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TB-500 Side Effects: What Is Known and What Is Not

Creator: Nadia Okonjo, Health Writer
Published: 2026-08-27

Most of what circulates as a TB-500 side effect list is neither reported nor ruled out. The honest version starts with how thin the human safety data is — and with why a seller's clean record is not one.

Nadia Okonjo, Health Writer August 27, 2026 7 min read

A woman in a linen shirt and trousers fastens her cuff in a sunlit living room

The short version

There is no approved TB-500 product and therefore no regulator-compiled side-effect list. What exists is a small human safety file on the parent protein thymosin beta-4, a large animal literature, and clinical experience.
Two early-phase studies — 40 healthy volunteers at 42 to 1,260 mg intravenously (2010) and 84 healthy volunteers at 0.05 to 25 µg/kg (2021) — reported no serious adverse events and no dose-limiting toxicities, with events mild to moderate and no more frequent than placebo in the later study.
Injection-site redness, bruising and soreness are the most commonly reported effect, and a share of it is the injection technique rather than the drug.
The angiogenesis concern is mechanistic and taken seriously: overexpressing thymosin beta-4 in mouse melanoma cells raised tumour blood-vessel counts 4.4-fold and lung metastases from 10.9 to 46.7 nodules, which is why a cancer history can end the conversation at intake.
"No reported side effects" on a seller's page describes the absence of a reporting system, not the absence of risk — and some of what people experience comes from the vial's contents rather than the molecule.
TB-500 is dispensed as a compounded medication, which is different from an FDA-approved product: the formulation offered here is not FDA-approved, and a licensed provider decides whether to prescribe it at all.

TB-500 side effects cannot be listed the way a package insert lists them, because no regulator has ever compiled that list. What exists instead is a small human safety file on the parent protein, thymosin beta-4; a large animal literature; and the clinical experience of prescribers. Injection-site reactions are the most commonly reported effect, and most of the rest is mechanistic rather than observed. The line on almost every peptide seller's page — no reported side effects — describes the absence of a reporting system, not the absence of risk.

That last point is the whole article. What TB-500 has been studied for sits in TB-500 for injury recovery, and how it differs from the other repair peptide in BPC-157 vs TB-500. This page is about tolerability and risk.

Where a TB-500 side effects list actually comes from

Three places, and they are not equally strong.

Human trials of thymosin beta-4. TB-500 is a synthetic peptide based on the actin-binding region of thymosin beta-4, not the protein itself. The parent has been through early-phase human studies; the fragment has not, in any comparable way. The human numbers below therefore belong to the parent, given intravenously, and reach the fragment only by inference.

Animal and cell work, extensive and the source of most mechanistic concerns, plus clinical experience and user report — real, but uncontrolled, so nobody knows how often any of it happens.

What does not exist is an approved product — so no label with an adverse-reaction table, no Phase 3 safety database, no placebo comparison at scale.

What the human safety data reports

Two studies carry most of the weight, both in healthy volunteers.

A 2010 Phase 1 study gave four cohorts of ten healthy subjects a single intravenous dose of placebo or synthetic thymosin beta-4 at 42, 140, 420 or 1,260 mg, then the same daily dose for 14 days. Adverse events were infrequent and mild or moderate, with no dose-limiting toxicities and no serious adverse events (Ruff et al., Ann N Y Acad Sci 2010).

A first-in-human study of recombinant human thymosin beta-4 enrolled 84 healthy volunteers across single doses of 0.05 to 25 µg/kg and multiple doses of 0.5 to 5.0 µg/kg daily for ten days. Again no serious adverse events and no dose-limiting toxicities; events were mild or moderate and resolved untreated, and their incidence and severity were equal in the thymosin beta-4 and placebo groups. Injection-site bruising and phlebitis are among those recorded (Wang et al., J Cell Mol Med 2021).

Read those precisely. Roughly 120 people, the parent protein rather than the fragment, intravenous rather than subcutaneous, days to a fortnight rather than months. Reassuring as far as it goes, and it does not go far.

Injection-site reactions, and what they usually are

This is the effect people actually encounter: redness, a small raised welt, itching, bruising or soreness at the site for a day or so. It appears in the trial record and in ordinary practice.

A good share of it is the injection rather than the drug — cold solution straight from the fridge, a blunted needle, injecting too fast, or returning to the same square inch of skin. That is no reason to ignore it. A reaction that spreads, comes with fever, produces pus or worsens after 48 hours is not a routine local reaction, and is a reason to contact the prescriber rather than wait it out.

Reported versus theoretical

The two get blended on every page that sells the compound.

Effect	Where it comes from	Status
Injection-site redness, bruising, soreness	Human trials and clinical experience	Reported
Headache, transient fatigue in the first days	User report and clinical experience	Reported, never quantified
Light-headedness shortly after a dose	Anecdotal only	Reported, never quantified
Effect on tumour growth or spread	Animal and human tissue work on the parent protein	Theoretical, taken seriously
Effect on platelets or clotting	Thymosin beta-4's abundance in platelets	Theoretical
Harm in pregnancy or breastfeeding	No data of any kind	Unknown — reason enough to decline

The angiogenesis question, and why a malignancy history matters

The mechanism that makes TB-500 interesting for repair is cell migration and the growth of new blood vessels — two of the things a tumour needs.

The concern is not invented. Mouse melanoma cells engineered to overexpress thymosin beta-4 showed a mean 2.3-fold increase in cell migration and a 4.4-fold increase in blood vessels in the resulting tumours, with VEGF induction alongside; mice given those cells intravenously developed a mean of 46.7 metastatic lung nodules against 10.9 in controls (Cha et al., J Natl Cancer Inst 2003). A tissue-microarray survey later found thymosin beta-4 upregulated in osteosarcoma, colorectal carcinoma and oesophageal cancer (Jo et al., Appl Immunohistochem Mol Morphol 2011).

Now the precision peptide marketing leaves out in both directions. Those are overexpression models and expression surveys of the parent protein. No study shows that injecting the synthetic fragment causes cancer in a person, and none has looked. What the work establishes is a plausible mechanism — which is why a malignancy history is a real conversation at intake rather than a formality, and why a provider may decline on it alone.

TB-500 is also one half of the Wolverine peptide stack, paired there with BPC-157. Everything here applies to that blend too.

Why "no reported side effects" means absent data, not absent risk

For an approved medicine, the side-effect list is manufactured deliberately. Thousands of participants are followed in controlled trials, every event is recorded whether or not anyone thinks it is drug-related, rates are compared against placebo, and surveillance continues after launch. The list exists because someone was obliged to build it.

TB-500 has none of that machinery. The FDA does run a channel for compounded medicines — its Compounding Incidents Program collects adverse-event and product-quality reports and calls that reporting a critical mechanism for identifying potential quality problems. A channel only works with a named pharmacy and a named prescriber in the chain. A vial from an anonymous website has neither, so nothing that happens to the person who injects it is ever counted.

There is a second reason a seller's clean record is worth little: some of what people experience is not the molecule. Grey-market vials sold without a prescription have turned up under-filled, over-filled, degraded, non-sterile and carrying endotoxin. All of those produce something a person would reasonably call a side effect, and none of it is TB-500 — are peptides safe has that comparison in full.

What a prescriber weighs before writing it

Not a checklist you can pass. What comes up:

Cancer history — active, recent or in close family, for the angiogenesis reasons above.
Pregnancy or breastfeeding, where there is no data at all.
Bleeding disorders or anticoagulant use, on the theoretical platelet question.
Everything else you take, and whether anything overlaps.
Competitive sport — thymosin beta-4 and its derivatives, TB-500 among them, sit on the World Anti-Doping Agency's Prohibited List at all times.
Whether the request is coherent — what the problem is, and whether something better supported fits it.

On regulatory status: there is no FDA-approved TB-500 product in the United States, and none for thymosin beta-4. Through Promise it is dispensed as a compounded medication, which is different from an FDA-approved product: the formulation offered here is not FDA-approved. In July 2026 the FDA's Pharmacy Compounding Advisory Committee voted 8–6–1 to recommend TB-500 for the 503A bulk drug substances list; that vote is advisory and rulemaking has not concluded. A licensed provider may still prescribe a compounded formulation — that decision is between you and your doctor.

A licensed provider in Promise's prescriber network reviews every request and writes a prescription only where it is appropriate for the person asking. Not everyone qualifies.

How to read a thin safety file

A short list of known side effects beside a long list of unknowns is not a safe drug, and it is not a dangerous one either. It means the question has not been asked properly yet — so the sensible response is to bring your own history to someone qualified to weigh it, rather than trust a page that had no way of knowing. What you are hoping to address, everything else you take, any cancer or bleeding history, whether you compete under an anti-doping code. If TB-500 does not fit, that is worth knowing too.

Common questions

What are the side effects of TB-500?

Injection-site reactions are the most commonly reported: redness, a small welt, itching, bruising or soreness for a day or so. Headache and transient fatigue in the first few days are described by users but have never been quantified against placebo. Beyond that the profile is characterised from pharmacology rather than from trials, because no large safety study of the fragment exists.

Is TB-500 safe?

Nobody can answer that with the confidence the question implies. Early-phase human studies of the parent protein, thymosin beta-4, reported no serious adverse events and no dose-limiting toxicities across roughly 120 healthy volunteers given it intravenously. That is genuinely reassuring and genuinely small — different molecule form, different route, days to weeks rather than months.

Is TB-500 FDA-approved?

No. There is no FDA-approved TB-500 product in the United States and none for thymosin beta-4 either, and the compounded preparation dispensed through Promise is not FDA-approved. Compounded medications are not reviewed by the FDA for safety, effectiveness or quality. A licensed provider may still prescribe one where they judge it appropriate — that decision is between you and your doctor.

Can TB-500 cause cancer?

No study has shown that, and no study has properly asked. The concern is mechanistic: thymosin beta-4 promotes cell migration and new blood-vessel growth, and overexpressing it in mouse melanoma cells raised tumour vessel counts 4.4-fold and lung metastases from 10.9 to 46.7 nodules. That is an overexpression model, not a person injecting the fragment — but it is why a cancer history is a serious question at intake.

Why do peptide sellers say TB-500 has no reported side effects?

Because nobody is collecting reports. An approved drug has trial safety databases and post-marketing surveillance behind its label. An anonymous seller has no obligation and no mechanism to record what happens after the vial ships. The phrase describes the absence of a reporting system rather than a clean safety record.

Who should avoid TB-500?

That is a prescriber's call, not a rule, but the histories that most often lead to a decline are an active or recent malignancy, pregnancy or breastfeeding, and bleeding disorders or anticoagulant use. Competitive athletes should raise it first: thymosin beta-4 and its derivatives sit on the World Anti-Doping Agency's Prohibited List at all times.