Thymosin Alpha-1 Side Effects: Delayed-Onset Adverse Events Explained

By HealthRX.com Editorial Team

Published Jan 15, 2025Updated Nov 7, 2026Last reviewed Jul 14, 2025

6 linked public sourcesEditorial, sourcing, and corrections policy

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Delayed side effect timing for thymosin alpha-1 is not established

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At a glance

Approved in the US / No; FDA found no US application [1]
Only side effect in one trial / Discomfort at injection sites [3]
FDA adverse-event database / Zero reports in an April 2024 search [1]
FDA on safety information / Inadequate to judge the extent of possible risks [2]
Half-life / About 1.8 to 2.6 hours; no accumulation over 5 daily doses [1]
Pooled trial signal / Fewer serious adverse events than comparators, low certainty [4]
Manufacturer-linked figures / 'Infrequent and mild' over 17 million doses [1]
Italian Zadaxin label / Not for children; contraindicated in pregnancy and breastfeeding [1]

What the trials recorded

The largest safety comparison is a 2026 Cochrane review of 10 randomised trials in 1,349 adults with chronic hepatitis B, published 1991 to 2018. Thymosin alpha-1 may have caused fewer serious adverse events than the comparison treatments (risk ratio 0.72, 95% CI 0.53 to 0.99; 5 trials, 1,056 people; low certainty) and fewer non-serious events (RR 0.47, 95% CI 0.27 to 0.83; very low certainty) [4].

Individual trials were small. One hepatitis B trial gave 1.6 mg under the skin twice weekly for 26 weeks in one group and 52 weeks in another. The trial reported no significant side effects [5]. In a 33-patient trial the only side effect reported with thymosin alpha-1 was local discomfort at injection sites [3].

The widest figures come from authors tied to the manufacturer: more than 4,400 people in trials and more than 17 million marketed doses of Zadaxin, with adverse experiences 'infrequent and mild' [1]. That is company-linked data, not independent surveillance.

Questions worth asking

Choose the questions you want to keepCould a side effect from thymosin alpha-1 show up days after an injection?What side effects should I watch for after starting thymosin alpha-1?Has FDA evaluated thymosin alpha-1 for safety in the United States?Which injection-site reactions are normal and which need medical attention?Should I report unusual symptoms to FDA's adverse-event reporting system?How could thymosin alpha-1 affect my immune system if I have an autoimmune condition?

What FDA has raised

Thymosin alpha-1 has four US orphan drug designations, which come from a separate process and are not approvals [1]. In its compounding review FDA raises possible immunogenicity with certain routes of administration, peptide impurities and characterization problems, and safety information 'inadequate for the agency to sufficiently understand the extent of any safety issues' [2]. The Pharmacy Compounding Advisory Committee voted 4 yes and 17 no on both the free base and the acetate in December 2024; members who voted no cited no compelling evidence of clinical effectiveness and safety [6].

What the Zadaxin label says

Italy's label for the finished 1.6 mg Zadaxin injection is not a US label [1]. The Italian label gives 1.6 mg under the skin or into muscle twice weekly for 4 weeks starting at the first vaccination, repeated from week 8 to week 12 at the second vaccination. It says the product is not for children and is contraindicated in pregnancy and breastfeeding. It calls for caution in people with allergies or autoimmune disease and with other immune-modulating drugs [1].

Check what you know

Think you have it? Try 4 quick questions and see what the article says.

1. In the 33-patient trial, which side effect was the only one reported with thymosin alpha-1?

Question 1 of 4

Score: 0 right, 0 of 4 answered

Frequently asked questions

How soon after an injection do side effects appear?

The trial and FDA material do not state when a side effect might appear after an injection. One trial reported injection-site discomfort as the only side effect [3].

Is thymosin alpha-1 FDA-approved?

No. No US application was found for it, and its four orphan drug designations are a separate process from approval [1].

Which side effects were reported in trials?

Local discomfort at injection sites was the only side effect reported with thymosin alpha-1 in a 33-patient trial [3], and a 2026 Cochrane review found fewer serious adverse events than comparators (RR 0.72, 95% CI 0.53 to 0.99; 5 trials, 1,056 people; low certainty) [4].

Is it a concern for people with autoimmune disease?

Italy's Zadaxin label calls for caution in people with allergies or autoimmune disease and with other immune-modulating drugs [1]. FDA also cites a possible risk of immunogenicity for certain routes of administration when the peptide is compounded [2].

References

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U.S. Food and Drug Administration.
Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks | FDA.
Andreone P, Cursaro C, Gramenzi A, Zavagliz C, Rezakovic I, Altomare E, et al. A randomized controlled trial of thymosin-alpha1 versus interferon alfa treatment in patients with hepatitis B e antigen antibody--and hepatitis B virus DNA--positive chronic hepatitis B. Hepatology. 1996.
Naing C, Ni H, Aung HH, Aye SN, Nikolova D, Poovorawan Y, et al. Thymosin-ɑ1 for people with chronic hepatitis B. Cochrane Database Syst Rev. 2026.
Chien RN, Liaw YF, Chen TC, Yeh CT, Sheen IS. Efficacy of thymosin alpha1 in patients with chronic hepatitis B: a randomized, controlled trial. Hepatology. 1998.
U.S. Food and Drug Administration.

Evidence overview for Thymosin Alpha-1 Side Effects: Delayed-Onset Adverse Events Explained