Thymosin alpha-1 access: approved abroad, not in the US

The short answer: there is no lawful way to get thymosin alpha-1 in the United States today outside a clinical trial. It is approved as Zadaxin (thymalfasin) in more than 30 countries, but FDA has never approved it. [1] US compounding pharmacies lost their basis to make it when FDA placed it in 503A Category 2 in 2023, and the nomination has since been withdrawn, so it is not on the list of bulk substances pharmacies may compound. [2] [3]

This guide explains what "approved abroad" means for someone in the US, what the human evidence shows, the forms you will see marketed, and how thymosin alpha-1 differs from thymalin. It does not give dosing.

What is thymosin alpha-1?

Thymosin alpha-1 is a synthetic copy of a 28 amino acid peptide found in the thymus. It supports T cell maturation and function, and has been used for roughly four decades in hepatitis B and C, as a vaccine adjuvant, and in other infections. [1] Unlike most peptides in our catalog, it has randomized human trials, which is why our evidence grade for it is human RCT.

What the human trials show

The evidence is real but mixed, and it depends on the condition.

• Chronic hepatitis B: in a randomized trial of 98 patients in Taiwan, complete viral response at 18 months occurred in 40.6% of patients in one treatment arm versus 9.4% of untreated controls, with no significant side effects. [4]
• Severe sepsis, 2013: a single-blind trial of 361 ICU patients found 28 day mortality of 26.0% with thymosin alpha-1 versus 35.0% with standard care, a difference that did not reach statistical significance. [5]
• Sepsis, 2025: the larger, double-blind, placebo-controlled phase 3 TESTS trial in 1,106 adults at 22 centers in China found no difference in 28 day mortality (23.4% versus 24.1%, hazard ratio 0.99), and no secondary or safety outcome differed significantly. [6]
• Vaccine response: in a pilot study of patients on hemodialysis, it enhanced the immune response to a pandemic influenza vaccine and was well tolerated. [7]

The sepsis result is the one to weigh most. The smaller 2013 trial pointed to a benefit, and the larger, better-blinded 2025 trial did not confirm it. [5] [6] The immune function page puts these results next to other immune-targeted options.

Approved abroad but not in the US: what that means

FDA approval is country-specific. A drug's approval in another country does not make it lawful to sell or prescribe in the US, and FDA has not approved thymalfasin. [1]

That leaves three US routes that people ask about, and none of them works today.

1. A prescription for an approved product. There is no FDA approved thymosin alpha-1 product for a doctor to prescribe.

2. A compounded prescription. A 503A pharmacy can compound from a bulk substance only if it is part of an approved drug, has a USP monograph, or is on the 503A bulks list. [3] FDA placed thymosin alpha-1 in Category 2 in 2023, citing immunogenicity risk and a lack of safety information for compounded product. FDA's page, current as of April 22, 2026, now lists it among nominations that were withdrawn: out of Category 2, but with no path onto the bulks list. [2] It was not among the peptides the Pharmacy Compounding Advisory Committee reviewed in July 2026. [8]

3. Bringing it in from abroad. FDA states that in most circumstances it is illegal for individuals to import drugs for personal use, and specifically that a drug approved in another country but unapproved in the US is illegal to import. FDA staff may use discretion only in narrow situations, such as a serious condition with no effective treatment available in the US, under conditions FDA sets out. [9] We do not cover workarounds. If you have a serious condition, the right first step is a US specialist.

The route that does exist: clinical trials. ClinicalTrials.gov lists registered studies of thymalfasin, most of them in cancer, hepatitis B, and critical care, and most of the studies currently recruiting are in China. [10] Enrollment depends on each trial's criteria.

Our thymosin alpha-1 regulatory timeline records each FDA action, the thymosin alpha-1 cost page explains why there is no lawful US price, and the state legal guide covers how state rules interact with federal compounding law. For the general process with peptides that do have a lawful path, see how to get peptides prescribed.

Why US status and foreign approval diverge

Seeing a drug approved in dozens of countries and unavailable in the US can feel like a paperwork gap. It is more than that. US approval requires an application to FDA with the evidence FDA asks for, and a foreign approval does not substitute for it. [9] The evidence also depends on the use. The hepatitis B trial was positive, the largest sepsis trial was not, and many of the uses people ask about online, such as general immune support, have no randomized trial behind them. [4] [6] A foreign label for hepatitis B says nothing about those uses.

Thymosin alpha-1 supplement and nasal spray forms

Every trial described above used subcutaneous injection. [4] [6] We found no published study of thymosin alpha-1 taken by mouth or as a nasal spray, so there is no evidence that capsules, "supplements," or sprays deliver the peptide. Products sold for research use are not lawful for human use, and FDA's Category 2 concern about immunogenicity from impurities applies with more force to products no regulator has inspected. [2]

Thymalin vs thymosin alpha-1

The names are similar and both come from thymus research, but they are different products.

• Thymosin alpha-1 is one defined, synthetic 28 amino acid peptide, approved as a drug in more than 30 countries, with randomized trials. [1]
• Thymalin is a mixture of peptides extracted from calf thymus, registered as a drug in Russia. Its main human evidence is long-term, non-randomized follow-up of elderly patients given thymalin with a pineal extract, reported by its developers. [11] It has never been reviewed by FDA.

A third thymic peptide, thymulin, is covered in our thymosin alpha-1 vs thymulin comparison. And despite the shared name, thymosin alpha-1 is unrelated to thymosin beta-4, the protein behind TB-500; see thymosin alpha-1 vs TB-500 and our explainer on TB-500 vs thymosin beta-4.

What's actually offered, and what that means for you

Licensed compounding pharmacies, clinics, and telehealth prescribers openly offer many unapproved peptides, and prescribers write prescriptions for them. If you see thymosin alpha-1 offered that way, the offer does not change its status: it is not FDA approved, its 503A nomination was withdrawn, and it is not on the bulks list, so there is no lawful compounding path. [2] [3]

Enforcement is uneven. FDA mostly acts through warning letters to compounders and online sellers, its Category 2 safety list, and import alerts that let it detain peptide shipments, rather than stopping every pharmacy. [2] [14] [15] [16] [17] State boards of pharmacy oversee pharmacies day to day, and their rules differ. [13]

What changes for you:

• No FDA review of the product. FDA does not verify a compounded drug's safety, effectiveness, or quality before it is sold. [13]
• Identity, purity, sterility, and dose accuracy vary. FDA cited immunogenicity risk and inadequate safety information for compounded thymosin alpha-1. [2]
• Insurance rarely pays, because there is no approved indication to match, so you usually pay cash.
• Sport. Its WADA status is unclear; ask your anti-doping organization first. [12]

What to check: a 503A pharmacy licensed in your state, a real evaluation by a licensed prescriber rather than an online form, and a certificate of analysis for the specific batch. These checks lower some risks, but they do not make an unlawfully compounded product lawful or safe. No approved US product contains thymosin alpha-1; a registered clinical trial is the lawful route described above.

Athletes and WADA

We record thymosin alpha-1's status on the WADA Prohibited List as unclear. [12] Athletes should check with their anti-doping organization before using any unapproved substance.