TB-500 side effects and benefits: what the evidence shows

The short answer: nobody knows what side effects TB-500 causes in people, because it has never been studied in people. FDA's July 2026 review of TB-500 found no published report of it being given to humans and no animal toxicity studies. [1] The safety results you see quoted online, mild side effects and no serious ones, come from trials of full-length thymosin beta-4, a larger protein that FDA says is not the same substance. [2] [3]

The benefits claims follow the same pattern. Faster wound healing, heart repair, and tendon recovery are thymosin beta-4 results, mostly in rodents. For TB-500 itself, the indexed record is two metabolism studies in animals and one cell study in which it did not close scratch wounds. [8] Our evidence grade for TB-500 is animal only.

This page sets out what each claim rests on, what FDA flagged as risks, and where TB-500 stands legally. It does not cover dosing, and no human dose of TB-500 has been established.

What is TB-500, and why does that matter for side effects?

TB-500 is a synthetic copy of a 7 amino acid segment (LKKTETQ, positions 17 to 23) of thymosin beta-4, with an acetyl group added to one end. Thymosin beta-4 is a 43 amino acid protein found in nearly every cell in your body. Doping laboratories confirmed the acetylated fragment in a product sold as TB-500. [7] [9]

This matters because a fragment does not inherit its parent protein's safety record. FDA's briefing states that thymosin beta-4 and TB-500 are not the same substance, and it notes that different active ingredients have been sold under the same common name. [1] So a product labeled TB-500 may contain the fragment, the full protein, or something else, and any side effect report has to start with the question of what was actually injected. Our TB-500 vs thymosin beta-4 explainer covers the chemistry in detail.

What side effects has TB-500 caused in people?

None has been documented in a study, because no study has given TB-500 to people. FDA's literature search found no article in which TB-500 was administered to humans and no human pharmacokinetic study (how the body absorbs and clears it) by any route. [1] FDA also identified no animal toxicity studies and no carcinogenicity studies. [1]

That leaves three kinds of information, each with clear limits.

• Self-reports. People who use TB-500 describe injection site irritation, headache, and tiredness online. These reports have no control group, cannot separate the peptide from anything else the person was taking, and cannot confirm what the vial contained. A 2026 review of peptides used in sports medicine concluded that rigorous human safety data for unapproved peptides such as TB-500 are scarce and that there is potential for serious harm. [15]
• Animal and lab data. In the 2024 rat and cell study, TB-500 was broken down into shorter fragments and showed no toxicity to cultured fibroblasts. [8] A cell assay with no toxicity signal is not a safety study in a living animal, let alone a person.
• FDA's risk review. This is the most specific safety information that exists for TB-500, covered in the next section.

What risks did FDA flag?

FDA evaluated TB-500 for the July 23 to 24, 2026 meeting of its Pharmacy Compounding Advisory Committee and proposed that it not be added to the 503A bulks list. [1] The briefing named these concerns:

• Immune reactions. FDA said compounded TB-500 may pose a risk of immunogenicity, meaning the immune system reacts to the peptide, for certain routes, because of aggregation (the peptide clumping) and peptide related impurities. [1]
• No safety testing. No human exposure data, no animal toxicity studies, and no carcinogenicity studies. [1]
• Uncertain identity. Different active ingredients have been sold under the same name, so you cannot rely on the label. [1]

Products sold as research chemicals add a separate layer of risk. They are not made to pharmaceutical standards and are not verified for identity, purity, or sterility. FDA has issued warning letters stating that products labeled research use only and sold for human use are unapproved new drugs. [19] Our peptide injection safety explainer covers contamination and sterility risks that apply to any injected peptide outside a pharmacy.

Is thymosin beta-4 safe? The trials people cite for TB-500

When someone says TB-500 is "well tolerated in human trials", they are almost always citing thymosin beta-4 studies. Those trials are real, but they tested a different molecule, by different routes, for different purposes.

• Intravenous safety, 2010. A randomized, placebo-controlled phase 1 study gave synthetic thymosin beta-4 into a vein to 40 healthy adults. Adverse events were infrequent and mild to moderate, with no serious events and no dose-limiting toxicity. [2]
• Intravenous safety, 2021. A randomized, double-blind phase 1 study of recombinant thymosin beta-4 in 84 healthy volunteers in China reported mild to moderate adverse events and no serious ones. [3]
• Venous leg ulcers, 2010. A placebo-controlled phase 2 trial of a topical form in 73 patients found safety comparable to placebo. [4]
• Dry eye, 2015. A placebo-controlled phase 2 trial of eye drops in 72 adults observed no adverse events over 28 days. [5]

These results describe short exposures to the full-length protein in small groups. None tested injections under the skin or into muscle, which is how TB-500 is sold, and none followed people long enough to see rare or delayed harms. They do not count toward TB-500's evidence grade.

Does TB-500 cause cancer?

No study shows that TB-500 causes cancer. No study has looked. FDA found no carcinogenicity study of TB-500. [1]

The concern comes from the biology of thymosin beta-4. The same activities that help wounds close, cell migration and new blood vessel growth, are also used by tumors to grow and spread. In colorectal cancer research, cancer cells with more thymosin beta-4 were more invasive, and higher levels were found in tumors that had spread to distant organs. [13] In mouse and cell experiments, thymosin beta-4 was needed for a gene program linked to metastasis, and higher expression tracked with poorer survival in several human cancer datasets. [14]

These are findings about the body's own thymosin beta-4 inside tumors. They do not show that an injected fragment starts a cancer. They are the reason our TB-500 page lists active or past cancer as the situation where this theoretical concern applies most, and why the absence of long-term human data matters.

What does the research claim TB-500 does?

The benefits marketed for TB-500 are faster healing of tendons, muscles, ligaments, and skin, and sometimes heart repair. Here is where each claim comes from.

• Faster wound healing. In rats, full-length thymosin beta-4 increased skin regrowth over wounds by 42% at day 4 and up to 61% at day 7 versus saline. Not a TB-500 study. [11]
• Heart repair. In mice with a blocked heart artery, full-length thymosin beta-4 improved heart cell survival and heart function. Not a TB-500 study. [12]
• Skin ulcer healing. In two phase 2 trials of topical thymosin beta-4 in leg and pressure ulcers, patients who healed did so almost a month faster. Not a TB-500 study. [6]
• Repair from the fragment. The same 7 amino acid sequence without the acetyl group sped wound repair in aged mice. TB-500 is the acetylated form, and FDA says this result cannot be directly extrapolated to it. [10]
• Wound closure in cells. The only healing test of TB-500 itself: in cultured fibroblasts, TB-500 did not significantly close scratch wounds, while one of its breakdown products did. [8]

Only the last item tested TB-500. FDA identified no animal study testing whether TB-500 promotes wound healing. [1] Even the full-length protein has a mixed human record: the dry eye trial missed both of its primary endpoints, though some secondary measures improved. [5]

TB-500 benefits: what has been shown in humans?

Nothing, for TB-500. That is not the same as saying it does not work. It means the question has not been tested. The honest summary for each common goal:

• Tendon and ligament injuries: we found no human trial of TB-500 or thymosin beta-4 for these injuries. The tendon injury page lists the treatments that have been tested in people.
• Muscle recovery: we found no human trial of either peptide.
• Skin wounds: small phase 2 trials of topical thymosin beta-4, not TB-500. The wound healing page ranks the options by evidence.

A 2026 sports medicine review discusses the placebo effect as a driver of perceived peptide benefits and how social media amplifies it. [15] That is why self-reported improvement without a control group cannot show benefit, as our guide on how to read a peptide study explains.

Is TB-500 safer combined with BPC-157?

There is no data on any combination. TB-500 is most often sold with BPC-157, the pairing covered on our Wolverine stack page, and no trial has tested the two together or measured whether side effects add up. Our BPC-157 vs TB-500 comparison shows that neither has a controlled human trial for injury healing. The TB-500 vs GHK-Cu comparison and the thymosin alpha-1 vs TB-500 comparison set out how the evidence differs between peptides that are often grouped together.

Is TB-500 legal?

TB-500 is not an FDA approved drug for any use. In April 2026, FDA moved it from the 503A Category 2 list (compounding ingredients flagged for significant safety risks) to its list of withdrawn nominations, which places it in no 503A category. [16] It is not on the 503A bulks list, so a 503A pharmacy has no federal basis to compound it from bulk. [17]

At the July 2026 advisory committee meeting, news reports said the committee voted to recommend listing TB-500, but FDA's own briefing recommended against listing, and FDA has not posted the vote or minutes. [18] FDA has proposed no rule to add it. Our PCAC vote explainer covers what the meeting did and did not change.

Products sold as research chemicals for human use are unapproved drugs. [19] For tested athletes, WADA names thymosin beta-4 and TB-500 under section S2, prohibited at all times. [20]

The TB-500 regulatory timeline tracks each dated decision, and the TB-500 cost page explains why there is no lawful price to compare.

What would change this answer?

Our grade for TB-500 would move only when a study gives TB-500 itself to people and reports the results, ideally a randomized, placebo-controlled trial with side effects recorded and the product's identity confirmed. A human pharmacokinetic study and the animal toxicity studies FDA found missing would be the first steps. [1] More thymosin beta-4 trials would not change the TB-500 grade, because they test a different substance. We will update this page and the TB-500 entity page when that evidence appears.