Is Thymosin Alpha-1 FDA approved?

No. It is approved as Zadaxin (thymalfasin) in more than 35 countries for chronic hepatitis B and immune indications, and it holds US orphan-drug designation for some indications, but it is not FDA approved. In the US it is used as a research peptide or via compounding.

What is the pharmaceutical Thymosin Alpha-1 dose?

The established Zadaxin regimen for chronic hepatitis B is 1.6 mg by subcutaneous injection twice weekly, with injections spaced about 3–4 days apart, typically for 6–12 months. Higher doses have been studied but are not standard.

Does Thymosin Alpha-1 have strong evidence?

Relative to most peptides, yes — it has decades of pharmaceutical use abroad and Phase 3 trials in viral hepatitis. But evidence is indication-specific: a large 2025 Phase 3 sepsis trial was negative for its primary endpoint, so the data are not uniformly positive across all proposed uses.

Dosage Guide · Approved abroad · not FDA-approved

Thymosin Alpha-1 Dosage: Zadaxin's Schedule vs US Status

Last updated May 19, 2026 · Reviewed by Grey Peptides Editorial Board · ✓ Primary-sourced

← Thymosin Alpha-1 encyclopedia entry · See also: TB-500 dosage · Regulatory tracker

🌐 Status: approved abroad (Zadaxin), not by the FDA

Thymosin Alpha-1 (thymalfasin) is a genuine approved medicine in 35+ countries, marketed as Zadaxin for chronic hepatitis B and immune indications — one of the few peptides here with decades of pharmaceutical use and Phase 3 data. But it is not FDA approved (it holds only US orphan-drug designation), and in the US it is a research or compounded product. The dosing below is the regulated Zadaxin schedule, documented for education.

The short version

Thymosin Alpha-1 is a 28–amino-acid peptide that the thymus uses to help tune immune function; it activates T cells, NK cells, and dendritic cells. Its evidence base is unusually deep for this field — it has been studied in trials involving thousands of patients and used pharmaceutically abroad for decades. The honest framing is "approved there, not here," and the dosing that is actually anchored in regulatory data is the Zadaxin hepatitis B schedule.

The pharmaceutical (Zadaxin) schedule

The most established, regulatory-grade regimen is the chronic hepatitis B protocol:

Parameter	Zadaxin (hepatitis B) label
Dose	1.6 mg (≈900 µg/m²)
Frequency	Twice weekly (≈3–4 days apart)
Route	Subcutaneous
Duration	~6–12 months

A useful "do not overdose" point from the clinical record: the 1.6 mg dose was optimized in Phase 3 trials, and higher amounts (3.2–6.4 mg) studied did not consistently show superior immune outcomes. More is not reliably better. Community immune-support protocols sometimes cite slightly lower maintenance amounts, but the figure with the strongest backing is the 1.6 mg twice-weekly hepatitis B schedule.

Evidence is indication-specific

It is worth being precise about what the evidence supports. The strong data are in chronic viral hepatitis and certain immune/oncology adjunct settings. The compound has also been studied in sepsis, COVID-19, and as a vaccine adjuvant — but a large 2025 Phase 3 sepsis trial was negative for its primary endpoint. So "well-studied" does not mean "proven for everything"; the benefit is tied to specific indications.

The US picture

In the US, Zadaxin is not available because it lacks FDA approval; Thymosin Alpha-1 is accessed as a research peptide or through compounding pharmacies. The FDA has tightened scrutiny of compounded peptides and raised manufacturing-quality and immunogenicity questions, and Thymosin Alpha-1 has been part of the 503A/PCAC review discussions. See the Regulatory Status Tracker.

Frequently asked questions

Is it the same as TB-500 (thymosin beta-4)?

No. Both are thymosins by name, but Thymosin Alpha-1 (immune modulation) and thymosin beta-4 / TB-500 (tissue repair) are different peptides with distinct biology. See the TB-500 dosage page.

Why isn't it FDA approved if it's used in 35+ countries?

Much of its US history is orphan-drug designations and earlier development that did not culminate in commercial FDA approval, plus more recent compounding-quality scrutiny — not a simple "approved but hidden" situation.

Can a US clinician prescribe it?

Not as the approved drug Zadaxin; US access is via compounding under prescription or as a lab-use peptide, which is why product verification matters.

Sources

Zadaxin (thymalfasin) prescribing information — chronic hepatitis B, 1.6 mg twice weekly; approvals in 35+ countries.
Goldstein AL, et al. Thymosin alpha-1 immunology and clinical development.
TESTS phase 3 sepsis trial (negative primary endpoint), 2025; U.S. FDA orphan-drug records and compounded-peptide statements.

Medical disclaimer: Education only, not medical advice. Thymosin Alpha-1 is not FDA approved; US-sourced product is research/compounded. Dosing reflects the regulated Zadaxin schedule, not a recommendation. Consult a licensed clinician.