TB-500 (thymosin beta-4): what the studies say

What it is

Thymosin beta-4 is a natural peptide of 43 amino acids found in almost every cell of the body. Its main job is to hold single units of actin, the protein that cells use to move and change shape, and it is the most abundant actin-binding peptide in mammalian tissue (Philp 2003). After an injury it is released by platelets and immune cells (Goldstein 2012). TB-500 is a product name rather than a scientific one. When doping-control laboratories analysed a product sold as TB-500, they found that its key ingredient was a 7-amino-acid piece of thymosin beta-4, the actin-binding region (amino acids 17 to 23), with an acetyl group attached at one end (Ho 2012, Esposito 2012). Some products sold under the name contain the full peptide instead. The studies below used full-length thymosin beta-4 unless stated otherwise. Neither form is an approved medicine anywhere (Tham 2026).

What the studies looked at

• Wounds in rats. Full-thickness skin wounds treated with thymosin beta-4 on the skin or by injection (Malinda 1999), surgical incisions and sponge implants to study scarring (Ehrlich 2010), and a repaired Achilles tendon in 32 rats comparing TB-500, BPC-157 and both (Biçer 2026).
• Hearts in mice and pigs. Mice whose coronary artery was tied off to mimic a heart attack (Bock-Marquette 2004, Smart 2007, Smart 2011), and pigs, 9 per group, with a coronary artery blocked for 60 minutes (Hinkel 2008).
• The actin-binding fragment. Cell migration and vessel-sprouting assays comparing the whole peptide with its 7-amino-acid segment (Philp 2003), and hair growth in rats and mice (Philp 2004).
• People. 40 healthy volunteers in 4 cohorts of 10 received intravenous thymosin beta-4 or placebo, first once and then daily for 14 days (Ruff 2010). 73 patients with venous leg ulcers were randomised to a gel at three strengths or placebo at 8 sites in Italy and Poland (Guarnera 2010). 9 patients with non-healing corneal defects received eye drops on compassionate grounds (Dunn 2010). 72 people with moderate to severe dry eye were randomised to 0.1 % eye drops or placebo for 28 days (Sosne 2015).
• Horses. A racing laboratory in Hong Kong and a doping-control laboratory in Belgium developed tests to detect the TB-500 fragment in blood and urine (Ho 2012, Esposito 2012).

What the studies say

Wounds and scars in rats. Thymosin beta-4 increased the regrowth of skin over the wound by 42 % at day 4 and by as much as 61 % at day 7, wounds contracted at least 11 % more than controls, and more collagen and new vessels were seen. In a dish, as little as 10 pg stimulated keratinocyte migration 2 to 3-fold (Malinda 1999). Incisions treated with the peptide were narrower and healed with minimal scarring, with well-organised collagen and few of the contractile cells that cause scars, and sponge implants that received 100 µg on days 2, 3 and 4 showed the same pattern at day 14 (Ehrlich 2010). In the tendon study, rats given TB-500 at 60 µg per kg per day for four weeks had a higher load to failure than controls (p < 0.05) and better tissue scores (p = 0.016) (Biçer 2026).

The fragment. The 7-amino-acid actin-binding motif showed almost the same activity as the whole peptide in endothelial migration and vessel-sprouting assays at about 50 nM, while fragments lacking the motif were inactive (Philp 2003). The full peptide also increased hair growth in rats and mice by activating stem cells in the hair follicle (Philp 2004).

Hearts. In mice, treatment after coronary ligation raised the survival signals ILK and Akt, improved early survival of heart muscle cells and improved heart function (Bock-Marquette 2004). It made the outer layer of the adult heart release progenitor cells that formed vessels (Smart 2007), and after injury some of these cells became new heart muscle cells (Smart 2011). In pigs, 15 mg infused into a heart vein reduced the infarct to 37 % of the area at risk against 54 % in controls, and the damaged wall moved again (Hinkel 2008).

People. Intravenous doses of 42 mg to 1260 mg, once and then daily for 14 days, caused infrequent, mild or moderate adverse events with no dose-limiting toxicity (Ruff 2010). In the venous ulcer trial the safety profile matched placebo at every strength, and the 0.03 % gel was associated with complete healing within 3 months in about 25 % of patients, mostly those with small or moderate wounds (Guarnera 2010). Of the 9 corneal patients, the 6 with broad defects healed, the 3 with small spotted defects did not change, and 1 showed thinning of the cornea (Dunn 2010). In the dry eye trial neither primary endpoint, discomfort or corneal staining, differed from placebo at day 29. Discomfort in the test chamber fell 27 % more with the drops on day 28 (P = 0.0244), some staining scores improved, and no adverse events occurred (Sosne 2015). A review by the peptide's developers reported that phase 2 trials in pressure ulcers, stasis ulcers and epidermolysis bullosa wounds showed faster repair (Kleinman 2016).

Horses. The Hong Kong laboratory detected the acetylated fragment at 0.02 ng per ml in plasma and 0.01 ng per ml in urine and confirmed it in horses after a single 10 mg dose, the first identification of TB-500 in post-administration samples (Ho 2012).

What the researchers flagged

• Which molecule. A 2026 orthopaedic review stressed that TB-500 is a distinct 7-amino-acid fragment that is often conflated with full-length thymosin beta-4 without evidence that the two behave the same, and that direct evidence for muscles, tendons and joints is limited to cell and rodent studies (Tham 2026).
• Human trials used the whole peptide. The published human studies used full-length thymosin beta-4 as eye drops, a gel or a hospital infusion. None of them tested the fragment sold as TB-500 (Tham 2026).
• Mixed trial results. The dry eye trial missed both primary endpoints (Sosne 2015), and the corneal series was uncontrolled compassionate use in 9 people (Dunn 2010).
• Product quality. Reviewers raised concerns about the identity, purity, concentration and sterility of peptides sold online, and noted that human safety data are scarce (Tham 2026, Mendias 2026).
• Sport. The 2026 Prohibited List bans "Thymosin-β4 and its derivatives e.g. TB-500" as growth factors, class S2, at all times (WADA 2026), and racing laboratories developed the detection methods above to control its misuse in horses (Ho 2012).