TB-500 Side Effects and Dosing: Whose Safety Data Are They?

Nobody knows the side effects of TB-500 in people, and no dose of it has been tested in people or approved. The side-effect lists and dose ranges quoted for TB-500 come from trials of thymosin beta-4, the 43-amino-acid protein it was cut from, given by IV drip, as a skin gel or as eye drops. TB-500 is a seven-amino-acid fragment of that protein, and FDA found no study in which it was given to a human.^[2] Under a federal policy dated October 28, 2026, a 503A pharmacy can now make TB-500 acetate for injection under the skin or into muscle, on a prescription, without facing HHS enforcement. That is a decision not to enforce, not an approval, and HHS’s own reasoning calls the safety concerns about TB-500 “inferential rather than observed.”^[1]

What TB-500 is and what it does

HHS identifies TB-500 acetate by its sequence, Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln, usually written Ac-LKKTETQ.^[1] Those seven amino acids are the stretch of thymosin beta-4 that grips actin, the scaffolding protein cells use to move, which is why the fragment is sold for tendon, muscle and wound repair. The case for it rests on cell and animal work. A July 2026 rat study found that repaired Achilles tendons were stronger after four weeks of daily TB-500 than without it, and that adding BPC-157 gave no extra benefit.^[10] A 2024 cell study found that TB-500 itself did not speed wound closure, though one of its breakdown products did.^[9]

A review in the Journal of the American Academy of Orthopaedic Surgeons, published online on October 27, 2026, sums up the confusion. It calls TB-500 a distinct fragment that is “frequently conflated with full-length TB-4 despite the absence of validated biologic or clinical equivalence,” and concludes that routine clinical use cannot be recommended.^[11] That conflation matters most when you look for side effects.

Where the side-effect data come from

The human safety numbers attached to TB-500 online come from trials of the whole thymosin beta-4 protein. Those trials are real, their results are public, and each one recorded side effects at a specific dose by a specific route.

Read together, the trials show a protein that caused few problems over two weeks to three months, at rates close to placebo. The Chinese phase 1 trial also checked for antibodies against the drug: one of 108 samples from the single-dose volunteers tested positive, and it turned negative on follow-up.^[4] In the 10-day arm, one volunteer on the 5 µg/kg dose had a brief episode of vision loss, logged among the mild-to-moderate events. A tumor marker, SCC, rose slightly and temporarily in both the drug and placebo groups. The authors noted that lab research has linked thymosin beta-4 both to tumor spread and to anti-tumor effects, and that no tumor-related events occurred in their trial.^[4]

What the table cannot tell you is how TB-500 behaves under the skin or in a muscle. On ClinicalTrials.gov, every registered trial of the full protein gives it by vein, on a wound or in the eye. None uses the two routes HHS now allows for TB-500. The newest, a Chinese heart-attack trial posted in May 2026 and not yet recruiting, still gives the protein by IV at 10 or 20 µg/kg.^[7] The longest treatment in the trials above ran 84 days, and all of them used pharmaceutical-grade material made for a trial.

What is known about TB-500’s own safety

FDA’s 2026 review for its compounding advisory committee found no clinical studies and no human exposure data for TB-500 by any route, and no acute or repeat-dose toxicity studies in animals. Its conclusion: “potential safety risks associated with the use of TB-500 in humans are unknown.”^[2] The risks FDA named were about the product as much as the peptide.

• Immune reactions. FDA said it is “concerned about the potential for immunogenicity” of TB-500 injected under the skin or into muscle, because peptides can clump into aggregates and carry peptide-related impurities, and no data on either were available.^[2]
• Wrong substance. TB-500 is a common name, and FDA found several salts and derivatives sold under it. Inconsistent naming, FDA wrote, means patients “may be dosed with a different BDS [bulk drug substance] than the physician ordered.”^[2] Even the July 2026 rat study describes its TB-500 as “synthetic thymosin beta-4.”^[10]
• Missing tests. The certificates FDA reviewed lacked results for impurities, aggregates, microbial quality and bacterial endotoxin, the checks that show whether an injectable is free of microbes and of the bacterial toxins that cause fever.^[2] Our guide to reading a peptide certificate of analysis shows what a complete one includes.

Two lab studies have watched TB-500 break down in human serum in a test tube. Enzymes trim it from the far end, leaving shorter acetylated pieces.^[2][8][9] In the 2024 study, neither TB-500 nor its fragments harmed cultured fibroblasts, the cells that build connective tissue, and the only wound-healing activity came from a fragment, Ac-LKKTE, rather than TB-500 itself.^[9] In horses given 10 mg under the skin, blood levels peaked within two hours and could no longer be measured after six to ten.^[2] Nobody has measured any of this in a person.

Why HHS allowed it anyway

FDA’s drug center recommended against adding TB-500 to the list of substances pharmacies may compound, and the advisory committee voted in favor.^[1] HHS sided with the committee on an interim basis. Its written justification for TB-500 has three parts: certificates from FDA-registered manufacturers now supply the test results FDA found missing, pharmacy use is now documented, and on safety,

“the safety concerns CDER identified are inferential rather than observed, the parent protein has a controlled human safety record across several routes, and the fragment is rapidly truncated in human serum.”^[1]

— HHS, Enforcement Discretion Policy Regarding 503A Compounding Using Certain Peptide Bulk Drug Substances, Appendix B

Each clause holds up only so far. “Inferential rather than observed” is accurate as far as it goes. A harm can only be observed in a study, and FDA found no study of TB-500 in people. The parent protein’s record is real, yet its several routes are IV, gel and eye drops, while the policy covers TB-500 only as an injection under the skin or into muscle.^[1][3][5][6] And if the fragment is cut up quickly in blood, much of what the body meets may be its breakdown products, which have been tested only in cell dishes and rats.^[9]

The policy does set conditions, and several bear on side effects. TB-500 must be the acetate form, compounded as the only active ingredient, so premixed TB-500 and BPC-157 vials fall outside it (see peptide blends and stacks). The raw material needs a full certificate of analysis from an FDA-registered maker, the only device allowed is an FDA-cleared syringe, and each vial needs a patient-specific prescription. The pharmacy must report serious adverse events to FDA’s MedWatch program and to its state, and HHS reserves the right to pull back if a safety problem or a quality problem shows up.^[1] The policy does not cover research-use-only sellers. Our full read of it is in the HHS peptide compounding policy.

TB-500 dosage: what the studies used

No TB-500 dose has been tested in people, approved by FDA or set by the HHS policy. The policy says nothing about how much or how often; that decision rests with the prescriber who writes the patient-specific prescription.^[1] These are the only doses on record, each tied to its study.

These figures do not convert into a TB-500 dose. Even the two IV studies of the same protein disagree by a wide margin. The highest Chinese dose, 25 µg/kg, comes to a few milligrams for an adult, less than a tenth of the 42 mg at the bottom of the American range.^[3][4] A milligram of a seven-amino-acid fragment and a milligram of a 43-amino-acid protein are different amounts of different molecules. Rat doses given into the abdomen do not translate to a person injecting under the skin.

What a TB-500 dosage calculator does

Online TB-500 calculators do reconstitution arithmetic: enter the milligrams in a vial and the milliliters of water added, and they return the syringe units for a chosen dose. The math is fine. The chosen dose is the problem, because no study supports any dose of TB-500 in people, and a calculator only scales whatever number it is given. It also assumes the vial holds what its label says. Research-use vials are sold in 2, 3, 5, 10 and 15 mg sizes, under a name that FDA found covers more than one substance.^[2] Under the HHS policy, a compounded vial is made for one patient on a prescription, from tested raw material, and carries a pharmacy label.^[1]

Reporting a reaction, and TB-500 in sport

If you have a reaction to a compounded TB-500 product, tell the prescriber and the dispensing pharmacy. The policy requires pharmacies to send serious adverse events to MedWatch and to put the reporting procedure on the patient labeling.^[1] Those reports are the only route by which TB-500’s side effects in people will ever be counted, and a pattern in them is the kind of evidence that could make HHS reconsider. What lab tests can and cannot show for people taking peptides is covered in peptide blood work and monitoring.

For athletes, the HHS policy changes nothing. The World Anti-Doping Agency’s list for 2027, which takes effect January 1, names TB-500 as a derivative of thymosin beta-4 in section S2.3, growth factors, a class prohibited in and out of competition.^[12] A prescription does not make it permitted in sport. BPC-157, its usual partner, has its own safety record, covered in BPC-157 side effects. Our TB-500 provider page tracks which clinics, if any, we can confirm sell it lawfully.