In late June, the U.S. Equestrian Federation announced an extraordinary rule change to allow the addition of hair testing for certain prohibited substances as a complement to the existing blood and urine testing protocols. When the hair testing went into effect on July 1, the organization published the list of substances that it would be used to detect. 

It includes three types of drugs—barbiturates, selective androgen receptor modulators (SARMs) and anticonvulsants. Of those, barbiturates and SARMs are completely prohibited, while there are four specific anticonvulsants that are prohibited.

Veterinarians say the drugs on the new list should never show up in horses that are being ridden and competing. One, the barbiturate pentobarbital, which is commonly used in humane euthanasia, should never show up in any living animal, and the others are only very rarely used in horses. Although there might be times—under very specific circumstances—where a veterinarian would reach for one of them for a legitimate medical reason, those situations would not arise in horses who are healthy enough for a show ring, experts said.

Hair testing goes farther back in time than blood and urine testing do—potentially 12 to 18 months, depending on the hair sample—to detect the presence of a drug. (Read more about the science behind hair testing here.) The rumored use of pentobarbital in show horses was the primary impetus for the addition of hair testing, USEF Chief Veterinary Officer Dr. Stephen Schumacher, DVM, said. Although pentobarbital would be detected in blood samples if it was given to a horse at a show, “the interest in looking further back is because we’ve also had some information that people may be using this in their horse a little further out from competition and still gaining that kind of advantage,” he said.

“What that rule change allowed us to do is just provide another tool in our arsenal to try and protect horses, frankly—especially when we had information about this substance [pentobarbital] potentially being used. There is no therapeutic use for this; this is a drug that’s used for euthanizing horses.”

Schumacher added that some USEF members have voiced concerns that, with hair testing, they could be unfairly implicated in drugging if they purchase a horse that was given prohibited substances in the past. However, he said, USEF is already equipped to investigate such cases, because some drugs can show up in blood or urine testing for longer periods of time as well. Reserpine, for example, can be picked up in blood samples a long time after it is administered.

Here’s some additional information about the specific prohibited drugs that hair testing is being used for and what they do:

Selective Androgen Receptor Modulators (SARMs)

SARMs can be thought of like anabolic steroids, said Dr. Emilee Lacey, VMD, MS, a large animal internal medicine specialist at Palm Beach Equine Clinic (Florida). Both drugs bind to androgen (sex hormone) receptors in the body, but SARMs bind to those in muscle and bone, while anabolic steroids bind to receptors throughout the body.

“[SARMs] won’t have as many androgen receptor side effects, like classic androgen steroids would. Those drugs will bind pretty indiscriminately to androgen receptors across the body,” said Lacey. “The point of [SARMs] in horses would be to really increase muscle mass, increase bone mass, so it really is a performance enhancing medication.

“Now I think that there’s a time and a place to use those drugs, particularly let’s say you have the equine athlete that maybe has gone through an injury and is really down on muscle mass, or even a horse that’s been going through some rehabilitation processes. But never would we recommend those drugs to be used regularly. [They’re] very, very, very rarely used,” she said. 

Horses with a legitimate therapeutic need for these drugs wouldn’t be in any condition to be ridden or shown, Lacey emphasized. “I’m talking severe decrease in muscle mass, something with a very low body condition score. And even then, we’d be pretty cautious to reach for those drugs.”

Barbiturates

Most horse owners are familiar with the barbiturate pentobarbital, which is commonly used today to perform humane euthanasia, but originally the drug had some other medical uses. Newer drugs have largely replaced it in that capacity, however. 

“It’s really like an old anesthetic class of drugs. We really don’t use them much,” Lacey said. 

“Phenobarbital, another example you’ll see on [the USEF list of prohibited substances], can actually be used as an anticonvulsant as well,” she added. “However, if a horse is taking that medication, [the owner] probably should be working very closely with their veterinarian, because [the horse is] taking it because they’ve had seizures. It’s an anti-seizure medication. So again, these drugs really are something that a horse in active work shouldn’t be receiving.”

Seizure disorders in horses are extremely rare, much more rare than in dogs and cats, Lacey said. But one known equine seizure disorder is juvenile idiopathic epilepsy in Arabian foals from Egyptian lineage. It generally occurs in horses under a year old, and although the cause is not known it’s thought that there’s a genetic component.

“A lot of the time these horses will grow out of this disease process, but they may be on a tapering course of phenobarbital over that time, until they grow out of having seizures,” she said. “So there are medical reasons that we may choose to put a horse on phenobarbital, for example. But again, anytime a horse is on this drug, the owner really should be working pretty closely with their veterinarian.”

Barbiturates, particularly pentobarbital, also have a huge environmental impact, Lacey said.

“There are some risk factors of euthanizing a horse with pentobarbital and then the animal’s body being available for wildlife to come and consume,” she said. “So again, there’s just a lot of risk around giving that medication, not only as a euthanasia solution, but even more so if they’re giving it to a live animal. This is definitely a drug that should not be given to a live animal.”

Anticonvulsants

Four specific anticonvulsant drugs are prohibited by USEF: felbamate, phentermine, phenytoin and valproate.

Like barbiturates, anticonvulsants “are going to be prescribed when an animal has seizure issues and neurologic disease,” Lacey said. “So, again, a horse really shouldn’t be competing if they’re on those drugs, without veterinarian advice, that is.”

Phenytoin can sometimes be used for neuropathic pain, Lacey said.

“It’s unusual,” she said. “I think we just have many more, better drugs. That drug is probably the one that would be used for horses that basically have pain. … It’s used more in humans than it is in horses, but I’ve used it in horses that have been self-traumatizing due to really itchy skin or really diseased skin.”

Lacey noted that while there might be some rare instances where these drugs would be prescribed for a legitimate reason, in the interest of safety, those reasons would—or should—generally preclude a horse from being shown in the first place.

“I’m not as concerned with a horse receiving this medication if they have a medical need for it,” she said. “I’m more concerned for the safety of the horse and the rider, if the horse has a seizure condition and someone’s trying to ride it.”

”The biggest thing is, I think, hey, this list of drugs came out to raise awareness, but also to protect the horse and rider at the end of the day. So I don’t think it’s something to be scared about,” Lacey added. “Rather, it’s something that I think the USEF is doing a good job by monitoring and making sure that [people are] not promoting the use of prohibited drugs, but really just keeping the horse and rider in mind, because, again, any of these drugs are medications that really shouldn’t be given to an actively [working] horse.”