What it is

Thymosin alpha-1 is a 28-amino-acid peptide with an acetyl group at its N-terminus, first isolated from calf thymus tissue in 1977 [2]. The chemically made version is called thymalfasin and has the same amino acid sequence as the natural peptide [2]. It is sold outside the United States under the brand name Zadaxin by SciClone Pharmaceuticals as a 1.6 mg solution for injection [2]. According to the company’s 2014 annual report it is approved in countries in the Asia-Pacific region, Latin America, Eastern Europe and the Middle East, claims FDA said it could not independently verify in full [2]. It is not approved in the United States, Japan, or Europe apart from Italy, and there is no USP monograph for it [2]. Compounded injectable and nasal-spray versions have been marketed online in the US [2].

How it works

Thymosin alpha-1 is an immunomodulator: FDA’s review describes its effects as mediated at least in part through Toll-like receptor 9 on dendritic and other immune cells [2]. In animals given the peptide intravenously it disappears from blood within minutes, so any effect is thought to come from triggering immune signalling rather than from sustained circulating levels [2]. The peptide is said to distribute within the extracellular fluid volume, based on a volume of distribution of roughly 30–40 litres in healthy volunteers [1]. FDA noted that dose-response relationships have not been well studied, which limits how precisely the mechanism can be tied to a given dose [2]. Because the label uses studied outside the US are in infections and cancer, the mechanism is usually framed as boosting or rebalancing immune responses rather than acting directly on a disease process [2].

What the evidence shows

Thymosin alpha-1 has a large but uneven human literature. A 2024 narrative review counted more than 30 trials involving over 11,000 people, mostly in hepatitis B and C, COVID-19, cancer and autoimmune conditions, and judged it well tolerated [7]. FDA reached a different conclusion when it reviewed the same evidence for 12 proposed uses, including hepatitis B, hepatitis C, HIV, COVID-19, vaccine adjuvant use, melanoma, hepatocellular carcinoma, non-small-cell lung cancer, sepsis, infections after stem-cell transplant, COPD and chronic fatigue syndrome [2]. FDA found a “lack of evidence to support the effectiveness” of subcutaneous thymosin alpha-1 for those uses, noting that the studies were inconclusive, small and had design deficiencies, and that no US clinical guideline recommends it [2]. Several PCAC members who voted against listing agreed there was no compelling evidence of clinical effectiveness and safety for the uses reviewed [3]. In short, approval elsewhere does not mean the evidence met FDA’s bar.

Regulatory status

Thymosin alpha-1 is not an FDA-approved drug, and FDA has approved no product containing it, despite at least one website advertising a compounded version as “FDA-approved” [2]. In September 2023 FDA moved more than a dozen peptides, thymosin alpha-1 among them, into 503A Category 2, substances that may present significant safety risks, citing immunogenicity, impurities and limited human data [6]. FDA’s current page lists thymosin alpha-1 among substances formerly in Category 2 whose nominations were withdrawn, with the note that its safety information is “inadequate” to understand the extent of any safety issues [5]. FDA then took the substance to its Pharmacy Compounding Advisory Committee on December 4 2024 [8][4]. FDA proposed that it not be added to the 503A bulks list, and the committee voted 4 in favour and 17 against for both the free base and the acetate [3]. As of September 24 2026 we found no FDA action adding it to any bulks list.

Half-life and what it means for a level curve

In nine healthy volunteers given thymosin alpha-1 by subcutaneous injection, the peptide was absorbed quickly with peak levels 1–2 hours after the dose, and the elimination half-life was short, under 3 hours [1]. FDA summarises the same study as a serum half-life of about 2 hours, with no accumulation after daily dosing for 5 days [2]. The reference half-life in STACK is 2 hours. That is the value FDA cites; the primary abstract says “less than 3 hours” without a single figure, so treat 2 hours as approximate. With a half-life this short, a level halves every couple of hours and is essentially gone within a day, matching the finding that repeated doses do not build up [1]. Steady state arrives after about five half-lives, roughly 10 hours, so each injection stands alone. STACK’s curve is an estimate from this published half-life, not a blood level. Rat intravenous half-lives of 1.9–3 minutes are not used [2].

Reported side effects

In most clinical studies thymosin alpha-1 has not been associated with significant adverse events attributable to the peptide, and the most common reactions were local irritation, redness and discomfort at the injection site [2]. Adverse events reported in individual studies included flares of the liver enzyme ALT in people with chronic hepatitis B, thyroid-stimulating hormone abnormalities in people with hepatitis C, nipple pain, and fatal immune haemolytic anaemia and engraftment failure in stem-cell transplant recipients [2]. Labels from countries where it is sold carry warnings or contraindications for children, pregnant and breastfeeding women, people with autoimmune disease and immunosuppressed patients [2]. FDA’s adverse event database held one report, a man on peginterferon and thymosin alpha-1 who was hospitalised with anxiety, atrial fibrillation and a transient fall in TSH, confounded by the interferon [2]. FDA also flagged an unstudied immunogenicity risk from aggregates and impurities in compounded injections [2]. Call your prescriber about injection-site reactions, jaundice or new symptoms.

Storage and handling

For thymosin alpha-1 free base, FDA’s review reports the raw peptide is recommended to be stored desiccated below −18 °C, and that once reconstituted the solution is stable for 2–7 days at 4 °C or longer if frozen below −18 °C [2]. For the acetate salt, the manufacturer recommends a sealed container at 2–8 °C in a refrigerator or freezer, and it is reported stable below −20 °C [2]. FDA notes the peptide is sensitive to formulation and environmental conditions that can cause aggregation and degradation, and that its water solubility is limited to about 2 mg/mL [2]. For a compounded product, the pharmacy’s label governs; follow its storage temperature, protect vials from light, and do not use a solution that is cloudy. STACK’s reconstitution calculator handles the concentration arithmetic from your label’s numbers and does not suggest a dose.

What STACK tracks for it

STACK logs each thymosin alpha-1 dose as your prescriber wrote it, with the date, time and injection site, and draws an estimated level curve from the 2-hour reference half-life, which you can override with your own estimate [1]. Because the curve returns to zero between injections, STACK’s dose history is more useful than the curve for this compound. You can log the reason your prescriber chose it and track markers they care about, such as infection frequency or lab results they share with you. Side-effect logging covers injection-site reactions, fatigue, and symptoms of liver or thyroid change, which are the categories FDA’s review identified [2]. STACK also tracks vial inventory, reconstitution dates and the days-in-fridge count against the 2–7 day stability window FDA reports for reconstituted solution [2].

The reference half-life in STACK

STACK's simulator and the app use 2 h for Thymosin alpha-1: ~2 h (reported as under 3 h). Measured in: nine healthy adult volunteers, subcutaneous, single and 5-day repeated dosing.

Thymosin alpha-1

2 h

Reference half-life of about 2 hours is the figure FDA cites from Rost et al. 1999 (the abstract says "less than 3 hours"); treat it as approximate. Editable. Educational only — not medical or dosing advice.

“Elimination half-life of less than 3 hours after subcutaneous injection in nine healthy volunteers; FDA summarises the same study as about 2 hours.” — Rost KL, Wierich W, Masayuki F, et al. Pharmacokinetics of thymosin alpha1 after subcutaneous injection of three different formulations in healthy volunteers (1999). International Journal of Clinical Pharmacology and Therapeutics · PubMed 10027483. View source

Simulate Thymosin alpha-1 Plan with Thymosin alpha-1

Estimate only. Modelled from the published half-life you selected and the doses you logged — not a measured blood level. Do not use it to make medication decisions.

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Thymosin alpha-1

What it is

How it works

What the evidence shows

Regulatory status

Half-life and what it means for a level curve

Reported side effects

Storage and handling

What STACK tracks for it

The reference half-life in STACK

Thymosin alpha-1

Free tools for Thymosin alpha-1

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