Thymosin alpha 1 is unusual on the research-peptide shelf for one reason: it has been tested the way a drug is tested. Under the name thymalfasin, and the brand Zadaxin, it is licensed as a hospital drug in China and a number of other countries, and it has a large double-blind placebo trial with a counted adverse-event table. It has no approval in the United States and no FDA label.
That makes its side-effect record better than almost anything else sold beside it, and it also makes the gaps easier to see. Everything below was measured in hospital patients receiving a licensed product, not in healthy people injecting a research vial. The amounts used in those trials, and where they come from, are on our thymosin alpha 1 dosage page; this page is about harms.
The placebo trial: TESTS
TESTS randomised 1,106 adults with sepsis at 22 hospitals in China between September 2016 and December 2020. Half received 1.6 mg of thymosin alpha 1 by subcutaneous injection every 12 hours for seven days; half received a matching placebo. 1,089 were analysed. The results were published in the BMJ in January 2025 (PMID 39814420; registration NCT02867267).
| Measure, within 90 days | Thymosin alpha 1 (n = 542) | Placebo (n = 547) |
|---|---|---|
| At least one adverse event | 360 (66.4%) | 370 (67.6%) |
| At least one serious adverse event | 145 (26.8%) | 160 (29.3%) |
| Died by day 28 | 127 (23.4%) | 132 (24.1%) |
Across both arms the most common adverse events were anaemia (10.7%), fever (9.6%), abdominal distension (5.4%) and coagulation disorders (4.8%). The authors report that no unexpected serious adverse event related to thymosin alpha 1 occurred and that no safety outcome differed significantly between the groups.
Two things qualify that reading. First, an adverse-event rate of two in three is the background noise of an intensive-care unit; a drug with a modest effect on any single event would be hard to see against it. Second, the trial was supported in part by SciClone Pharmaceuticals, which markets Zadaxin, and several authors report grants or consultancy fees from the company.
The subgroup that went the wrong way
TESTS found no overall effect on 28-day mortality. Its prespecified subgroup analysis did find a split by age:
| Subgroup | Hazard ratio for 28-day death (95% CI) |
|---|---|
| Under 60 | 1.67 (1.04 to 2.67) |
| 60 and over | 0.81 (0.61 to 1.09) |
| Interaction | P = 0.01 |
A hazard ratio of 1.67 means more deaths on thymosin alpha 1 than on placebo in younger patients. The authors call it a potential differential effect. Subgroup results inside a neutral trial are frequently chance, and this one has not been tested again. It is reported here because it is the only finding in the controlled record that points towards harm, and because the people most likely to use thymosin alpha 1 from the research market are under 60.
There is also a small discrepancy worth knowing about for anyone checking the paper. The PubMed abstract gives the overall hazard ratio as 0.99 (0.77 to 1.27, P = 0.93). The full text on PubMed Central gives 0.97 (0.76 to 1.24, P = 0.82). The death counts are identical in both. Neither version changes the conclusion, but quoting one figure as "the" result without saying which version it came from is how errors spread.
The dialysis trial: no placebo, more sepsis on the drug
The second registered trial with posted results is NCT04428008, a pilot run in 2020–2021 to see whether thymalfasin prevented COVID-19 in people on haemodialysis. The treated group received 1.6 mg twice weekly after dialysis for eight weeks; the control group received standard care with no injection.
| Measure, through six months | Thymalfasin (n = 91) | Standard care (n = 98) |
|---|---|---|
| Any serious adverse event | 28 | 26 |
| Deaths | 3 | 7 |
| Sepsis (serious) | 4 | 0 |
| Respiratory failure (serious) | 4 | 2 |
| COVID-negative pneumonia (serious) | 4 | 1 |
| COVID-19 (serious) | 5 | 7 |
No other adverse event reached the trial's 5% reporting threshold in either arm. Dialysis patients are frail and get infections often, the numbers are small, and an open-label design with no placebo cannot separate a drug effect from chance. The sepsis imbalance is reported here because it is in the registry, not because it establishes anything.
A third registered study, a 488-patient melanoma trial in which thymosin alpha 1 was added to dacarbazine and interferon (NCT00911443, sponsored by sigma-tau), is listed as having results but posts no adverse-event table. Its 2010 publication states only that adding thymosin alpha 1 "did not lead to any additional toxicity" (PMID 20194853).
FDA's adverse-event database, read report by report
Counts for thymosin alpha 1 in FDA's adverse-event reporting system vary widely online, and the reason is the name. On 5 October 2026 the openFDA endpoint returned 37 reports for "thymosin alpha", 197 for "thymalfasin" and 14 for "Zadaxin" — 248 unique reports once duplicates across the three searches were removed.
What those 248 reports contain:
- 187 (75%) list thymosin alpha 1 only as a concomitant drug — something the patient was also taking, not something the reporter suspected.
- 61 list it as a suspect. In none of them is it the only drug; the companions most often listed are methylprednisolone, hydroxychloroquine, lopinavir, dacarbazine and interferon — the pattern of COVID-19 and melanoma treatment.
- 195 of the 248 came from China, where thymalfasin is a licensed hospital drug; 8 came from the United States.
- Among the 61 suspect reports, the most frequent terms are "off label use" (21), liver injury (8), "product use in unapproved indication" (5) and COVID-19 (5). Six record a death.
A spontaneous report records that someone associated an event with a product. It does not establish cause, and with every suspect report here involving several other drugs given to seriously ill people, attribution is not possible from the database. The figure that matters is the proportion: in three of every four reports, thymosin alpha 1 was simply present. Why research-market compounds generally produce so few reports at all is covered on our peptide side-effects overview.
What FDA has said
FDA has not approved thymalfasin, and DailyMed holds no label for it. Thymosin alpha 1 sits on FDA's list of bulk drug substances that may present significant safety risks in compounding (content current as of 22 April 2026), with this entry: compounded drugs containing it "may pose significant risk for immunogenicity for certain routes of administration and may have complexities with regard to peptide-related impurities and API characterization."
Immunogenicity — the body making antibodies against the peptide — is a stated risk, not a measured rate. We found no study that tested antibody formation in people using research-market thymosin alpha 1. The licensed trials excluded people with active autoimmune disease, organ or bone-marrow transplants and haematological cancers, so the record says nothing about them.
What is not established
- Any adverse-event rate in healthy adults, at any dose, by any route.
- Effects of use longer than the trials' seven days (TESTS) or eight weeks (dialysis).
- Whether the under-60 mortality signal in TESTS is real.
- Whether a research-market vial labelled thymosin alpha 1 contains thymalfasin at the stated amount; identity and purity are what a certificate of analysis is supposed to show.
- Effects in pregnancy, which every trial excluded. FAERS holds two reports of spontaneous abortion naming the compound, both with other suspect drugs.
What thymosin alpha 1 is and where it was first isolated is in Peptide Lexicon's thymosin alpha 1 entry. It is a different molecule from thymosin beta-4, whose fragment is sold as TB-500; that record is on our TB-500 side-effects page. Anyone who has used thymosin alpha 1 and has symptoms should tell a doctor what the vial said and where it came from.
Sources and dates
- Wu J, et al. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. BMJ. 2025;388:e082583. PMID 39814420; PMC11780596 (full text, adverse-event and subgroup sections, competing-interests statement). Read 2026-10-05.
- ClinicalTrials.gov NCT04428008, A Pilot Trial of Thymalfasin (Ta1) to Prevent COVID-19 Infection in Renal Dialysis Patients — adverse-events module. Read via API v2, 2026-10-05.
- ClinicalTrials.gov NCT00911443 (sigma-tau) — results modules listed, no adverse-events module. Read 2026-10-05.
- Maio M, et al. Large randomized study of thymosin alpha 1, interferon alfa, or both in combination with dacarbazine in patients with metastatic melanoma. J Clin Oncol. 2010;28(10):1780–7. PMID 20194853.
- openFDA drug adverse-event endpoint, medicinal product "thymosin alpha", "thymalfasin" and "Zadaxin" — all reports retrieved, de-duplicated by report ID and split by drug characterisation (suspect vs concomitant). Run 2026-10-05.
- FDA. Certain Bulk Drug Substances for Use in Compounding That May Present Significant Safety Risks, thymosin-alpha 1 entry. Content current as of 2026-04-22, read 2026-10-05.
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