What is TB-500? Thymosin beta-4, the research and FDA status

Quick answer TB-500 is a lab-made piece of a natural protein called thymosin beta-4, and FDA's reviewers say the two aren't the same substance. In July 2026 an FDA advisory committee looked at it, and news reports say the committee recommended adding it to the list of ingredients pharmacies may compound, against the advice of FDA's own staff. Today nothing has changed: TB-500 is not FDA-approved and not on that list, so licensed pharmacies can't compound it. The thing to watch is a proposed rule from FDA, which hadn't appeared as of October 2026.

TB-500 in plain terms

TB-500 has become popular in gym and sports circles, usually mentioned alongside BPC-157 and with more confidence than the evidence allows.

FDA's reviewers describe it as a synthetic piece of thymosin beta-4: amino acids 17 to 23 of the protein, with an acetyl group added to one end. Analytical chemists in a doping-control laboratory reported that structure in a TB-500 product they examined (Esposito and colleagues, 2012). FDA's safety page lists it as "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500."

The name is loose. FDA's briefing says TB-500 is a common name, not an official one, and that different salts and derivatives, including different active forms, are sold under it. FDA treats that as a patient-safety problem, because a person could receive a different substance than the one a prescriber meant.

Thymosin beta-4 itself is a natural protein of 43 amino acids, made in many cell types; TB-500 is not made by the body. Research on the full protein is not research on TB-500, and results for one can't be assumed for the other.

What the research measured

There is very little research on TB-500 itself, and none in people.

• FDA's reviewers found no clinical study and no case report of TB-500 in people.
• Most peer-reviewed work on the fragment is chemistry: identifying it and detecting it for anti-doping tests.
• The human trials people cite tested full-length thymosin beta-4, which FDA says is not the same substance.

Detection. The doping-control paper identified the fragment in a product suspected of use in sport and proposed a way to detect it in blood plasma and urine (Esposito and colleagues, 2012).

Animal studies. FDA's reviewers found studies in horses and rats of how TB-500 is absorbed and broken down, but no toxicity studies of any kind. A related 2003 mouse study applied full-length thymosin beta-4 to skin wounds in diabetic and aged mice, and a seven-amino-acid peptide with the TB-500 sequence but no acetyl group to wounds in aged mice (Philp and colleagues, 2003). FDA's reviewers say a result for the version without the acetyl group can't be applied directly to TB-500.

The full protein. One published human trial tested thymosin beta-4 eye drops in nine people with severe dry eye, in a small randomized, placebo-controlled phase 2 study (Sosne and colleagues, 2015). That is the whole protein, in a different form, for a different condition, in a very small group.

Full citations are in the Sources list below.

Where TB-500 stands with FDA

TB-500 is not an FDA-approved drug and is not on FDA's 503A bulks list, so licensed pharmacies can't compound it; FDA removed it from its interim Category 2 on April 22, 2026 after the nominators withdrew their nominations, which FDA says is not a determination about the substance (how the categories work). In July 2026 FDA's reviewers proposed against adding it to the list, news reports (CNN and AJMC, July 31, 2026) say the advisory committee voted to recommend adding it anyway, and only a final rule after notice-and-comment rulemaking would change its status, with no proposed rule published as of October 6, 2026 (the vote and the rulemaking path).

Questions to ask anyone offering it

With no lawful pharmacy route today, anyone offering TB-500 should be able to answer these plainly.

• On what legal basis is it supplied, given that it is not on the 503A bulks list?
• Which exact substance is it: TB-500 free base, an acetate salt, or full-length thymosin beta-4?
• Who made the ingredient, and does its certificate of analysis test for impurities and bacterial endotoxins?
• Which state-licensed pharmacy prepares it, and can you look up that license yourself?
• Has a licensed prescriber reviewed your health history, and what do they say about the lack of human data?

"Research use only" labels don't answer any of these. FDA has said in writing that a "research use only" label doesn't change its view when a seller's website shows the product is meant for people. What the label means is explained here.

Is this right for me?

TB-500 can't be prescribed through a licensed pharmacy today, so this isn't a decision a provider can make with you yet. Here is what you can do instead:

• If you want to hear whether that changes, use the sign-up at the end of this page. It is for availability news only, and joining reserves nothing.
• Take the reason you're asking to your own doctor, who can talk through approved treatments.
• Pass on "research use only" products; Safety information below explains why.

If FDA's rules change, a licensed provider in your state would still review your health history, medications and goals, and decide. The answer may be no.

How it works

1. Enter your email below. The form at the end of this page asks for your email address and state, and nothing about your health.
2. Hear from us only if it becomes available through a licensed provider. We write about availability and nothing else, and you can unsubscribe anytime.
3. A licensed provider still decides. Being on the list is not a reservation or a prescription. If FDA's rules change, a provider licensed in your state reviews your health history first and can decline.

FAQ

Is TB-500 just thymosin beta-4? No. TB-500 is a synthetic, acetylated seven-amino-acid fragment of it, and FDA's reviewers say the two are not the same substance.

Can a doctor prescribe TB-500? A prescription doesn't create a lawful source. Licensed pharmacies can't compound it, and that would take a final rule adding it to the bulks list. It is not in FDA's interim Category 1 either.

Is TB-500 banned in sport? FDA's briefing notes that it is on the World Anti-Doping Agency's prohibited list. That is a sports rule, separate from FDA's rules.

How is TB-500 different from BPC-157? They are unrelated peptides with different research histories, in the same regulatory position. The comparison is here.

What this means for you

As of October 2026, licensed pharmacies can't legally compound TB-500 for patients: it isn't on FDA's 503A bulks list, and only FDA rulemaking changes that. If FDA lists it one day, the legal route would be a prescription from a licensed provider in your state, filled by a licensed pharmacy; compounded medications are not FDA-approved. Until then, products sold online as "research use only" aren't made for people.

The July 2026 FDA peptide vote, explained covers all seven peptides the committee reviewed and what has to happen next.

Safety information

There is no FDA-approved, labeled TB-500 product to describe. FDA's safety-risks page says, word for word: "Compounded drugs containing thymosin Beta-4, Fragment (LKKTETQ) may pose risk for immunogenicity for certain routes of administration due to the potential for aggregation as well as peptide-related impurities. FDA has not identified any human exposure data [sic] drug products containing Thymosin Beta-4, Fragment. FDA lacks important information regarding any safety issues raised by this drug, including whether it would cause harm if administered to humans."

Immunogenicity is the risk of an immune reaction; aggregation means the peptide clumping together. FDA's briefing adds three specific gaps:

• No human data. No clinical study or case report, so safety in people is unknown. FDA found no adverse-event reports in its main database either, and cautions that this settles nothing, because reporting is voluntary.
• Unclear form. The nomination named the free base but included a certificate of analysis for the acetate.
• Thin quality testing. A representative public certificate of analysis reported appearance, identity and purity, but not impurities, bacterial endotoxins or clumping. FDA says those tests matter for an injected product.

Products sold as "research use only" add their own risks: no licensed pharmacy prepares them for a named patient, no prescriber reviews your history, and nothing assures you of what is in them. Everlon never sells "research use only" products. Compounded medications are not FDA-approved, which would stay true even if a pharmacy could one day prepare TB-500.

If you have used a product sold under this name and something feels wrong, stop, contact a doctor, and consider reporting it to FDA's MedWatch program. If this is a medical emergency, call 911.

Sources

• Esposito S, Deventer K, Goeman J, Van der Eycken J, Van Eenoo P. Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500, a product suspected to possess doping potential. Drug Testing and Analysis. 2012;4(9):733-8. PMID: 22962027. PubMed, accessed 2026-10-06
• Sosne G, Dunn SP, Kim C. Thymosin β4 significantly improves signs and symptoms of severe dry eye in a phase 2 randomized trial. Cornea. 2015;34(5):491-6. PMID: 25826322. PubMed, accessed 2026-10-06
• Philp D, Badamchian M, Scheremeta B, Nguyen M, Goldstein AL, Kleinman HK. Thymosin beta 4 and a synthetic peptide containing its actin-binding domain promote dermal wound repair in db/db diabetic mice and in aged mice. Wound Repair and Regeneration. 2003;11(1):19-24. PMID: 12581423. PubMed, accessed 2026-10-06 (abstract only)
• FDA. FDA Briefing Document: TB-500-related bulk drug substances (TB-500 free base and TB-500 acetate). Pharmacy Compounding Advisory Committee, July 23-24, 2026. fda.gov, accessed 2026-10-06
• FDA. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks (content current as of 04/22/2026). fda.gov, accessed 2026-10-06
• FDA. Bulk Drug Substances Nominated for Use in Compounding Under Section 503A of the FD&C Act (interim categories list). Updated May 14, 2026. fda.gov, accessed 2026-10-06
• FDA. Bulk Drug Substances Used in Compounding Under Section 503A of the FD&C Act (content current as of 05/14/2026). fda.gov, accessed 2026-10-06
• FDA. Guidance for industry: Interim Policy on Compounding Using Bulk Drug Substances Under Section 503A of the FD&C Act (January 2025). fda.gov, accessed 2026-10-06
• FDA. July 23-24, 2026: Meeting of the Pharmacy Compounding Advisory Committee. fda.gov, accessed 2026-10-06
• FDA. Pharmacy Compounding Advisory Committee; Notice of Meeting; Establishment of a Public Docket; Request for Comments. Federal Register. 2026;91:20465. FR Doc. 2026-07361, Docket FDA-2025-N-6895. April 16, 2026. federalregister.gov, accessed 2026-10-06
• FDA. Warning Letters (index; the five CDER letters to online peptide sellers dated August 24, 2026, posted September 1, 2026). fda.gov, accessed 2026-10-06
• CNN. FDA advisers support easing restrictions on six peptides. Here's what those drugs are and what happens next. July 31, 2026 (news report). cnn.com, accessed 2026-10-06
• AJMC. FDA Panel Backs 6 Peptides for Compounding. July 31, 2026 (news report). ajmc.com, accessed 2026-10-06
• 21 CFR 216.23, Bulk drug substances that can be used to compound drug products in accordance with section 503A. ecfr.gov, accessed 2026-10-06