TB-500 Side Effects and Safety: What's Known, What Isn't, and the Thymosin Beta-4 Mix-Up

TB-500 side effects are mostly an unknown, because no published study has tested TB-500 itself in people. TB-500 is a seven-amino-acid fragment, Ac-LKKTETQ, cut from the 43-amino-acid protein thymosin beta-4^[2][1]. The reassuring safety data often quoted for TB-500 come from trials of full-length thymosin beta-4 given intravenously, as eye drops or on the skin^[5][6][7]. A 2026 orthopedic review put it plainly: TB-500 is “frequently conflated with full-length TB-4 despite the absence of validated biologic or clinical equivalence”^[4]. Under a draft HHS enforcement policy announced October 28, 2026, HHS says it does not intend to take action against 503A pharmacies that compound TB-500 as subcutaneous or intramuscular injections under ten conditions^[1]. The policy is not final and is not binding on HHS or the public, and comments are open until November 13, 2026^[14]. It is enforcement discretion, not a finding that TB-500 is safe, and TB-500 is not FDA-approved.

The short answer

• There is no human safety data for TB-500. We found no published clinical study of Ac-LKKTETQ in people. A 2026 sports-medicine review describes rigorous human safety data for unapproved peptides, TB-500 among them, as scarce^[11].
• The human data people cite are for thymosin beta-4. A placebo-controlled phase 1 study gave intravenous thymosin beta-4 to healthy volunteers for 14 days with infrequent, mild or moderate adverse events and no serious ones^[5]. That result belongs to a different, much larger molecule.
• The theoretical concern is cell migration. Thymosin beta-4 is associated with the metastatic potential of tumor cells, and in a cell study, thymosin beta-4 fragments increased ovarian cancer cell migration and invasion^[9]. No human data link TB-500 to cancer; no human data rule it out either.
• Product quality is the most documented risk. French anti-doping chemists found that the contents of products sold as TB500 and TB1000 were not consistently what they claimed to be^[10].
• The draft HHS policy adds safeguards, not proof. Compounded TB-500 must be a single ingredient from a tested source, given by subcutaneous or intramuscular injection, with serious adverse events reported to FDA MedWatch^[1].

TB-500 is not thymosin beta-4

Thymosin beta-4 is a naturally occurring 43-amino-acid protein that binds actin, a building block of the cell skeleton^[9]. TB-500 first appeared as a veterinary preparation. Its key ingredient is the peptide LKKTETQ, a short stretch of thymosin beta-4 associated with actin binding, cell migration and wound healing, with an acetyl group added to the start^[3]. Belgian anti-doping chemists identified it as the N-terminal acetylated 17-23 fragment of thymosin beta-4, Ac-LKKTETQ^[2]. HHS Appendix B gives TB-500 acetate the same sequence and a molecular weight of 949.1 g/mol^[1].

What the thymosin beta-4 trials show, and why they do not settle TB-500

The strongest human safety study is for thymosin beta-4. In a randomized, placebo-controlled trial, four cohorts of 10 healthy volunteers received single intravenous doses of 42, 140, 420 or 1,260 mg, then the same dose daily for 14 days. Adverse events were infrequent and mild or moderate, with no dose-limiting toxicity and no serious adverse events^[5]. In a phase 2 trial of thymosin beta-4 eye drops in 9 patients with severe dry eye, the drops were reported safe and well tolerated^[6]. A review of two phase 2 trials in stasis and pressure ulcers reported faster healing in the patients who did heal^[7].

Those results are reassuring for thymosin beta-4 in those settings. They do not transfer to TB-500 for three reasons. TB-500 is a different molecule, and HHS itself notes “the fragment is rapidly truncated in human serum”^[1], so its breakdown products are not the same as the full protein's. The trials were short: 14 days intravenously, 28 days of eye drops^[5][6]. And none of them used subcutaneous or intramuscular TB-500, the routes the HHS policy covers and the way it is sold.

What happens to TB-500 in the body

A 2024 Korean doping-control study is the closest look at TB-500's fate. In human serum, enzyme systems and rats given TB-500, the main metabolite was Ac-LK, the highest-concentration product in rats over the first 6 hours, and Ac-LKK was a long-lived metabolite detectable up to 72 hours. Neither TB-500 nor its metabolites were toxic to fibroblasts in the lab, and only one metabolite, Ac-LKKTE, showed significant wound-healing activity; the authors suggested earlier wound-healing reports for TB-500 may reflect that metabolite rather than the parent peptide^[8]. Horse studies confirmed TB-500 and its metabolites can be detected in plasma and urine after a single 10 mg dose^[3].

For a reader, this has two implications. Short-term cell toxicity has not shown up in lab tests. And because the body breaks TB-500 into several fragments, the question of what it does in a person is really a question about those fragments, none of which has been studied in humans.

Possible side effects and risks

Reported side effects

There is no clinical study listing TB-500 side effects, so any list of “common side effects” you see online comes from user reports, not trials. The thymosin beta-4 phase 1 trial is the nearest controlled record, and it found infrequent mild or moderate events^[5]. Injection-site reactions, infection from non-sterile product and allergic reactions are general risks of any injected compound, more so with unregulated product.

Cancer and cell growth

This is the most discussed theoretical concern. Thymosin beta-4 is an actin-binding protein associated with the metastatic potential of tumor cells through stimulating cell migration. In a 2021 cell study, full-length thymosin beta-4 and three fragments, including one spanning residues 12 to 26 that contains the TB-500 sequence and others lacking the actin-binding site, all increased migration, invasion and proliferation of SKOV3 ovarian cancer cells^[9]. A 2026 review of unregulated injectables lists pathological angiogenesis and carcinogenesis among the potential risks of unapproved peptides including TB-500^[12]. These are cell and mechanism findings. They do not show that TB-500 causes cancer in people, but they are a reason for caution in anyone with a current or past cancer. Our review of whether peptides cause cancer covers this in more depth.

Product quality and contamination

This risk is documented, not theoretical. When French anti-doping chemists analyzed products sold online as TB500 and TB1000, they found the contents were not consistently what earlier descriptions said^[10]. The 2026 orthopedic review flagged concerns about identity, purity, concentration, sterility and manufacturing consistency of commercial TB-500^[4]. The HHS conditions target exactly these problems: a bulk source from an FDA-registered manufacturer, a certificate of analysis covering identity, purity, potency, endotoxin, heavy metals, microbial contamination and impurities, and a single active ingredient^[1].

Blends and stacks

TB-500 is often sold premixed with BPC-157 (the “Wolverine” stack) or in four-peptide blends such as KLOW. Condition 7 of the HHS policy allows only one active ingredient per compounded product, so premixed blends are outside the policy^[1]. In the one rat study that compared the two peptides and their combination on Achilles tendon repair, the combination added no benefit over either alone^[13]. Combining two compounds with no human safety data does not make either safer. See the BPC-157 and TB-500 stack and peptide stacking safety.

Athletes and drug testing

TB-500 has been treated as a suspected doping agent for over a decade: anti-doping chemists developed methods to detect it in plasma and urine, including in racehorses^[2][3]. The 2026 orthopedic review advises clinicians to counsel athletes about anti-doping restrictions^[4]. Competitive athletes should check their sport's current rules before using it.

What the draft HHS policy means for TB-500 safety

• Route: TB-500 acetate by subcutaneous or intramuscular injection only^[1].
• Source: bulk peptide from an FDA-registered manufacturer with a lot-level certificate of analysis^[1].
• Prescription: patient-specific, filled by a State-licensed pharmacy, a Federal facility or a licensed physician^[1].
• Reporting: compounders must report serious adverse events, whether or not considered drug-related, and product quality problems to FDA MedWatch and the states^[1].
• Revisiting: HHS may revisit the policy if a safety signal or product quality issue arises^[1].

The reporting condition is the closest thing TB-500 has had to a safety surveillance system. It does not make TB-500 safe; it means harms in the compounded supply should now be counted. The full policy is in the HHS 503A peptide policy explained, and the announcement is covered in our news report.

Bottom line

TB-500's safety profile in people is unknown. The human trials often quoted are for full-length thymosin beta-4, given by other routes for days to weeks^[5][6]. The fragment is broken down quickly into several smaller pieces whose effects in people are unstudied^[8]. The theoretical cancer concern rests on cell studies of thymosin beta-4 and its fragments^[9], and the documented risk is product quality^[10]. The HHS policy narrows that last risk for compounded product and adds adverse-event reporting, but it is not approval. Our TB-500 provider page lists every seller we confirmed sells TB-500 as a single-ingredient product, shows the form each one sells, and labels any form the policy does not cover; blends are left off. A listing does not confirm that the pharmacy meets the policy’s other conditions, so check them yourself: a certificate of analysis, a single active ingredient, the “compounded drug product, not for resale” label line, subcutaneous or intramuscular use, a 510(k)-cleared syringe as the only device, and a State-licensed 503A pharmacy. For the other peptides on the HHS list, see BPC-157 and its side-effects review, semax nasal spray and KPV by route. If you were hoping TB-500 would help with weight, see TB-500 and weight loss.