Thymosin Alpha-1 Compounding Pharmacy: 503a vs 503b Explained

Is thymosin alpha-1 on FDA's bulk substance lists?

FDA lists thymosin alpha-1 in no category for state-licensed pharmacies and physicians. For outsourcing facilities, it is in category 3, 'nominated without adequate support'; FDA's category 1 policy does not apply [1][2].

FDA keeps a separate bulks list for each pathway, so the same substance can land in a different place on each [1][2].

What the peptide is, and where it is approved

Thymosin alpha-1 is a 28-amino-acid peptide with an acetylated N-terminal end, first isolated from thymosin fraction 5, a thymus extract; thymalfasin is the synthetic version with the same sequence [5]. It is described as an immune modulator that augments T-cell function [7]. Zadaxin is the brand of the finished 1.6 mg injection sold outside the US and is not interchangeable with bulk powder [5].

There is no FDA-approved thymosin alpha-1 product. A Drugs@FDA search turns up no US application; FDA's December 2024 review notes four US orphan drug designations, which come from a separate process and are not approvals [5]. FDA lists national authorizations in Italy (adjuvant to influenza vaccination in immunocompromised people), India (chronic hepatitis B in adults 18 or older with compensated liver disease and virus replication) and Hong Kong [5]. Two reviews by authors tied to the Zadaxin developer report approval in more than 30 countries (2007) and over 35 countries (2009) [8][9]. FDA separately says it could not independently verify the broad regional approval claims in a company annual report [5]. The European Medicines Agency has granted only an orphan designation for hepatocellular carcinoma, not a marketing approval [5].

Why FDA's advisers voted against adding it

On December 4, 2024, FDA's Pharmacy Compounding Advisory Committee voted 4 yes and 17 no, with no abstentions, on both thymosin alpha-1 free base and thymosin alpha-1 acetate; FDA had proposed that the peptide not be added to the pharmacy bulks list [4]. Members who voted no cited no compelling evidence of clinical effectiveness and safety, while some who voted yes said patients should have access [4].

Does FDA still treat it as a safety concern?

Thymosin alpha-1 once sat in category 2, the group of bulk substances FDA says may present significant safety risks, and it now appears in the 'nominated but withdrawn' table because the nominators withdrew it [6]. Withdrawal did not retract the agency's stated concerns: a significant risk of immunogenicity for certain routes of administration, peptide-impurity and characterization problems, and safety information that FDA called inadequate to understand the extent of any safety issues [6].

Does the October 2026 HHS peptide policy cover it?

No. HHS's October 2026 draft enforcement-discretion policy names six peptides, BPC-157, epitalon, KPV, MOTS-c, Semax and TB-500, and thymosin alpha-1 is not one of them [3].

What dose does the Italian Zadaxin label describe?

There is no US-approved dose [5]. The Italian label for Zadaxin 1.6 mg powder describes subcutaneous or intramuscular use 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, is contraindicated in pregnancy and breastfeeding, and calls for caution in people with allergies or autoimmune disease and with other immune-modulating drugs [5].

After subcutaneous injection in healthy volunteers, thymosin alpha-1 peaks at about 0.7 to 2 hours and clears with a half-life of about 1.8 to 2.6 hours, with no accumulation after daily dosing for five days [7][5].

What is known about its safety

FDA's search of its adverse-event database on April 22, 2024 returned zero reports for thymosin alpha-1 [5]. Authors tied to the manufacturer 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, which is company-linked data [5].

In a 2026 Cochrane review of hepatitis B trials, 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 evidence) and fewer non-serious adverse events (risk ratio 0.47, 95% CI 0.27 to 0.83; very low-certainty evidence) [10]. The same review, covering 10 randomized trials and 1,349 adults, found possible lower all-cause mortality (risk ratio 0.53, 95% CI 0.29 to 0.96; 3 trials, 907 people) but rated the certainty very low, with little to no effect on quality of life or liver histology, and the reviewers said they were not sure the drug helps [10].