What is TB-500?

What sources report

What TB-500 is

TB-500 is a synthetic heptapeptide, a chain of seven amino acids: Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH, or Ac-LKKTETQ in one-letter code.¹ The "Ac" marks an acetyl group on the first amino acid, leucine.¹ PubChem lists the free base as C38H68N10O14, molar mass 889.0 g/mol, under compound ID 62707662.⁸ Its CAS registry number is 885340-08-9.²

"TB-500" is a common name, not an official United States Adopted Name.¹ FDA treats the free base and its acetate salt as different active ingredients.¹ It notes that several suppliers use the free-base CAS number for the acetate too.¹ Products have also appeared under the TB-500 name as other salts, derivatives and even different active molecules.¹ A measured mass separates the two TB-500 forms: 889.0 g/mol for the free base, 949.1 g/mol for the acetate.⁸¹

Where it came from

Thymosin beta-4, the parent protein, was sequenced in 1981 from calf thymus and has 43 amino acids.⁹ FDA says TB-500 appears to have first been made from it in 2003.¹ A veterinary preparation followed in 2011, marketed to boost performance in horse and greyhound sports.¹ Doping-control chemists in 2012 described TB-500 as a synthetic version of an active region of thymosin beta-4.¹⁰ That same year, another lab used high-resolution mass spectrometry to confirm that a TB-500 product contained Ac-LKKTETQ.¹¹

How it is thought to work

The parent protein grips single actin molecules in a 1:1 complex and stops them from joining into filaments.¹² FDA says this actin binding appears essential to building actin filaments and maintaining the cell skeleton, which regulates cell shape and movement.¹ TB-500 contains LKKTET, the six-amino-acid stretch known as the actin-binding region of thymosin beta-4.¹ That the LKKTETQ fragment buffers actin the same way is only a hypothesis.¹ FDA says it remains to be determined whether actin binding contributes to the effects reported for this fragment.¹

TB-500 is broken down by trimming amino acids one at a time from its tail (C-terminal) end.¹ This pattern was seen in treated horses and rats and in lab tests with human serum and human liver and kidney preparations.¹ One fragment, Ac-LKKTE, may carry the activity: in a 2024 fibroblast study it alone showed significant wound-healing activity, and TB-500 itself did not.¹³

Evidence so far

Evidence for TB-500 itself comes from animals, cells and one small case series.¹⁷ In an exploratory 2026 study of 32 rats (8 per group) after Achilles tendon repair, TB-500 was associated with better tissue scores and stronger tendons at 4 weeks.¹⁴ FDA found no human data and concluded there is a lack of evidence to evaluate TB-500 for wound healing.¹ Full list: TB-500 benefits and uses. The healing research overview places it beside related compounds.

TB-500, thymosin beta-4 and thymosin alpha-1

Three similar names cover three different molecules.

FDA says TB-500 and thymosin beta-4 are not the same substance, even though several websites use the names interchangeably.¹ Small human trials have tested full-length thymosin beta-4, not TB-500: a single-dose IV study in 40 healthy volunteers, eye drops for dry eye, and a topical form for venous ulcers.¹⁷¹⁸¹⁹ FDA lists thymosin alpha-1 separately from the TB-500 fragment, with its own safety summary.³ More on that peptide: thymosin alpha-1.

TB-500 and BPC-157

BPC-157 is a 15-amino-acid fragment of a protein first isolated from human gastric juice.²⁰ TB-500 comes from a different parent protein, thymosin beta-4.¹ FDA's 2026 briefing documents found five clinical studies that used BPC-157 and none in which TB-500 was given to people.²⁰¹ FDA staff recommended against adding either to the 503A Bulks List, but the advisory committee voted to recommend both in July 2026.¹²⁰²¹ The two also come blended, as in the Wolverine stack, but one rat tendon study found the pair did no better than either peptide alone.¹⁴ More: BPC-157.

Side effects

Side effects of TB-500 in people are unknown, because FDA found no clinical studies or human exposure data by any route.¹ FDA's adverse event database held no TB-500 reports through March 26, 2025.¹ FDA warns that injected peptides like TB-500 may trigger immune reactions, from antibodies with no symptoms to life-threatening reactions.¹ Full list: TB-500 side effects.

Dosage information

No human dose of TB-500 is established, and FDA found no human studies of how the body absorbs or responds to it.¹ Published doses come from rats, horses and cell cultures, plus one uncontrolled 3-patient case series.¹⁴¹⁷ With no clinical studies or human exposure data, the safety of any of these doses in people is unknown.¹ Full table: TB-500 dosage.

Reconstitution and storage

TB-500 is supplied as a powder and dissolved before lab use.¹ Vendor data cited by FDA report that the free base dissolves in water at 50 mg per mL.¹ Vendor storage data, also cited by FDA, give the free base powder 1 year at -20°C and a solution 1 month at -20°C.¹ No USP or National Formulary standard covers TB-500.¹ Lab steps and concentration math: How to reconstitute TB-500. The 5 mg research vial and its specifications are listed under TB-500 for sale.

Regulatory status

United States

TB-500 is not FDA-approved.¹ There is no USP or National Formulary monograph for TB-500 or its acetate, and neither is part of any approved drug.¹ No outsourcing facility has reported compounding drugs containing TB-500 to FDA.¹

FDA's safety-risk page lists "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500" among substances once in category 2.³ That category covers bulk substances that may present significant safety risks, and TB-500's nominators later withdrew it.³ FDA's summary there says compounded TB-500 may carry a risk of immune reactions for certain routes.³ It adds that FDA has found no human exposure data.³ As of FDA's May 14, 2026 update, TB-500 sits in none of the three 503A nomination categories.⁵²²

On April 16, 2026, FDA announced a Pharmacy Compounding Advisory Committee meeting in the Federal Register (docket FDA-2025-N-6895).²³ The notice listed TB-500 free base and TB-500 acetate for discussion, and the committee took them up on July 23, 2026.²⁴²⁵ FDA's briefing document proposed not adding either form to the 503A Bulks List.¹ The committee voted 8 to 6, with 1 abstention, in favor of adding both.²¹ The vote is a nonbinding recommendation: FDA says it will not make a final determination until it has considered the committee's input.¹ A physician quoted by Healio said formal rulemaking runs into 2027.²¹ One dissenting member raised concern that TB-500 is being used in combination with other peptides.²¹

In a June 12, 2023 warning letter, FDA cited an online seller of injectable and nasal-spray "TB-500".²⁶ Despite "research purpose only" labeling, FDA said evidence from the seller's website showed the products were intended for human use.²⁶ It said they were not generally recognized as safe and effective, which made them new drugs.²⁶

Other countries

FDA's 2026 review found TB-500 in neither the European nor the Japanese pharmacopeia.¹ It found no approved TB-500 products in Canada, Australia, the United Kingdom, Belgium, Ireland, France, Norway, Germany, Spain or Italy.¹ It found none authorized for use in the European Union by the European Medicines Agency.¹

Sport

WADA's 2026 Prohibited List took effect on January 1, 2026.⁶ It names thymosin beta-4 and its derivatives, such as TB-500, under S2.3: growth factors and growth factor modulators.⁶ Substances in that class are banned at all times, in and out of competition, and are non-Specified.⁶ USADA says TB-500 remains prohibited under S2 on the 2027 list, which takes effect January 1, 2027.²⁷ That list also names peptides in general as prohibited under class S0.²⁷ In September 2025, USADA announced a four-year period of ineligibility for a US triathlete who possessed and used BPC-157 and TB-500.²⁸