At a glance: Thymosin alpha-1 is a 28-amino-acid peptide first found in the thymus, the gland that trains immune cells. As thymalfasin (Zadaxin) it is approved in about 35 countries, mainly for hepatitis B, but never by FDA. Unlike most peptides, it has been tested in large trials, but the biggest ones were disappointing: no survival benefit in a 1,106-patient sepsis trial and no clear benefit in hepatitis C or COVID-19. In December 2024, FDA advisers voted 17 to 4 against adding it to the US compounding list.
What is thymosin alpha-1?
Thymosin alpha-1 (Ta1) is a natural peptide, 28 amino acids long with an acetyl group on one end. It was isolated in the 1970s from thymus tissue by researchers including Allan Goldstein, who proposed that shortages of thymus hormones contribute to some immune deficiencies. The synthetic drug version is called thymalfasin.
SciClone Pharmaceuticals developed it as Zadaxin. China approved it in 1995, and it is now used in many Asian countries and elsewhere for hepatitis B, and in some places for hepatitis C, liver cancer and as a vaccine booster in people with weak immune systems.
How does thymosin alpha-1 work?
Thymosin alpha-1 is described as an immune modulator: it seems to strengthen weak immune responses and help rebalance overactive ones, rather than simply boosting or suppressing immunity.
- Toll-like receptors: it activates these "alarm" sensors on immune cells, especially dendritic cells, which coordinate immune responses
- T cells: it promotes the maturation and activity of T cells, the immune cells that fight viruses and tumors
- Cytokine balance: it shifts immune signaling molecules, which in theory could help both weakened immunity and harmful overactivation
- Antiviral and anti-tumor activity in lab and animal studies
What the research shows
| Use | Study | Main finding |
|---|---|---|
| Hepatitis B | Chien et al., 1998: 98 patients | Higher viral response at 18 months with treatment (about 27% to 41%) than without (9%) |
| Hepatitis B | Mutchnick et al., 1999: 97 patients, placebo-controlled US trial | Response 14% vs 4% on placebo, not statistically significant |
| Hepatitis C | 2012 trial: 552 patients, added to standard therapy | Cure rate 12.7% vs 10.5%, not statistically significant |
| Sepsis | TESTS trial (Wu et al., BMJ 2025): 1,106 adults in China | 28-day death rate 23.4% vs 24.1% on placebo; no benefit |
| COVID-19 | Meta-analysis of 9 studies, 5,352 patients (2023) | No effect on death rate; an early small study suggesting benefit was not confirmed |
| Melanoma and other cancers | Small phase 2 studies, combined with chemotherapy or immunotherapy | Some encouraging signals, but no large confirmatory trials |
Several positive early studies were small or open-label; larger placebo-controlled trials have mostly failed to confirm benefits.
Why it gets attention
- Immune support: marketed for frequent infections, recovery from illness and "immune resilience"
- Long COVID and chronic fatigue: popular in clinics for these conditions, without supporting trials
- Foreign approvals: its approval in dozens of countries gives it more credibility than most peptides
- Cancer support: studied as an add-on to cancer treatment and vaccines
How it is used
Where approved, thymalfasin is a prescription injection under the skin. In the US it is not available as an approved drug, and products sold online are labeled "for research use only." This page does not provide dosing guidance.
What about side effects?
Thymosin alpha-1 has a reassuring safety record in clinical trials. In the large sepsis trial, side effects did not differ meaningfully from placebo, and hepatitis studies found it better tolerated than interferon.
- Common: discomfort, redness or swelling at the injection site
- Autoimmune and transplant concerns: because it stimulates immune activity, it may not be suitable for people with autoimmune diseases or organ transplants on immune-suppressing drugs
- Product quality: unregulated research products may be impure, mislabeled or non-sterile
- Pregnancy and breastfeeding: not well studied
Regulatory status
- Approved in about 35 countries as Zadaxin (thymalfasin), mainly for hepatitis B.
- Not FDA-approved. US trials in hepatitis B produced inconclusive results, and FDA has granted only orphan drug designations (which are not approvals) for conditions such as melanoma and liver cancer.
- It was removed from FDA's "Category 2" compounding list after its nomination was withdrawn. At the December 4, 2024 advisory meeting, FDA recommended against adding it to the compounding list, and the committee agreed, voting 17 to 4 for each form, citing a lack of convincing evidence of effectiveness.
- It was not part of the July 2026 peptide review, so US pharmacies have no approved pathway to compound it from bulk powder.
The bottom line
Thymosin alpha-1 stands out among peptides: it is a real medicine in many countries, it has been tested in thousands of patients, and it appears safe. But when the big, well-controlled trials were run, in sepsis, hepatitis C and COVID-19, it did not beat placebo. Its benefits in hepatitis B are modest and mixed. In the US it remains unapproved, and FDA advisers have voted against compounding it.
Frequently asked questions
What is Thymosin Alpha-1?
A 28-amino-acid peptide first isolated from the thymus gland, which plays a role in immune function.
How is Thymosin Alpha-1 taken?
Subcutaneous injection.
Is Thymosin Alpha-1 FDA approved?
Not approved in the US; approved in several other countries.
That means no regulator has reviewed its safety, effectiveness or manufacturing quality for this use.
What has research shown about Thymosin Alpha-1?
- Chronic hepatitis B and C treatment.
- Improving response to vaccines in people with weak immune systems.
What side effects have been reported?
- Injection site redness and discomfort
- Joint aches
Serious or less common risks are listed in the Side Effects section above.
What is still unknown about its safety?
Most of it. Thymosin Alpha-1 has not been through the trials that establish long-term safety in people, and products sold online are made by unregulated labs. Independent testing of online peptides has repeatedly found contents that do not match the label.
References
- Dominari A, et al. Thymosin alpha 1: a comprehensive review of the literature. World Journal of Virology. 2020. View source
- FDA: Minutes of the December 4, 2024 Pharmacy Compounding Advisory Committee meeting (thymosin alpha-1 votes). View source
- FDA: Bulk drug substances nominated for use in compounding under section 503A (thymosin alpha-1 removed from Category 2). View source
- Peptide Journal: Thymosin alpha-1 clinical evidence review (hepatitis B, TESTS sepsis trial, COVID-19). View source
- Health Works Collective: Thymosin alpha-1 got the big trials most peptides never get. View source
- AUTM Better World Project: Zadaxin, a therapeutic developed in the United States that benefits many in Asia. View source