Thymosin alpha-1: a licensed medicine abroad, not in North America

The short answer

• Thymosin alpha-1, also called thymalfasin or Tα1, is a 28-amino-acid immunomodulatory thymic peptide, sequence-identical to the natural peptide first isolated in 1977.
• Evidence tier: Approved, but outside North America. Italy licenses it as an adjuvant to influenza vaccination in immunocompromised people. Sponsor-derived reporting claims approval in more than 35 countries, chiefly for chronic hepatitis B and C; the FDA said it could not independently verify that list.
• Human trials for sepsis and for necrotising pancreatitis, and in both cases the trials were large, properly designed, and negative. A tier records that the evidence exists, not that it came out well.
• No human data at all for long COVID, ME/CFS, Lyme disease, or “immune support” in healthy people. Those are the uses it is mostly sold for.
• Not approved by the FDA. Not authorized in Canada, and named in a Health Canada advisory of 1 April 2025 on peptides seized from a Calgary clinic.
• It is not thymosin beta-4 or TB-500, despite the shared word.

What it is and how it works

Thymosin alpha-1 is 28 amino acids long with an acetylated N-terminus. At that length it counts as a drug rather than a biologic in the US, where the biologic threshold sits above 40 amino acids. There is no USP monograph for it. It is not the same thing as older thymic extracts such as thymosin fraction 5, thymomodulin or thymulin, and it is emphatically not thymosin beta-4, and therefore not TB-500, a different peptide with a different mechanism and a different anti-doping status.

The mechanism is better characterised than most peptides in this space. In mouse and human cell work, Tα1 triggers maturation and IL-12 production by myeloid dendritic cells through MyD88-dependent Toll-like receptor signalling. TLR9 is the best-supported receptor, and downstream it activates NF-κB through a TRAF6-dependent pathway.

Here is the detail that undercuts the marketing. Tα1 also induces indoleamine 2,3-dioxygenase in dendritic cells, which favours regulatory T cells and tolerance. In human monocyte-derived dendritic cells it amplified responses to viral TLR3 and TLR7/8 agonists but dampened responses to bacterial TLR2 and TLR4 stimulation (Giacomini 2015). “Immune booster” is a misdescription. The preclinical picture is context-dependent modulation, in both directions.

What is shown in humans is narrower still: changes in immune surrogates in some small studies. What has not been shown is that those changes produce fewer infections, lower mortality or better outcomes when a trial is large enough to tell.

The trials that were designed to settle it

Two phase 3 trials tested Tα1 properly, in conditions where immune modulation had an obvious rationale.

Sepsis. TESTS randomised 1,106 adults across 22 Chinese centres, double-blind and placebo-controlled. Twenty-eight-day mortality was 23.4% with Tα1 and 24.1% with placebo, a hazard ratio of 0.99 with a confidence interval from 0.77 to 1.27. No secondary or safety outcome differed either (Wu 2025). The earlier, smaller, single-blind ETASS trial had found a borderline signal (26.0% versus 35.0%, p=0.062) in 361 patients. TESTS was three times larger, double-blind, and did not confirm it. A post-TESTS meta-analysis found a pooled benefit that disappeared when the analysis was restricted to high-quality or multicentre trials.

Necrotising pancreatitis. The TRACE trial randomised 508 patients, double-blind. Infected pancreatic necrosis occurred in 15.7% versus 18.1%, p=0.48 (Ke 2022).

The hepatitis story is similar, just older. A Taiwanese randomised trial in 98 patients found 40.6% complete virological response at 18 months against 9.4% in untreated controls (Chien 1998). A US phase 3 double-blind trial in 97 patients found 14% versus 4%, p=0.084, and its authors concluded plainly that their results did not confirm the earlier efficacy findings (Mutchnick 1999). A 2026 Cochrane review pooled 10 randomised trials and 1,349 participants and rated nearly every outcome very low to low certainty (Naing 2026).

Even where the licence stands, the indication has moved on. Chronic hepatitis B is treated with entecavir or tenofovir, and hepatitis C with direct-acting antivirals, which replaced the interferon regimens Tα1 was tested alongside.

What it is sold for, and what exists for those uses

This is the honest part of the article. The gap between the licensed indications and the marketed ones is almost total.

Marketed use	What actually exists
Long COVID	Ex vivo experiments on lymphocytes from patients. No clinical trial.
ME/CFS or chronic fatigue	The FDA reviewed this in December 2024 and found no clinical study at all.
Lyme disease	Nothing.
”Immune support” in healthy people	Nothing.
Acute COVID-19	Conflicting retrospective cohorts and one 49-patient open-label pilot trial with no significant clinical difference. Meta-analyses split.
Vaccine response	Small exploratory studies using influenza vaccines no longer standard, measuring antibodies rather than infections.

The vaccine line deserves a note, because it is the licensed indication in Italy. The supporting studies enrolled 90 older men with the 1986 influenza vaccine (Gravenstein 1989) and 99 haemodialysis patients (Carraro 2012). Meanwhile, high-dose influenza vaccine is 24.2% more effective than standard dose in adults 65 and over, and the adjuvanted recombinant zoster vaccine showed 97.2% efficacy in adults 50 and over. Those answer the same clinical problem with randomised efficacy data on clinical outcomes.

One oncology signal is worth naming. In 488 patients with metastatic melanoma, adding Tα1 to dacarbazine produced more responses and overall survival of 9.4 versus 6.6 months, hazard ratio 0.80, p=0.08 (Maio 2010). That is a signal, not a result.

Doses that were studied

Label doses may be quoted, so here they are with their indications attached. The international prescribing information for chronic hepatitis B specifies 1.6 mg subcutaneously twice weekly for 6 to 12 months. The Italian label for influenza-vaccine adjuvant use in immunocompromised people specifies one 1.6 mg vial subcutaneously or intramuscularly twice weekly for 4 weeks from the day of the first vaccination, repeated from week 8 to week 12 with the second. The label says not to give it intravenously. Trial doses clustered around the same figure: 1.6 mg twice weekly in hepatitis, 1.6 mg every 12 hours for 7 days in the sepsis trials.

The pharmacokinetics raise a question nobody has answered. After subcutaneous injection, peak concentration arrives at about 2 hours and the serum half-life is about 2 hours, with no accumulation over repeated dosing (Rost 1999). Dosing a 2-hour peptide twice weekly assumes the immune effects substantially outlast the exposure. That is plausible, and it has never been demonstrated by a dose-response study. The FDA noted the gap explicitly.

Clinic and vendor practice borrows the vial strength and repeats it anywhere from daily to twice weekly, sometimes as a nasal spray, for indications that were never studied. The dose resembles the label; nothing else about it does.

Safety and the regulatory picture

Tolerability is genuinely good. The label describes adverse events as infrequent and mild, mainly injection-site discomfort. In TESTS, adverse events occurred in 66.4% on Tα1 versus 67.6% on placebo, and serious adverse events in 26.8% versus 29.3%. ALT flares have been reported in hepatitis B treatment, and TSH abnormalities when combined with interferon.

The contraindications matter. The international label contraindicates deliberate immunosuppression, such as organ transplantation, unless benefit clearly outweighs risk; in a phase 1/2 study after haploidentical stem cell transplantation, one of six recipients died of immune haemolytic anaemia (Perruccio 2010). The Italian label contraindicates pregnancy and breastfeeding, says not to use it in children, and requires a case-by-case decision in autoimmune disease. No carcinogenicity studies exist.

On regulation, as of September 2026: the FDA has approved no Tα1 product. It sat in 503A Category 2 from 2023; the nomination was withdrawn in September 2024, and the FDA’s advisory committee voted 4 yes to 17 no against adding it to the 503A Bulks List on 4 December 2024. With no USP monograph and no Bulks List entry, it cannot lawfully be compounded from bulk under 503A. A Health Canada Drug Product Database search on 25 September 2026 returned no product for thymalfasin, thymosin or Zadaxin. The FDA also documented websites advertising compounded injectable and nasal Tα1, one of which falsely described it as FDA-approved.

For athletes the situation is genuinely ambiguous. Tα1 is not named on the 2026 WADA list, and the S0 category covers substances with no current approval by any governmental health authority, which thymalfasin has. No explicit WADA ruling was found. The practical hazard is product identity: a grey-market “TA1” vial mislabelled with another thymic peptide, including TB-500, which is prohibited under S2, would produce a positive test.

The bottom line

Thymosin alpha-1 is a real licensed medicine in some countries, whose licensed indications have been overtaken by better drugs and whose two best trials were negative. In Canada and the US it has almost no legitimate outpatient role as of September 2026. The person asking for immune support usually needs a diagnosis, not a modulator; the interventions with outcome data for that problem are vaccines, glycaemic control and sleep. See peptides and cellular repair for how the adjacent claims grade out.

Education only. This page is educational. Most compounds referenced are not approved for human use, and a “research use only” label carries no legal status of its own. Nothing here constitutes medical advice, diagnosis, or treatment.

Sources

• 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.
• Ke L, et al. Immune enhancement in patients with predicted severe acute necrotising pancreatitis: a multicentre double-blind randomised controlled trial. Intensive Care Med, 2022.
• Naing C, et al. Thymosin-ɑ1 for people with chronic hepatitis B. Cochrane Database Syst Rev, 2026.
• Mutchnick MG, et al. Thymosin alpha1 treatment of chronic hepatitis B: results of a phase III multicentre, randomized, double-blind and placebo-controlled study. J Viral Hepat, 1999.
• Giacomini E, et al. Dual effect of Thymosin α 1 on human monocyte-derived dendritic cell in vitro stimulated with viral and bacterial toll-like receptor agonists. Expert Opin Biol Ther, 2015.
• 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.
• Rost KL, et al. Pharmacokinetics of thymosin alpha1 after subcutaneous injection of three different formulations in healthy volunteers. Int J Clin Pharmacol Ther, 1999.
• FDA, Pharmacy Compounding Advisory Committee presentation and summary minutes on thymosin alpha-1 related bulk drug substances, 4 December 2024; Health Canada advisory on unauthorized injectable peptide drugs seized in Calgary, 1 April 2025; AIFA Zadaxin patient leaflet (Italy).