Thymosin Alpha-1: Compounding Pharmacy Research-Only vs Medical-Grade Peptides

What is actually different between research-only and medical-grade thymosin alpha-1?

FDA has not approved thymosin alpha-1 in the United States [1]. The peptide is a 28-amino-acid chain with an acetylated N-terminal end, first isolated from thymosin fraction 5, a thymus extract; thymalfasin is the synthetic version with the same sequence [1].

The only finished, authorized product is Zadaxin, a 1.6 mg injection sold outside the US. It is not interchangeable with bulk thymosin alpha-1 powder [1].

FDA's posted concerns about the bulk substance considered for compounding cover immunogenicity for certain routes of administration, peptide impurities and characterization problems, and safety information inadequate for the agency to understand the extent of any safety issues [5].

Does FDA allow compounding of thymosin alpha-1?

No FDA or HHS action allowing it was found [6]. The specifics:

• On FDA's list for state-licensed pharmacies and physicians (updated 14 May 2026), thymosin alpha-1 appears in none of the three categories [2].
• On FDA's list for outsourcing facilities (updated 21 March 2025) it is in category 3, 'nominated without adequate support' [3]. FDA's category 1 policy, generally no action against compounders if conditions are met, does not apply to it [2][3].
• On FDA's page for bulk substances that may present significant safety risks, it now appears in the 'nominated but withdrawn' table, after having been in category 2. The nominators withdrew; FDA's safety concerns remain posted [5].
• On 4 December 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. Members who voted no cited no compelling evidence of clinical effectiveness and safety; some who voted yes said patients should have access [4]. FDA proposed that it not be added to the pharmacy bulks list [4].
• Thymosin alpha-1 is not among the six peptides in HHS's October 2026 draft enforcement-discretion policy (BPC-157, epitalon, KPV, MOTS-c, Semax, TB-500) [6].

What does FDA say the risk is?

FDA's posted concerns are 'significant risk for immunogenicity for certain routes of administration', peptide-impurity and characterization problems, and safety information 'inadequate for the agency to sufficiently understand the extent of any safety issues' [5]. These concerns were raised in FDA's review of the bulk substance nominated for compounding [5].

Is thymosin alpha-1 approved anywhere?

Not in the United States. No US application for it appears in Drugs@FDA, and its four orphan drug designations are a separate process from approval [1]. FDA's December 2024 review lists national authorizations in Italy (Zadaxin as an 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 [1]. A SciClone 2014 annual report describes approvals across parts of Asia-Pacific, Latin America, Eastern Europe and the Middle East, mostly for hepatitis B and as a vaccine adjuvant, and FDA could not independently verify those broader claims [1]. The European Medicines Agency holds only an orphan designation, for hepatocellular carcinoma, which is not a marketing approval [1].

Two reviews by authors tied to the Zadaxin developer put the count at 'over 35 countries' in 2009 and 'more than 30 countries' in 2007; those figures are company-linked [8][9].

What the hepatitis B trials actually found

A 2026 Cochrane review pooled 10 randomised trials with 1,349 adults, published between 1991 and 2018 in Bangladesh, China, Italy, Korea, Singapore and Taiwan. All-cause mortality was possibly lower with thymosin alpha-1 (risk ratio 0.53, 95% CI 0.29 to 0.96; 3 trials, 907 people) but the certainty was very low. There was little to no effect on quality of life or liver histology, the effect on hepatitis B-related illness was very uncertain, and the reviewers said they were not sure it helps [10].

Two individual trials show the spread. Lim and colleagues randomised 98 adults with HBeAg-positive chronic hepatitis B to 24 weeks of lymphoblastoid interferon plus thymosin alpha-1 (n=48) or plus placebo (n=50); HBeAg loss at 72 weeks was 45.8% versus 28.0% (difference 17.8%, 95% CI -1.2 to 35.3, P=0.067), which was not statistically significant [11]. A very small 1991 pilot gave thymosin fraction 5 or thymosin alpha-1 to 7 people and placebo to 5; HBV DNA cleared in 6 of 7 versus 1 of 5 (P<0.04) [12].

A 2001 pharmacy review reported a serum half-life of about 2 hours and noted that hepatitis B had been evaluated in 195 patients across four trials, with HBV DNA clearance of 40.6% after 6 months and 25.6% after 12 months of thymosin alpha-1 versus 9.4% of untreated controls in one randomised trial [13].

These results come from defined trial products. They are not a measurement of any particular compounded or research vial.

Dose, label cautions and what is known about safety

There is no US-approved dose [1]. The Italian Zadaxin label gives 1.6 mg subcutaneously or intramuscularly twice weekly for 4 weeks starting at the first vaccination, repeated from week 8 to week 12 at the second vaccination. 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].

FDA's search of its adverse-event database on 22 April 2024 returned zero reports for thymosin alpha-1 [1]. Zero reports reflects what was reported, not a safety finding. 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 'infrequent and mild'; that is company-linked data [1]. In the 2026 Cochrane review of hepatitis B trials, thymosin alpha-1 may have had 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 events (risk ratio 0.47, 95% CI 0.27 to 0.83; very low-certainty evidence) [10].

Absorption and clearance are short: after subcutaneous injection in healthy volunteers, peak levels arrived at about 0.7 to 2 hours, the half-life was about 1.8 to 2.6 hours, and there was no accumulation after daily dosing for five days [13][1].

Is thymosin alpha-1 banned in sport?

It is not named on WADA's 2026 Prohibited List [7]. WADA does name thymosin-beta-4 and derivatives such as TB-500 under S2.3 (growth factors), which is a different peptide [7]. WADA's S0 category covers substances with no approval from any governmental health authority, and thymosin alpha-1 is approved in some countries [7][1].

Bottom line

The dividing line is regulatory status, not a purity grade. Thymosin alpha-1 has no FDA approval, no place in the compounding categories that allow it, and no support from the advisory committee [1][2][3][4]. A research vial is not an FDA-approved drug, and nothing in the trial literature measures what is in a given vial. Zadaxin is authorized in some other countries [1].