Thymosin Alpha 1 (Thymalfasin): What the Evidence Shows

Thymosin alpha 1 is unusual among the peptides clinics sell: it is a real drug with real human trials. Sold as thymalfasin under the brand Zadaxin, it is licensed in more than 30 countries, mainly for hepatitis B and as a vaccine booster.^[7] It has never been approved in the US, and the trials that should have settled its value came up short: two US hepatitis C trials missed, and in 2025 the largest sepsis trial found no difference in deaths.^[6][13] For what US sellers promote it for, from “immune optimization” to long COVID, there is almost no evidence at all. In December 2024, FDA’s compounding advisers voted 17 to 4 against letting pharmacies make it.^[2]

What thymosin alpha 1 is

Thymosin alpha 1 (often written Tα1 or Ta1) is a 28-amino-acid peptide first isolated in 1977 from a crude extract of calf thymus, the gland where T cells mature. Thymalfasin is the lab-made version with the same sequence, and Zadaxin is the finished injection sold abroad by SciClone Pharmaceuticals.^[1] It is classed as an immune modulator: the idea is that it nudges T-cell function, which is why it has been tested in chronic viral infections, sepsis, cancer and as an add-on to vaccines.

It is not the same thing as TB-500. Despite the shared “thymosin” name, TB-500 is a seven-amino-acid fragment of a different protein, thymosin beta-4, sold for tissue repair rather than immunity. FDA lists the two as separate substances with separate safety notes.^[3] We cover that one in what the evidence shows on TB-500.

Approved abroad, never approved in the US

SciClone’s own filings describe Zadaxin as approved in more than 30 countries, “primarily in China and countries in the Pacific Rim, Latin America, Eastern Europe, and the Middle East.”^[7] FDA’s 2024 review lists approvals mainly for hepatitis B and as a vaccine adjuvant, with some countries adding hepatitis C or use alongside chemotherapy. In Italy, for example, it is authorized only as an add-on to flu vaccination in people with weakened immune systems.^[1]

In the US, thymosin alpha 1 is not a component of any FDA-approved drug. It holds four US orphan drug designations, but FDA notes that orphan status is a set of incentives for developers, not an approval.^[1] SciClone did run US phase 3 trials, in hepatitis C. In December 2005 and June 2006 it reported results from two US phase 3 trials of Zadaxin added to pegylated interferon, and by its own account neither showed “a statistically significant clinical benefit” over interferon alone.^[6] The company said at the time that it had no approval in the US, Europe or Japan.^[6]

The human evidence, by condition

Hepatitis B and C: decades of trials, no clear win

Hepatitis B is where thymalfasin is most widely licensed, and where the evidence is largest. A US phase 3 trial enrolled 97 adults with active hepatitis B for six months of twice-weekly injections. Seven of 49 on thymalfasin (14%) cleared the virus, against two of 48 on placebo (4%), a gap that did not reach statistical significance. The investigators wrote that their results “do not confirm” earlier reports of benefit.^[8]

A Cochrane review published in September 2026 pooled 10 randomized trials with 1,349 people, published between 1991 and 2018 and run in Asia and Italy. Thymalfasin may have reduced deaths (risk ratio 0.53) and serious side effects, but the reviewers graded the evidence very low certainty for every outcome except serious adverse events, and concluded they were “not sure” it does any of these things.^[11] Most of those trials paired it with interferon or older antivirals. FDA added that thymalfasin is not in current US hepatitis B guidelines, and that using it instead of, or in a way that delays, an approved antiviral “could have serious negative sequelae.”^[1]

Hepatitis C followed the same arc. A 1998 US trial of 109 patients found more normal liver enzymes at the end of treatment when thymalfasin was added to interferon (37.1% vs 16.2%), but the lasting response was small (14.2% vs 8.1%).^[9] The two later US phase 3 trials missed,^[6] and a European phase 3 in 552 people who had failed earlier treatment found the same cure rate with or without thymalfasin; only a subgroup who finished treatment did better.^[10] Those authors concluded thymalfasin “seems to play no role in the primary therapy of the disease.”^[10] Hepatitis C is now cured with oral antivirals, so the question has largely moved on.^[1]

Sepsis: a promising trial, then a large one that found nothing

Sepsis looked like thymalfasin’s best chance. In the ETASS trial, run in six Chinese hospitals from 2008 to 2010, 26.0% of 181 patients given thymalfasin died within 28 days, against 35.0% of 180 controls. That result was borderline (relative risk 0.74, with a confidence interval that crossed 1), and the trial was single-blind.^[12]

The same research group then ran TESTS, a properly double-blind phase 3 trial at 22 centers in China. It randomized 1,106 adults with sepsis to thymalfasin or placebo injections every 12 hours for seven days. In the 1,089 analyzed, 28-day deaths were 23.4% on thymalfasin and 24.1% on placebo (hazard ratio 0.99, P = 0.93), and no secondary or safety outcome differed. The authors found “no clear evidence” that it reduces deaths.^[13] The trial was partly funded by SciClone, which makes the null result harder to dismiss.^[13]

A 2025 meta-analysis from the same group pooled 11 sepsis trials and found fewer deaths overall, but the benefit vanished when only high-quality or multi-center trials were counted, and the authors judged the total sample still too small to be sure.^[14] FDA’s review noted that every sepsis study it found was run in China and that sepsis guidelines do not mention thymalfasin.^[1]

COVID-19 and long COVID

Thymalfasin was widely used in China early in the pandemic, which produced a burst of studies that disagree with each other. A retrospective study of 76 severe patients in Wuhan reported deaths of 11.1% with treatment against 30.0% without.^[15] A larger retrospective study of 771 critically ill patients across 19 hospitals found a similar raw gap, but once the groups were matched for how sick they were, deaths were 51.0% and 52.9%, no real difference.^[16] That is the usual problem with retrospective data: who got the drug was not random.

The only randomized study in the US was a 49-patient open-label pilot in hospitalized patients with low oxygen. Recovery was somewhat more frequent with thymalfasin, but not significantly, though CD4 T-cell counts rose faster in one subgroup.^[17] A 2023 meta-analysis of eight studies reported lower mortality, with very high variation between studies, and said randomized trials “are still required.”^[18] FDA counted three of four meta-analyses it reviewed as finding no drop in deaths.^[1]

For long COVID, which is where much US marketing points, the evidence is thinner still. The main paper is an Italian lab study that exposed blood cells taken from people with long COVID to thymosin alpha 1 and measured immune markers; no one was treated.^[19] FDA found no clinical studies of thymosin alpha 1 in ME/CFS, the chronic fatigue condition that overlaps with long COVID.^[1]

“Immune optimization” and wellness use

FDA’s review collected how US websites sell thymosin alpha 1: to speed recovery from viral infections, to treat COVID-19, for chronic fatigue, Lyme disease, allergies and asthma, and to improve “overall wellness.”^[1] The pharmacy that nominated it for compounding also proposed “geriatric immune support” and Lyme disease; FDA found no clinical studies for Lyme disease and could not evaluate broad uses like general immune support.^[1]

That is the gap at the heart of this peptide. The trials that exist tested thymalfasin in very sick people, often alongside other drugs, and measured hard outcomes such as viral clearance or survival. None tested whether a healthy adult injecting it feels or functions better, gets fewer colds or recovers faster. A rise in T-cell counts on a lab report is a marker, not a benefit; in TESTS, immune markers did not translate into fewer deaths.^[13] Our peptide menu review sets thymosin alpha 1 beside the other peptides clinics sell and what each has been tested for.

Side effects and safety

Across the published trials, thymalfasin has been well tolerated. FDA found that the most common reactions were “local irritation, redness, or discomfort at the injection site,” at doses of 1 to 16 mg under the skin for up to a year. FDA’s adverse event database held one US report through June 2024, in a trial patient also on interferon.^[1] That is a reasonable record for the licensed product in trials. It says less about compounded or research-labeled vials, which are made differently and are rarely reported to FDA.

• Who should avoid it, per a foreign label. The Italian Zadaxin label says it should not be used in children, is contraindicated in pregnancy and breastfeeding, and calls for caution in people with allergies or autoimmune disease.^[1]
• Transplants and immune suppression. FDA flagged that in stem-cell transplant patients, an immune stimulant could worsen graft-versus-host disease or cause the graft to fail.^[1]
• Quality. FDA found no published data on impurities, clumping, microbial load or endotoxins for the raw ingredient, and raised the risk of immune reactions to an injected peptide.^[1]

“Compounded drugs containing thymosin-alpha-1 (Ta1) may pose significant risk for immunogenicity for certain routes of administration and may have complexities with regard to peptide-related impurities and API characterization. The safety-related information is inadequate for the agency to sufficiently understand the extent of any safety issues raised by the proposed compounded drug.”^[3]

— FDA, Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks

FDA’s review, the 2024 vote, and where it stands now

A pharmacy group nominated thymosin alpha 1 for the 503A bulks list, the short list of ingredients a state-licensed pharmacy may compound with when the substance is not in an approved drug and has no official monograph. It later withdrew the nomination, and FDA reviewed both the free base and the acetate salt on its own initiative across 12 conditions.^[1] Its conclusion was that there is “a lack of evidence to support the effectiveness” for all of them, and that the criteria weigh against listing either form.^[1]

On December 4, 2024, the Pharmacy Compounding Advisory Committee voted 17 to 4 against listing the free base and 17 to 4 against the acetate. Members voting no cited “no compelling evidence demonstrating clinical effectiveness and safety”; those voting yes said patients should have it as an option.^[2] Thymosin alpha 1 was not part of the July 2026 meeting at which advisers backed several other peptides; that session covered BPC-157, KPV, TB-500, MOTS-c, Semax, epitalon and emideltide.^[21]

Our peptide legal-status guide explains what each of these lists means, and 503A vs 503B pharmacies covers the two compounding channels. AOD-9604 was voted on at the same December 2024 meeting.

If you are offered thymosin alpha 1

Of the 153 telehealth sellers in our register with a structured peptide menu, one lists thymosin alpha 1. More is sold online as “research use only.” FDA’s position, set out in a December 2024 warning letter to a research-peptide seller, is that such a label does not stop a product being an unapproved drug when the seller’s own website shows it is meant for people.^[20] For a buyer, that means no prescriber, no pharmacist and no drug-quality standard behind the vial. If a clinic offers it, ask:

1. Which pharmacy makes it, and under what authority? Thymosin alpha 1 is not on the 503A bulks list, so the clinic should be able to explain the route.
2. Is it the free base or the acetate, and who tested it? FDA treats them as different ingredients.
3. What trial supports this use? For immune support, wellness or long COVID, there isn’t one.
4. Would it replace a proven treatment? For hepatitis B, hepatitis C or COVID-19, approved drugs exist, and FDA warns against delaying them.
5. Does my prescriber know? Anyone treating you, especially for autoimmune disease or after a transplant, should know you are taking an immune modulator.

You can see which peptides clinics sell, and what is known about each, on our peptides hub.