Racehorses and peptides: HISA rules, testing and injuries

What do HISA rules say about peptides?

Peptides sit in two Banned categories, and both apply at all times. HISA's anti-doping and medication control rules were approved by the FTC on 27 March 2023 and took effect on 22 May 2023. HISA's regulations page lists these as the rules in force, and HIWU enforces them.

Rule 4113 (S2) bans peptide hormones and growth factors, including growth factors that affect tendon or ligament repair and "regenerative capacity", plus other substances with a similar chemical structure or similar biological effect. HISA's Banned list names thymosin as an S2 peptide hormone, with the note "Lacks FDA approval". Rule 4111 (S0) bans any substance with no current approval by any government health authority for veterinary or human use.

Have the rules changed since 2023?

Changes are proposed, and the direction is stricter. HISA's site lists proposed changes to Rule Series 1000, 3000 and 4000 dated 18 November 2025. Rule updates need FTC approval, so check which version is in force when you read this.

The proposed S0 text ties approval to FDA's Green Book of animal drugs and its Orange Book of drugs for people. It also covers foreign products not approved by FDA for animals. It says compounding a substance that meets the Banned criteria is prohibited. Thymosin still appears as Banned in the proposed list. HIWU applies S0 in practice: in June 2025 it warned that adenosine and Hemo 15 are S0 Banned Substances with "no valid veterinary use in the racehorse".

Can a racehorse be tested away from the races?

Yes. A Thoroughbred becomes a Covered Horse at its first timed and reported workout and stays covered until it is retired. To make out-of-competition testing possible, trainers must file whereabouts if a horse moves to a private facility. They must also declare any Banned Substance given before the horse became covered.

HIWU says blood, urine or hair may be collected out of competition. Only Banned Substances are reported from those samples, which is exactly where peptides fall. Refusing or evading collection can itself be a violation.

Who is responsible, and what happens after a positive?

The Responsible Person is the trainer, or the owner if there is no trainer, and is strictly liable. Rule 3212 calls it a personal duty that cannot be handed to anyone else. Intent, fault or knowing use does not need to be shown. Possessing a Banned Substance, or giving it to a Covered Horse, is a violation on its own.

The horse is punished too. Under the 2023 rules a Covered Horse is ineligible for 6 months after an S2 violation and up to 14 months after an S0 violation. Read the wider competition rules for FEI and USEF.

Which injuries are racehorses known for?

Tendon injuries and stomach ulcers top the list. A 2025 study in Equine Veterinary Journal calls superficial digital flexor tendinopathy one of the most common musculoskeletal diseases in Thoroughbreds and a major cause of retirement. In that study, the injury was more likely after racing or trial racing than after a training gallop.

Ulcers are close to universal in training. A European consensus statement reports squamous gastric disease in about 37% of untrained Thoroughbreds, rising to 80 to 100% within 2 to 3 months of race training. Gaps of more than 6 hours between forage meals and high-starch diets raise the risk. Signs are vague, so gastroscopy is the only reliable way to diagnose ulcers.

• Heat, swelling or a bowed look at the back of the lower leg: stop work and call your vet today
• Sudden lameness after a race or gallop: call your vet today
• Poor appetite, dull coat, girthiness or falling performance: ask your vet about a scope

What your vet does instead

Your vet uses approved treatments and checks them against HISA rules before they are given. For tendons, that starts with an exam and ultrasound, then rest and a controlled return to work. See tendon injuries in horses. For ulcers, it starts with a scope and changes to feeding, and our gastric ulcer guide covers the rest.

Ask your vet to log every treatment, since attending vets must keep treatment records under the rules. If a substance is not on HIWU's lookup, that does not mean it is allowed. HIWU says it does not guarantee the lookup is complete. When unsure, ask HIWU in writing. A horse that has had a peptide should be declared and discussed with your vet before it trains or races.