Thymosin alpha-1 is a chain of 28 amino acids that the body makes, mostly in the thymus. A lab-made copy, Zadaxin, is an approved drug in more than 30 countries for hepatitis and weak immune systems. The FDA has not approved it.
Why scientists study it
- It switches on the TLR9 and TLR2 sensors on dendritic cells, which helps T cells mature and shifts the balance between Th1 and Th2 responses.
- Some countries, including Italy, China, Russia and parts of Southeast Asia, approve it as an add-on for long-term hepatitis B and C, but the US and UK do not.
- Its human safety record is among the largest for any peptide drug, built over decades of regulated medical use and several Phase II and III randomized trials.
Four studies to know
Garaci et al., 2003 · International Immunopharmacology
An early, influential review by Garaci and colleagues in Rome. It presents Thymosin Alpha-1 as a partner for cytokines and chemotherapy in cancer. It gathers the early clinical case and the reasoning on how it acts on the immune system. Read the paper
Sjogren & Rodrigues-Arenas, 2004 · J Gastroenterol Hepatol
A broad review of thymalfasin as an immune booster in long-term hepatitis B and C. It sums up the randomized trials behind its liver disease approvals in Italy, China and Southeast Asia. Read the paper
Liu et al., 2020 · Clinical Infectious Diseases
Liu and colleagues looked at 76 patients with severe COVID-19 in an observational study. Fewer of those given Thymosin Alpha-1 died, and their lymphocyte counts came back. PD-1 and Tim-3, two markers of worn-out CD8+ T cells, also reversed. Read the paper
Wu et al., 2025 · BMJ (TESTS trial)
Wu and colleagues ran a Phase 3 randomized trial at many centers. It enrolled 1,106 adults who had sepsis. It was double-blind and used a placebo. It is the largest and best current test of Thymosin Alpha-1 in critical care. Read the paper
Safety and legal status
- Countries that approve it, such as China, Italy, Russia and parts of Southeast Asia, have built up a large body of human data. It comes from many years of regulated use and several Phase II and III trials. Reports steadily describe it as well tolerated. The most common finding is a mild, short-lived reaction where the shot went in.
- The FDA has not approved it, and neither has the MHRA or the EMA. The FDA has granted only orphan drug status, for melanoma and for hepatitis B and C. Approval in some countries is not approval everywhere.
- It is not currently on the WADA 2025 Prohibited List, and that list changes each year. It is supplied only for laboratory and in vitro research. It is not for diagnosis, treatment or human consumption.