovery regimen following the January 2023 snowplow accident. Renner has publicly named BPC-157, Thymosin Alpha-1, Thymosin Beta-4, and MOTS-c as peptides in his recovery protocol. Coverage in NBC News and The Hollywood Reporter.
Personal anecdote, not clinical evidence. For educational purposes only.
Want to go deeper?
Mechanism, dosing, the international clinical-trial detail, side effects, and references. Click to expand.
Background and development
Thymosin Alpha-1 is a 28-amino-acid peptide naturally produced by the thymus gland, where it plays a role in T-cell maturation and immune balance. It was first isolated and characterized by Allan Goldstein and colleagues in the late 1970s. The synthetic version was developed as a pharmaceutical product by SciClone Pharmaceuticals (originally Thymosin BioSciences) under the brand name Zadaxin. It became one of the longest-marketed synthetic peptides in clinical use, with approval in 35+ countries beginning in the 1990s.
Mechanism of action
T-cell maturation and balancing
Thymosin Alpha-1 promotes maturation of T lymphocytes, particularly CD4+ helper T cells and CD8+ cytotoxic T cells. It enhances differentiation of immature thymocytes into functional T cells with appropriate receptor diversity.
Dendritic cell activation
The peptide acts on Toll-like receptors (particularly TLR9 and TLR2) on antigen-presenting cells, supporting their ability to process and present antigens to T cells. This is part of the rationale for its use as a vaccine adjuvant in immunocompromised populations.
Immune balancing rather than immune stimulation
An important distinction: Thymosin Alpha-1 is described as immune-modulating rather than purely immunostimulatory. In autoimmune contexts it may have a different profile than in immunocompromised contexts, the peptide tends to push the immune system toward a more balanced state.
Antiviral effects
The mechanism in chronic hepatitis B and C is thought to combine direct immune effects with potentiation of interferon-based regimens, hence the use of Thymosin Alpha-1 alongside interferons in many clinical protocols.
Internationally approved dosing
Approved internationally as a prescription medication. Dosing is established by the prescriber based on the indication.
Hepatitis B / C (international approval): 1.6 mg subcutaneous injection twice weekly for 6 to 12 months, often combined with interferon or other antiviral agents.
Vaccine adjuvant (immunocompromised): 1.6 mg twice weekly during vaccination period.
Cancer adjuvant (off-label or as part of clinical protocols): dosing varies; typically 1.6 mg twice weekly during chemotherapy or immunotherapy.
Side effects and safety profile
The post-marketing safety profile from three decades of international use is generally favorable. Reported adverse events:
Mild injection-site reactions, most common
Transient flu-like symptoms (uncommon)
Mild fatigue (uncommon)
Hypersensitivity reactions (rare)
Long-term safety data from international clinical use over decades supports the favorable tolerability picture. Theoretical considerations include effects on autoimmune conditions, in autoimmune disease, the immune-balancing effect may be helpful or unhelpful depending on the specific condition.
References
Sherman M, Sjogren MH, Creager RL, et al. (1998). "Combination therapy with thymosin alpha 1 and interferon for the treatment of chronic hepatitis C infection: a randomized, placebo-controlled double-blind trial." Hepatology, 27(4), 1128–1135. PubMed
Andreone P, Cursaro C, Gramenzi A, et al. (2001). "A double-blind, placebo-controlled, pilot trial of thymosin alpha 1 for the treatment of chronic hepatitis C." Liver, 21(5), 297–302. PubMed
Carraro G, Naso A, Montomoli E, et al. (2012). "Thymosin alpha 1 as an adjuvant for influenza vaccination in dialysis patients." Vaccine, 30(7), 1170–1175. PubMed
Garaci E, Pica F, Serafino A, et al. (2012). "Thymosin alpha 1 and cancer: action on immune effector and tumor target cells." Ann N Y Acad Sci, 1269, 26–33. PubMed
Wu M, Ji JJ, Zhong L, et al. (2020). "Thymosin α1 therapy in critically ill patients with COVID-19: a multicenter retrospective cohort study." Int Immunopharmacol, 88, 106873. PubMed
Goldstein AL, Hannappel E, Sosne G, Kleinman HK. (2012). "Thymosin: 50+ years of research." Ann N Y Acad Sci. PubMed
For educational and research purposes only. This is not medical advice. Thymosin Alpha-1 is approved as a prescription medication in 35+ countries but not by the FDA in the United States. Consult a licensed healthcare provider before considering any peptide. PeptideLibraryHub is independent and does not sell peptides or accept money from anyone who does. Information current as of May 2026.