Thymosin alpha-1 (TA1): benefits, side effects and what the trials show

Thymosin alpha-1 (Tα1) is a heat-stable, strongly acidic peptide of 28 amino acids. Goldstein and colleagues isolated it from a calf thymus extract known as thymosin fraction 5 and published its sequence in 1977 ^[1]. The drug form, thymalfasin, is made synthetically and marketed as Zadaxin. A 2005 company filing stated it was approved for sale in over 30 countries, primarily in Asia, the Middle East and Latin America ^[8].

Despite the similar name, thymosin alpha-1 is unrelated in structure and function to TB-500, which is a fragment of thymosin beta-4. Both came out of the same early thymus research, which is the source of the shared name.

Sellers of research peptides usually call it TA1, and the thymosin alpha-1 peptide sold in vials online as “TA1 peptide” is intended to be the same sequence as thymalfasin. It is not the same product: pharmaceutical thymalfasin is made and tested to medicine standards, while research vials are not checked by any regulator. For how that market works, see what research peptides are.

Thymosin alpha-1 is described as an immune modulator. The original 1977 paper proposed that peptides in thymosin fraction 5 help regulate the development and function of T cells, the white blood cells that mature in the thymus ^[1]. Later reviews describe it as modifying and restoring immune function and summarise its use in immunocompromised states, cancers, infections and as a vaccine enhancer; the evidence for these uses varies widely in quality ^[6].

Laboratory work has since tied it to the innate immune system. A 2004 study in Blood found that thymosin alpha-1 activated dendritic cells, which present foreign material to T cells, through Toll-like receptor signalling, and protected mice that had received blood stem cell transplants from aspergillosis, a fungal lung infection ^[9]. A 2001 pharmacy review summarised its proposed action as augmenting T-cell function, possibly by stimulating the differentiation of immature T cells in the thymus ^[10].

Thymosin alpha-1 has more human data than almost any other peptide sold as a research chemical, and the data are mixed. The largest and most rigorous trial, in sepsis, found no benefit. The hepatitis B evidence is suggestive but of low certainty. The cancer results come mostly from trials that added it to other treatments. The trials are summarised below.

Sepsis: the key trial is TESTS, a double-blind, placebo-controlled phase 3 study at 22 centres in China, published in the BMJ in 2025. Of 1,089 adults with sepsis analysed, 28-day mortality was 23.4% with thymosin alpha-1 and 24.1% with placebo, a difference that was not statistically significant; no secondary or safety outcome differed significantly. The trial was part-funded by SciClone Pharmaceuticals, which markets Zadaxin ^[2].

Chronic hepatitis B: a 2026 Cochrane review found 10 randomised trials with 1,349 participants. Thymosin alpha-1 may reduce all-cause mortality and serious adverse events, but the authors rated the certainty of evidence as very low for most outcomes and low for serious adverse events, and said they could not be sure of any benefit ^[3].

Older hepatitis trials, reviewed in 2001, give a sense of where that uncertainty comes from. One randomised controlled trial reported clearance of hepatitis B virus DNA in 40.6% of patients treated for six months and 25.6% of those treated for 12 months, against 9.4% of untreated controls. In hepatitis C, one trial found no difference from placebo in liver enzyme normalisation, while two others found better responses when thymosin alpha-1 was added to interferon ^[10].

Melanoma: in a randomised trial of 488 patients with metastatic melanoma, adding thymosin alpha-1 to chemotherapy (dacarbazine) with or without interferon alfa produced more tumour responses in two of the thymosin arms (10 and 12) than in the control arm (4); median overall survival was 9.4 versus 6.6 months, a difference that did not reach statistical significance (P = .08), and no extra toxicity was seen ^[4].

COVID-19: a 2023 meta-analysis of nine studies with 5,352 patients found no statistically significant effect of thymosin alpha-1 on mortality in hospitalised adults and concluded that the results did not support its use; subgroup analyses suggested possible benefit in older and more severe patients ^[5].

Interest in thymosin alpha-1 for autoimmune disease, including Hashimoto’s thyroiditis, rheumatoid arthritis and psoriasis, is not backed by clinical trials that we could find. The related research is observational or laboratory-based. A 2016 Italian study measured thymosin alpha-1 in blood and found lower levels in 120 people with psoriatic arthritis, 40 with rheumatoid arthritis and 40 with lupus than in 120 blood donors ^[11]. Lower levels in a disease do not show that giving the peptide would help.

In a 2018 laboratory study, adding thymosin alpha-1 to blood cells from people with relapsing-remitting multiple sclerosis reduced several inflammatory signals and expanded regulatory B cells, which dampen immune responses ^[12]. That was a test-tube experiment. Because the drug has been studied mainly to strengthen immune responses in infection and cancer ^[6], whether it would calm or worsen an autoimmune condition in a person is an open question that only a trial could answer.

Thymosin alpha-1 is marketed in bodybuilding circles for recovery and for staying well during hard training. We found no study of its effect on muscle, strength, recovery or athletic performance in people. Its trial record concerns infection, liver disease and cancer. We have not confirmed its status on the WADA Prohibited List, so athletes subject to testing should check with their national anti-doping organisation.

Trials measured outcomes on the timescale of the disease being treated, not the time to feel an effect. In sepsis, patients received a seven-day course and the main outcome was death at 28 days ^[2]. In the hepatitis B trial described above, viral clearance was assessed after six and 12 months of treatment ^[10]. The melanoma trial measured tumour response and survival over months ^[4]. None of these tells you how quickly a healthy person might notice anything, and there are no trials in healthy people.

The published trials gave thymosin alpha-1 by injection under the skin ^{[2] [10]}. According to a 2001 review, it is rapidly absorbed after injection, reaches peak blood levels within two hours, has a serum half-life of about two hours, and returns to baseline levels within 24 hours ^[10].

Thymosin alpha-1 nasal spray and oral products are also sold. We found no published study of either route in people, so there is no evidence on how much of the peptide is absorbed from them.

In the TESTS sepsis trial, patients received 1.6 mg of thymosin alpha-1 by subcutaneous injection every 12 hours for seven days. The authors noted that the conventional dose used for cancers and viral hepatitis is 1.6 mg twice weekly ^[2]. The melanoma trial tested doses of 1.6 mg, 3.2 mg and 6.4 mg alongside chemotherapy ^[4]. These are the doses used in the studies, not recommendations. The peptide calculator converts vial contents to concentrations only.

The 2001 review gives the hepatitis dose as 1.6 mg, equivalent to 900 micrograms per square metre of body surface, injected under the skin twice a week ^[10]. There is no published thymosin alpha-1 dosage when sick with a cold or flu, because no trial has tested it in otherwise healthy people with minor infections; the sepsis trial involved hospital patients with a life-threatening condition ^[2].

For vial arithmetic only: a 10 mg vial made up with 2 mL of liquid gives 5 mg/mL, and on a U-100 insulin syringe one unit (0.01 mL) then contains 0.05 mg. A 5 mg vial made up with 1 mL gives the same 5 mg/mL. This is arithmetic, not dosing advice.

In controlled trials of the pharmaceutical product, thymosin alpha-1 has generally been well tolerated. TESTS found no significant difference in safety outcomes between thymosin alpha-1 and placebo ^[2], the melanoma trial reported no additional toxicity ^[4], and the Cochrane review found that serious adverse events may be less frequent with thymosin alpha-1, based on low-certainty evidence ^[3].

The 2001 review reported that most studies observed only local irritation at the injection site ^[10]. These trials used short to medium courses in people with serious illness, often alongside other drugs, so they do not show what long-term use by healthy people would do.

Those findings apply to regulated thymalfasin. For compounded or research-grade material, the FDA has said that thymosin alpha-1 may pose a significant risk of immune reactions (immunogenicity) and of peptide-related impurities ^[7]. Research products are not made to pharmaceutical standards, which is why testing matters; see third-party peptide testing.

The two share a name only because both were first found in thymus extracts. Thymosin alpha-1 is a 28-amino-acid immune peptide that has been through phase 3 trials ^{[1] [2]}. Thymosin beta-4 is a different protein studied mainly for tissue repair, and TB-500 is marketed as a fragment of it; neither has comparable human trial data. They are not interchangeable, and results for one say nothing about the other. Our TB-500 guide covers the beta-4 evidence.

Thymalin is a Russian immunomodulatory drug made from a polypeptide extract of thymus, not a single defined peptide; its developers identify two dipeptides, KE and EW, as active components ^[13]. Its co-authors include Vladimir Khavinson, whose group also developed epitalon ^[13]. Thymosin alpha-1, by contrast, is a single synthetic 28-amino-acid peptide with international randomised trials ^{[2] [4]}. No study has compared the two.

Two other thymus peptides are often mentioned alongside it. Thymulin is a nine-amino-acid thymic hormone that needs zinc bound to it to be active, and has shown anti-inflammatory effects in the brain in animal studies ^[14]. Thymogen is a dipeptide, L-Glu-L-Trp, described as an immunostimulant drug ^[15]. Each is a distinct molecule with its own, generally smaller, evidence base.

Thymalfasin is not approved by the FDA. In the US, the FDA previously placed thymosin alpha-1 in category 2 of its interim compounding policy, the list of bulk substances that may present significant safety risks; the nomination was later withdrawn and the FDA now lists it among substances previously in category 2 ^[7]. It was not among the peptides considered at the FDA’s July 2026 compounding advisory committee meeting.

We could not confirm that thymalfasin is a licensed medicine in the UK, Canada, Australia or New Zealand. We have not confirmed its status on the WADA Prohibited List. For national rules on research peptides, see our legal status overview and the UK page.

In New Zealand, Medsafe’s classification database lists thymic peptide hormones as prescription medicines ^[16]. Under New Zealand law, possessing a prescription medicine without a reasonable excuse is an offence ^[17]; see peptide rules in New Zealand.

The serum half-life of injected thymosin alpha-1 is about two hours, according to the 2001 review ^[10].

Research-grade thymosin alpha-1 is supplied as a freeze-dried powder. Peptides are generally stored cold, dry and away from light, and reconstituted solutions are refrigerated and used within a limited period. See how to store peptides and bacteriostatic water.

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