Thymalin vs thymosin alpha 1 is a comparison between two very different products that both trace back to calf thymus. Thymalin is a mixture of small peptides extracted from calf thymus and registered in Russia as a prescription injection.[1] Thymosin alpha 1 is a single, defined 28-amino-acid peptide first isolated in 1977 and now made synthetically, sold as Zadaxin in a number of countries.[2][3] Neither is FDA-approved.
This guide compares where they come from, how they are used, the evidence behind them, doses, side effects and legal status. For background, see our thymalin and thymosin alpha 1 profiles and the longevity peptides hub.
At a glance
| Thymalin | Thymosin alpha 1 (thymalfasin) | |
|---|---|---|
| What it is | Peptide extract of calf thymus[1] | One 28-amino-acid peptide[2] |
| How it is made | Extracted from thymus of cattle under 1 year old[1] | Chemically synthesized[3] |
| Where approved | Russia, prescription[1] | Several countries, including Italy and India; FDA could not verify the full list[3] |
| Main uses abroad | Immune deficiency with infections, after chemotherapy or radiation[1] | Chronic hepatitis B; flu vaccine booster in people with weak immunity[3] |
| Typical dose | 5 to 20 mg into a muscle daily, 3 to 10 days[1] | 1.6 mg under the skin twice weekly[3] |
| Main evidence | Russian developer studies[6][7] | Randomized trials in several countries[8][10] |
| US status | Not approved; not on FDA compounding lists[13] | Not approved; FDA advisers voted against compounding[12] |
Where each one comes from
Thymalin was made by mild acid extraction of small peptides from calf thymus, and was put into medical practice as an “immunocorrector”.[4] Today’s product contains 10 mg of thymus extract per vial, with glycine.[1] Its developers report that the dipeptides Lys-Glu (KE) and Glu-Trp (EW) are active substances of the extract; Glu-Trp was later made synthetically as a separate Russian drug, Thymogen.[5][4]
Thymosin alpha 1 was isolated in 1977 by Allan Goldstein’s team from thymosin fraction 5, a crude calf thymus preparation, and its 28-amino-acid sequence was determined.[2] FDA notes that thymalfasin is a chemically synthesized version of it, and that Zadaxin is the finished product sold in other countries.[3]
So both started with calf thymus, but one is still an extract and the other is a single molecule made in a lab. Results for one cannot be applied to the other.
How they are used
- Thymalin’s Russian label covers immune deficiency in adults and in children from 6 months, for example with severe infections, lasting thymus problems, poor healing, or suppressed immunity after chemotherapy or radiation.[1]
- Thymosin alpha 1, according to FDA’s review, is approved in Italy as a flu vaccine booster for people with weak immune systems and in India for chronic hepatitis B. Singapore lists it for hepatitis B and C, and Mexico for hepatitis B and as a flu vaccine booster.[3] FDA said it could not independently verify all claimed approvals.[3]
Neither is approved anywhere for anti-aging, though both are marketed online for it.
The evidence compared
Thymalin
- Older adults: the developers followed 266 older people for 6 to 8 years and reported 2.0- to 2.1-fold lower death rates with thymalin than with controls. The abstract does not describe group sizes, how people were assigned, or death counts.[6]
- COVID-19: in a single-center, open-label trial of 92 hospital patients in St. Petersburg, 10 mg daily for 5 days was linked to faster falls in inflammation markers; no one died in either group, and CT scans worsened in 2 thymalin patients and 5 controls.[7]
These studies were run by the drug’s developers and have not been repeated by independent teams.
Thymosin alpha 1
- Hepatitis B (Taiwan): in 98 patients, a 26-week course of 1.6 mg twice weekly produced a complete response in 40.6% at 18 months, versus 9.4% of untreated controls. No significant side effects were observed, but none of the responders cleared the hepatitis B surface antigen, and a 52-week course did not clearly beat controls.[8]
- Hepatitis B (US phase 3): in a double-blind trial of 97 patients, complete response was 14% versus 4% on placebo, which was not statistically significant; the authors said the results did not confirm earlier benefit.[9]
- Sepsis (China, 2025): in 1,106 adults, 28-day mortality was 23.4% on thymosin alpha 1 and 24.1% on placebo, with no clear benefit.[10]
Dosing compared
| Thymalin[1] | Thymosin alpha 1[3] | |
|---|---|---|
| Route | Into a muscle | Under the skin (or into a muscle in Italy) |
| Adult dose | 5 to 20 mg once daily | 1.6 mg twice weekly |
| Course | 3 to 10 days; repeat after 1 to 6 months if needed | 6 months for hepatitis B; 4-week courses around flu vaccination in Italy |
Thymosin alpha 1 is absorbed quickly after injection under the skin, peaking in 1 to 2 hours, with a half-life under 3 hours.[11] We found no published pharmacokinetic data for thymalin. For more detail, see our thymalin dosage guide. A clinician sets the dose for any prescribed product.
Side effects compared
- Thymalin: the Russian label lists allergic reactions. It is not to be used in pregnancy or breastfeeding, and it carries cautions for some hormone conditions and the days around menstruation.[1]
- Thymosin alpha 1: labels abroad describe it as well tolerated, with mainly local discomfort at the injection site and rare redness, temporary muscle wasting, joint pain with hand swelling and rash. Labels warn against use in children, pregnancy and breastfeeding; one label notes temporary liver enzyme flares in hepatitis B, and advises against use in people deliberately immunosuppressed, such as transplant recipients.[3]
Both act on the immune system, so people with autoimmune disease or on immune-suppressing drugs should talk to a specialist first.
US legal status
- Thymosin alpha 1: not FDA-approved. In December 2024, FDA’s Pharmacy Compounding Advisory Committee voted 17 to 4 against adding it to the list of substances traditional (503A) pharmacies may use.[12] It does not appear on FDA’s May 2026 503A list.[13]
- Thymalin: not FDA-approved and absent from FDA’s 503A lists as of May 14, 2026.[13] In our reading, neither can legally be compounded by a 503A pharmacy.
For how these rules work, see 503A vs 503B compounding.
Related comparisons
Thymalin is often confused with thymulin, a different thymus hormone; see thymalin vs thymulin. For thymosin alpha 1 against an antimicrobial peptide, see thymosin alpha 1 vs LL-37. For broader context, see our guide to Khavinson peptide bioregulators and best peptides for immune support.
Questions to ask a clinician
- What condition are you treating, and is there an approved option with stronger evidence?
- Which product would I receive, and is it the registered medicine from another country?
- Do I have an autoimmune disease, transplant or medicine that makes immune stimulation risky?
- How will we judge whether it is working?
This is general information, not medical or legal advice.
