Thymosin Alpha-1

Thymosin alpha-1 (Tα1) is a heat-stable, strongly acidic peptide of 28 amino acids. Goldstein and colleagues isolated it from a calf thymus extract known as thymosin fraction 5 and published its sequence in 1977 ^[1]. The drug form, thymalfasin, is made synthetically and marketed as Zadaxin. A 2005 company filing stated it was approved for sale in over 30 countries, primarily in Asia, the Middle East and Latin America ^[8].

Despite the similar name, thymosin alpha-1 is unrelated in structure and function to TB-500, which is a fragment of thymosin beta-4. Both came out of the same early thymus research, which is the source of the shared name.

Thymosin alpha-1 is described as an immune modulator. The original 1977 paper proposed that peptides in thymosin fraction 5 help regulate the development and function of T cells, the white blood cells that mature in the thymus ^[1]. Later reviews describe it as modifying and restoring immune function and summarise its use in immunocompromised states, cancers, infections and as a vaccine enhancer; the evidence for these uses varies widely in quality ^[6].

Sepsis: the key trial is TESTS, a double-blind, placebo-controlled phase 3 study at 22 centres in China, published in the BMJ in 2025. Of 1,089 adults with sepsis analysed, 28-day mortality was 23.4% with thymosin alpha-1 and 24.1% with placebo, a difference that was not statistically significant; no secondary or safety outcome differed significantly. The trial was part-funded by SciClone Pharmaceuticals, which markets Zadaxin ^[2].

Chronic hepatitis B: a 2026 Cochrane review found 10 randomised trials with 1,349 participants. Thymosin alpha-1 may reduce all-cause mortality and serious adverse events, but the authors rated the certainty of evidence as very low for most outcomes and low for serious adverse events, and said they could not be sure of any benefit ^[3].

Melanoma: in a randomised trial of 488 patients with metastatic melanoma, adding thymosin alpha-1 to chemotherapy (dacarbazine) with or without interferon alfa produced more tumour responses in two of the thymosin arms (10 and 12) than in the control arm (4); median overall survival was 9.4 versus 6.6 months, a difference that did not reach statistical significance (P = .08), and no extra toxicity was seen ^[4].

COVID-19: a 2023 meta-analysis of nine studies with 5,352 patients found no statistically significant effect of thymosin alpha-1 on mortality in hospitalised adults and concluded that the results did not support its use; subgroup analyses suggested possible benefit in older and more severe patients ^[5].

In the TESTS sepsis trial, patients received 1.6 mg of thymosin alpha-1 by subcutaneous injection every 12 hours for seven days. The authors noted that the conventional dose used for cancers and viral hepatitis is 1.6 mg twice weekly ^[2]. The melanoma trial tested doses of 1.6 mg, 3.2 mg and 6.4 mg alongside chemotherapy ^[4]. These are the doses used in the studies, not recommendations. The peptide calculator converts vial contents to concentrations only.

In controlled trials of the pharmaceutical product, thymosin alpha-1 has generally been well tolerated. TESTS found no significant difference in safety outcomes between thymosin alpha-1 and placebo ^[2], the melanoma trial reported no additional toxicity ^[4], and the Cochrane review found that serious adverse events may be less frequent with thymosin alpha-1, based on low-certainty evidence ^[3].

Those findings apply to regulated thymalfasin. For compounded or research-grade material, the FDA has said that thymosin alpha-1 may pose a significant risk of immune reactions (immunogenicity) and of peptide-related impurities ^[7]. Research products are not made to pharmaceutical standards, which is why testing matters.

Thymalfasin is not approved by the FDA. In the US, the FDA previously placed thymosin alpha-1 in category 2 of its interim compounding policy, the list of bulk substances that may present significant safety risks; the nomination was later withdrawn and the FDA now lists it among substances previously in category 2 ^[7]. It was not among the peptides considered at the FDA’s July 2026 compounding advisory committee meeting.

We could not confirm that thymalfasin is a licensed medicine in the UK, Canada, Australia or New Zealand. We have not confirmed its status on the WADA Prohibited List. For national rules on research peptides, see our legal status overview and the UK page.

Research-grade thymosin alpha-1 is supplied as a freeze-dried powder. Peptides are generally stored cold, dry and away from light, and reconstituted solutions are refrigerated and used within a limited period. See how to store peptides and bacteriostatic water.

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