TB-500 Regret, Stopping, and Restarting: What the Evidence Actually Says

At a glance

• What it is / A synthetic 7-amino-acid fragment of thymosin beta-4, not the full protein [1]
• Human dosing studies / None found [1]
• Approved dose / None [1]
• Human safety data / Unknown; no toxicity or immunogenicity studies found [1]
• Regulatory status / Not FDA-approved; not in any category on FDA's interim-policy lists of bulk substances for compounding [1][2][3]
• Sport / Prohibited at all times by WADA under S2.3 [4]

Is there any evidence on stopping TB-500?

No washout period, taper, rebound flare, or lasting benefit after stopping TB-500 has been measured in people [1]. FDA found no human pharmacokinetic data by any route. Animal studies indicate that TB-500 given subcutaneously or intraperitoneally reaches the bloodstream and is broken down into smaller peptides; no animal studies by intramuscular injection were found [1].

What is known about regret

There is no research measuring how often people regret using TB-500. FDA found no published case reports and no TB-500 reports in its adverse-event database through March 26, 2025 [1]. Its food and supplement complaint system retrieved two cases about a blended TB-500 and BPC-157 product, with no safety assessment [1]. Few or no reports is not evidence that the product is safe.

One case in FDA's BPC-157 review describes a 40-year-old woman who used a research-only BPC-157 plus TB-500 product, 5 mg/5 mg, twice daily under the skin and developed widespread skin hyperpigmentation and gum darkening within a week; it recurred when she tried the product again two weeks later [5]. Because the product held two peptides, FDA said it could not tell whether either caused it [5].

Can you restart TB-500?

There is no approved dose and no human dosing study of TB-500 [1]. The nomination FDA reviewed proposed a 3 mg/mL lyophilized powder for injection, but that is a proposed product, not a recommended dose [1]. No human study establishes a restart schedule, a loading phase or a maintenance dose.

Regulatory status does not fill the gap. TB-500 is not FDA-approved, and it appears in none of the three categories on FDA's interim-policy lists for compounding [1][2][3]. FDA staff recommended against adding it to the pharmacy bulks list, citing no human data, unclear historical use, immunogenicity risk and the existence of approved wound treatments [1]. The Pharmacy Compounding Advisory Committee voted in favor of adding TB-500 on July 23, 2026, but its vote is advice only and not binding [6][7]. HHS's October 2026 enforcement-discretion policy is a draft that names TB-500 acetate for subcutaneous or intramuscular injection and says HHS does not intend to take enforcement action if its conditions are met [7][8]. It is not an approval, does not put TB-500 on a list, and HHS may revisit it [7].

Safety and sport restrictions

FDA's stated concern for injectable TB-500 is immunogenicity, an immune reaction, potentially amplified by peptide aggregation and impurities [1]. How often such reactions occur in people is unknown, because FDA found no study of TB-500 immunogenicity or aggregation [1].

WADA names thymosin-beta4 and its derivatives such as TB-500 in S2.3 of the Prohibited List. It is prohibited at all times, in and out of competition, on the 2026 list and the 2027 list [4][9].

Frequently asked questions

›What happens when you stop TB-500?

Not measured. TB-500 has no approved dose and no human study, so nothing describes what happens after stopping [1].

›Can stopping TB-500 cause a flare?

No published report describes one, which is not proof that stopping is harmless [1].

›How long should you wait before restarting?

No washout period has been measured [1]. A number from a seller protocol is not evidence.

›Is TB-500 legal to compound?

It appears in none of the three categories on FDA's interim-policy lists for compounding, and FDA staff recommended against adding it to the bulks list [1][2][3]. The July 2026 advisory committee vote was non-binding, and HHS's October 2026 enforcement-discretion policy is a draft that does not approve TB-500 [6][7].

›Is TB-500 banned in sport?

Yes. WADA's Prohibited List names thymosin-beta4 and its derivatives such as TB-500 in S2.3, prohibited at all times [4].

References

1. U.S. Food and Drug Administration. U.S. Food and Drug Administration
2. U.S. Food and Drug Administration. U.S. Food and Drug Administration
3. U.S. Food and Drug Administration. U.S. Food and Drug Administration
4. World Anti-Doping Agency. World Anti-Doping Agency
5. U.S. Food and Drug Administration. U.S. Food and Drug Administration
6. U.S. Food and Drug Administration. U.S. Food and Drug Administration
7. Official web page. Official web page
8. Official web page. Official web page
9. World Anti-Doping Agency. World Anti-Doping Agency