Thymosin Alpha-1 Side Effects: Severity Distribution by Patient Phenotype

What the trials recorded

The 2026 Cochrane review included 10 randomized trials and 1349 adults with chronic hepatitis B overall, but its serious-adverse-event comparison drew on five trials and 1056 people. That comparison found 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; low-certainty evidence) and fewer non-serious events (risk ratio 0.47, 95% CI 0.27 to 0.83; very low-certainty evidence) [2].

Smaller trials describe tolerability in general terms. In a 12-person placebo-controlled pilot, seven people received thymosin fraction 5 or thymosin alpha-1 and five received placebo twice weekly for six months, with no significant side effects observed in either group [4]. In a 33-person trial, some patients reported local discomfort at the injection sites, and the authors reported no other side effect in that trial [3].

What is known about severity by patient phenotype

The figures above are overall comparisons or general statements, not severity-grade breakdowns by patient phenotype [2][4][3]. FDA's April 22, 2024 search of its adverse-event database found zero reports for thymosin alpha-1 [1]. FDA's dashboard guidance says such reports cannot be used to estimate how often an event occurs or to establish that a drug caused it [5].

Company-linked authors report more than 4,400 people enrolled in trials and more than 17 million doses of Zadaxin given after marketing, with adverse experiences described as infrequent and mild [1]. That is company-linked data, and it is not broken down by severity grade or patient type.

Which patients does the label flag?

The Italian Zadaxin label covers a 1.6 mg finished injection given subcutaneously or intramuscularly twice weekly for 4 weeks from the first vaccination, repeated from week 8 to week 12. That finished injection is not interchangeable with bulk thymosin alpha-1 powder [1].

The label says the product is not for children, is contraindicated in pregnancy and breastfeeding, and calls for caution in people with allergies or autoimmune disease and with other immune-modulating drugs [1].

What has FDA said about compounded thymosin alpha-1?

Thymosin alpha-1 is not FDA-approved in the US. It appears in the nominated-but-withdrawn table on FDA's page about bulk substances that may present significant safety risks. FDA's stated concerns remain: significant risk of immunogenicity for certain routes of administration, peptide-impurity and characterization problems, and safety information the agency called inadequate to understand the extent of any safety issues [6].

FDA's Pharmacy Compounding Advisory Committee voted 4 yes and 17 no on both the free base and the acetate in December 2024. The peptide is not among those in HHS's October 2026 draft enforcement-discretion policy [7][8].