Both are sold to the same readers under the same promise — "thymus peptides for immunity" — and both trace back to the thymus glands of young cattle. That is where the resemblance ends. Thymalin is an extract, a mixture of peptides with no single defined sequence. Thymosin alpha 1 is one molecule, 28 amino acids long, now made synthetically. Their research records differ by an order of magnitude.
This page compares the two from their regulatory records and the literature, with dates. The label figures for each are on thymalin dosage and thymosin alpha 1 dosage; the trial safety record for the second is on thymosin alpha 1 side effects. Peptide Lexicon explains each molecule in its thymalin and thymosin alpha 1 entries. Nothing here is a recommendation; immune treatment decisions belong with a physician.
Two products from the same gland
| Thymalin | Thymosin alpha 1 | |
|---|---|---|
| What it is | Extract of calf and young-cattle thymus, 10 mg per vial with glycine | Synthetic 28-amino-acid peptide (INN thymalfasin) |
| Defined sequence | No — the label names an extract | Yes |
| Origin | Russian; its trial literature comes largely from the St Petersburg Institute of Bioregulation and Gerontology (Khavinson, Morozov) | Purified and sequenced from thymosin fraction 5 by Goldstein's group, 1970s |
| Registration | Russia, prescription, ЛП-№(005867)-(РГ-RU), 21 June 2024, held by Samson-Med | Zadaxin, marketed in a number of countries outside the US |
| US status | No FDA approval, no US label | No FDA approval, no DailyMed label; named on FDA's bulk-substance safety-risk list (22 April 2026) |
| Route on its label or programme | Intramuscular | Subcutaneous |
| Label side-effect section | "Possible: allergic reactions" (in full) | Trial adverse-event tables, e.g. the TESTS sepsis trial |
Thymalin from the Vidal reference (read 4 October 2026, the source of our thymalin dosage page). Thymosin alpha 1 from the 2001 Am J Health Syst Pharm review, the TESTS trial (BMJ 2025) and FDA's bulk-substance list, as cited on our thymosin alpha 1 side effects page. Origin of thymosin alpha 1: Low, Goldstein and colleagues, Ann N Y Acad Sci 1979, which describes several peptides of thymosin fraction 5 being "purified, sequenced and studied", thymosin alpha 1 among them.
The chemistry matters for anyone comparing claims. A thymalin result describes whatever mixture the extraction produced. A thymosin alpha 1 result describes one molecule at a known dose. A research vial sold as "thymalin" whose contents are a single synthetic sequence is not the product its Russian label describes — a point our dosage page makes and which no seller we have seen addresses.
The two literatures, counted
We ran the same searches for both names on 2 November 2026. Queries are printed so anyone can repeat them.
| Measure | Thymalin | Thymosin alpha 1 | Ratio |
|---|---|---|---|
| PubMed records (title/abstract) | 234 | 887 | 3.8× |
| Tagged "clinical trial" | 12 | 91 | 7.6× |
| Tagged "randomized controlled trial" | 1 | 64 | 64× |
| Records dated 2016–2026 | 11 | 243 | 22× |
| ClinicalTrials.gov studies naming it as an intervention | 0 | 66 | — |
Queries: thymalin[tiab]; ("thymosin alpha 1"[tiab] OR thymalfasin[tiab] OR "thymosin alpha1"[tiab]); each with AND clinical trial[pt], AND randomized controlled trial[pt] or AND 2016:2026[dp]. ClinicalTrials.gov API v2, query.intr = thymalin, thymalfasin and "thymosin alpha 1" (the last two return the same 66). Ratios are our arithmetic.
Three things stand out. Thymalin's literature is old and mostly closed: 11 of its 234 records are from the last decade, and the one record tagged as a randomised trial is the 2003 Khavinson and Morozov paper on thymus and pineal "bioregulators" in older people, whose English abstract does not state the dose given. Thymosin alpha 1's literature is current: 243 records in the last decade, including registered trials in sepsis, hepatitis B, dialysis and cancer immunotherapy. And the registry gap is total — trials submitted to US and most European regulators are registered on ClinicalTrials.gov, and none names thymalin.
The one paper that names both
Only one PubMed record mentions both names in its title or abstract: Belokrylov and colleagues, Drug Development and Industrial Pharmacy, 1998. It tested amino acids and peptide preparations in CBA mice and broiler chickens. Thymosin alpha 1, thymopentin and the peptide mixtures thymosin fraction 5 and thymalin "stimulate the immune response only at the site of subcutaneous injections"; thymalin and thymosin fraction 5 were also among the preparations that protected splenocytes from benzene and aflatoxin toxicity in vitro. It is an animal and cell study and compares no clinical outcome.
That is the entire direct comparison. No study in people has set thymalin against thymosin alpha 1, so any claim that one "works better" than the other has no trial behind it.
What each record says about safety
- Thymalin. The Russian instruction's side-effect section reads, in full, "possible: allergic reactions". It is contraindicated in pregnancy and breastfeeding. A one-line section is not evidence of safety; it reflects a label written without modern trial tables, and an extract of animal tissue carries the general questions of any biological material (source animals, purity, batch consistency) that the label does not address.
- Thymosin alpha 1. The largest placebo-controlled record is TESTS, 1,089 adults with sepsis in 22 Chinese centres: adverse events 66.4% vs 67.6% on placebo, serious events 26.8% vs 29.3%, no difference in 28-day mortality, and higher mortality in patients under 60 on the drug in a prespecified subgroup (hazard ratio 1.67). FDA names compounded thymosin alpha 1 for immunogenicity risk. Details and sources are on thymosin alpha 1 side effects.
What is not established
- Whether thymalin contains thymosin alpha 1 at all, or in what amount. Its label names an extract, not components, and we found no published compositional analysis of the registered product.
- Any human comparison of the two, for any outcome.
- Any US or EU regulatory review of thymalin's evidence.
Thymosin beta 4, the other well-known peptide from thymosin fraction 5, is compared with its research-market fragment on TB-500 vs thymosin beta 4; the safety picture across peptides is on are peptides safe.
Sources
- Thymalin (Тималин) instruction, Samson-Med, registration ЛП-№(005867)-(РГ-RU) of 21 June 2024 — Vidal drug reference, read 4 October 2026 (cited on /dosage/thymalin).
- Low TL, Thurman GB, … Goldstein AL. Current status of thymosin research. Ann N Y Acad Sci 1979;332:33–48. PMID 394636.
- Belokrylov GA et al. Immuno-, phagocytosis-modulating, and antitoxic properties of amino acids and peptide preparations. Drug Dev Ind Pharm 1998;24(2):115–27. PMID 15605441.
- Khavinson VKh, Morozov VG. Peptides of pineal gland and thymus prolong human life. Neuro Endocrinol Lett 2003;24(3–4):233–40. PMID 14523363.
- TESTS trial, BMJ 2025, PMID 39814420; FDA bulk drug substances that may present significant safety risks, current 22 April 2026 — as cited on /thymosin-alpha-1-side-effects.
- PubMed E-utilities and ClinicalTrials.gov API v2 searches as printed above, run 2 November 2026.