Veterinary Pharmacology for Large Animals
Written by Rebecca J. Anderson, PhD
In a brief six-month period, from December 2018 to June 2019, 30 horses died at the Santa Anita racetrack in California (home of Seabiscuit, the legendary champion). The horses had been euthanized after sustaining catastrophic musculoskeletal injuries either during their high-speed morning workouts or their afternoon races.1 Officials temporarily shut down the racetrack, while they investigated the track conditions and the drugs the horses had received.2 As an extra precaution, they initiated a ban on furosemide (Lasix). Although furosemide has legitimate uses, there was evidence that it could also enhance the horses’ performance.3
Many of the euthanized horses had routinely been given multiple drug treatments. In addition to furosemide, these included a bisphosphonate (a drug used to treat osteoporosis, but which may also have an analgesic effect on bone), the NSAID phenylbutazone, the corticosteroid dexamethasone (a drug to treat inflammation), and the muscle relaxant methocarbamol.1 Officials sought to determine whether those drugs may have endangered, rather than improved, the health of the horses.
Veterinary Pharmacology
Although American veterinary schools had been established in the mid-1800s, the beginnings of modern pharmacology in veterinary medicine date to 1954 when the textbook, Veterinary Pharmacology & Therapeutics, was first published. From that point forward, pharmacology was formally incorporated as a discipline in Colleges of Veterinary Medicine.3 In 1990, the US Pharmacopeia, for the first time, published standards and information for veterinary drugs.
In recent years, women have been appearing in increasing numbers in all professions. Specifically in veterinary medicine, four-times more women are now enrolling in veterinary schools than men.4 And the vast majority of veterinarians opt to care for pets and other household animals.5
Large animal veterinarians care for livestock and performance animals (mostly horses), and they must also comply with state and federal regulations regarding drug use.5,6 Unfortunately, demand far exceeds the number of available large animal veterinarians, especially in rural areas.5 To address this shortage, the US Department of Agriculture (USDA) announced a new program in 2025, which offers grants, loans, scholarships, and other incentives to veterinary schools and their students.
Large animal veterinarians often have a farming background, in addition to a special affinity for bigger animals. Rather than maintaining a clinic or animal hospital, they go to the animals—mainly on farms and ranches, but also at competition venues.5
For example, veterinarians who specialize in caring for racehorses may work primarily at racetracks.6 Similarly, licensed equine veterinarians care for horses while they are competing at non-racing equestrian events.7 Unlike most other veterinarians, equine veterinarians must balance the horse’s health with its ability to compete, which is the focus of the horse’s rider and owner.8
Other large animal veterinarians care for food-producing animals (cattle, pigs, and chickens). Poultry are not large animals, but because they are another food-producing species, they are often included in a large animal veterinarian’s practice.6 In addition to maintaining the animals’ health, food-animal veterinarians help to protect human health. By preventing animal illnesses and closely monitoring the drugs they administer, they ensure that diseases and veterinary drug residues do not pass to humans through meat, milk, eggs, and other food products.5
Some large animal veterinarians are employed by state or federal agencies and serve in a regulatory capacity. They may inspect horse husbandry conditions or food-animal production facilities to ensure and enforce compliance with drug regulations.7
Veterinarians prevent animal illnesses and closely monitor the drugs they administer to ensure that diseases and veterinary drug residues do not pass to humans through meat, milk, eggs, and other food products.
Treating Horses
Because racehorses experience severe physiological strain as a consequence of competitive racing, veterinarians routinely administer drugs to prevent or treat that strain. These include strong NSAIDs and bronchodilators. But in addition to their therapeutic value, some of these same drugs have commonly been administered to healthy horses to enhance their performance.9
Furosemide, which the California Horse Racing Board reviewed as part of the Santa Anita investigation, had commonly been administered at race time to reduce or prevent exercise-induced pulmonary hemorrhage, which is experienced by a high percentage of thoroughbred racehorses. Intense exercise was thought to increase pulmonary blood pressure, which ruptured lung capillaries and released blood into the lungs.10,11
Furosemide is a potent loop diuretic. By reducing plasma volume, it is thought that furosemide also reduces blood pressure in the lung capillaries and therefore lowers the risk of exercise-induced pulmonary hemorrhage.3,10 Furosemide’s benefits in preventing pulmonary hemorrhage are controversial, but there is substantial evidence that the drug can enhance racehorses’ ability to compete.3 Consequently, furosemide use has been restricted to exercise workouts and banned on race day in most states.3,12
After an in-depth review of veterinary and racetrack records at Santa Anita, the California Horse Racing Board and the Los Angeles District Attorney’s office found that all of the administered drugs were in compliance with the California Board’s regulations.1 Of concern, though, were the post-mortem exams, which showed evidence of chronic damage to muscles and bones in the euthanized horses. Those pre-existing injuries may have explained why some of the horses were receiving ongoing drug treatments, and also why racing with those injuries may have contributed to catastrophic damage.1
This example highlights a dilemma that all equine veterinarians face daily.1 That is, are they prescribing drugs for therapy? Or are they pharmacologically enhancing the horses’ ability to run faster, either intentionally or inadvertently?
What is considered a medication in one setting could be interpreted as doping in another. For example, giving anti-inflammatory and painkilling drugs to a horse to assist healing is entirely appropriate in veterinary practice, but administering those same drugs on race day would likely be improper. By masking pain, those drugs encourage a sore (and physically vulnerable) horse to compete beyond its natural ability.2,3,9
What is considered a medication in one setting could be interpreted as doping in another.
Doping
Doping is the colloquial term for illegitimate use of drugs to enhance performance.3 To ensure compliance with anti-doping rules, regulatory veterinarians collect and analyze blood or urine samples from horses prior to and/or immediately after participating in an event. The distinction between the presence of a drug given therapeutically versus doping can sometimes be a very fine line, as the owners of Dancer’s Image discovered.3
On May 4, 1968, Dancer’s Image surged from last place to finish first, in a thrilling win at the Kentucky Derby.2 Unfortunately, after the race, the stallion tested positive for phenylbutazone—also known in racing circles as “bute.” The Kentucky Derby did not allow the presence of phenylbutazone in horses on race day. Consequently, Dancer’s Image was disqualified, and the second-place finisher, Forward Pass, was declared the official winner.2
Phenylbutazone, an NSAID, is considered a valuable drug to treat sore horses during training.13,14 The drug’s anti-inflammatory and pain relief actions help to restore and maintain the fitness of horses debilitated by early injury to joints, tendons, or muscles.14 But by decreasing pain, phenylbutazone also potentially allows horses to run faster and increases the chance that they will aggravate an existing injury or suffer a new injury.13
Although the short-term benefits of phenylbutazone are undisputed, long-term, continuous use has been problematic. One study showed that horses with a high blood concentration of phenylbutazone suffered a higher incidence of racetrack injury.14 Another study, conducted in Argentina, found that phenylbutazone-treated thoroughbred horses suffered injuries requiring euthanasia at twice the rate, compared to horses that were drug-free.10 The investigators did not blame phenylbutazone for directly causing the injuries, but instead concluded that the drug masked pain and allowed the horses to race despite pre-existing injuries.10,13
With phenylbutazone in the horse’s system, it is difficult if not impossible for racetrack veterinarians to evaluate the degree of a horse’s lameness. For the same reason, many veterinarians refuse to conduct a pre-purchase lameness exam of a horse unless they receive evidence that the horse is free of NSAIDs and other anti-inflammatory drugs.14
Phenylbutazone has a long half-life, and the drug’s effects persist at least 24 hours after a therapeutic dose.12,14 Current US regulations, therefore, stipulate that phenylbutazone must not be given for at least 48 hours before a race.12,13 The veterinarian who treated Dancer’s Image apparently misjudged the impact of his dosing regimen.
International Regulations
The anti-doping regulations are especially strict for high-profile international equestrian events, such as the Olympics. The standards and practices, including anti-doping regulations, for international equestrian events are governed by the Fédération Equestre Internationale (FEI).
At the Paris Olympics in 2024, the Belgian equestrian team finished fourth. After the Games, on September 3, 2024, FEI officials notified the Belgian equestrian federation that the antidepressant trazodone had been found in the system of the mare, Dia Van Het Lichterveld Z, ridden by Tine Magnus of the Belgian equestrian team.15
The distinction between the presence of a drug given therapeutically versus doping can sometimes be a very fine line.
The source of trazodone was a supplement, RELAX PRO, made by Equine Care Group. During the Olympics, the Belgian team’s veterinarian had been remiss in not advising Magnus and her team of the dangers regarding supplement administration to the horses. On the contrary, he had enthusiastically recommended the use of 14 different supplements, including RELAX PRO. In a clear conflict of interest, that team veterinarian was employed by Equine Care Group, which sold and delivered the supplements to the Belgium team.15
Faced with disciplinary action, Magnus and her attorneys asked two prominent toxicologists to conduct their own tests of the RELAX PRO product used during the Olympics. They confirmed the presence of trazodone. They then purchased and tested other commercially available samples of RELAX PRO with the same lot number. Those samples also tested positive for trazodone, despite the product’s label, which stated that RELAX PRO was “doping-free.”15
The FEI investigators acknowledged that Magnus had kept meticulous records. She and her well-trained team were acutely aware of the anti-doping rules, diligently restricted treatments to drugs that benefited the horse’s welfare, and made every effort to prevent cross-contamination. However, during the Olympics, she relied on the advice of the team veterinarian and the reputation of the Equine Care Group.15
Unfortunately for Magnus, the FEI rules specified that she (as the rider) was ultimately responsible for all of her horse’s care.15 That included being the final arbiter on all drugs and procedures recommended by the Belgian team veterinarian, trainers, and team officials.
She was suspended for 5 months and fined $4700. Her horse was provisionally suspended for two months, and the Belgian equestrian team’s fourth-place finish was removed from the Olympic Games record book.15
Food-Production Animals
In the US, there are 1.5 billion chickens, 200 million turkeys, 86 million cattle, 75 million pigs, and 5 million sheep.3 Cattle, pigs, and poultry are considered “major” food-producing species, and the introduction of anti-parasitic and anti-microbial drugs, along with improved husbandry practices, has greatly expanded the availability and affordability of meat, eggs, and dairy products.16 Those drugs, along with NSAIDS, are among the essential drugs prescribed by food-animal veterinarians to combat disease, alleviate pain, and minimize economic losses. Many of these drugs are FDA-approved for treating food-producing animals.3
Nowadays, consumers place great (and sometimes unrealistic) demands on producers to ensure that veterinary drugs do not contaminate foods derived from animals.16 Of particular concern is the widespread use of anti-microbial drugs. Although these drugs greatly enhance food production by keeping animals healthy, they also accelerate bacterial resistance, not only in the animals but also in people who subsequently acquire drug-resistant infections.16
Consumers have placed great (and sometimes unrealistic) demands on producers to ensure that veterinary drugs do not contaminate foods derived from the animals.
Prior to 2017, farmers and ranchers could use antibiotics for growth promotion, not just for their labeled indication to treat infections.17 Exposing food-producing animals to antibiotics for long periods (i.e., more than 21 days) to promote growth in otherwise healthy animals raised concerns that these drugs would increase the risk of microbial resistance.5,17
For example, tylosin, an antibiotic related to azithromycin, was originally labeled for “continuous use” to treat liver abscesses in cattle. Although tylosin is used only in veterinary medicine, there is concern that treating cattle for 100 days or more can generate bacteria that are resistant not only to tylosin, but also to azithromycin and similar antibiotics used to treat human infections.17
The public’s demands have led to banning some anti-microbial drugs that were formerly used for decades in veterinary medicine and are still prescribed by physicians to treat patients.16 For example, the Food and Drug Administration (FDA) approved metronidazole in the early 1960s to treat various bacterial and protozoal infections.18,19 Although toxicity studies indicated that the drug was carcinogenic in rodents, short-term exposure in humans does not appear to increase the risk of developing cancer.19
Veterinarians also prescribe metronidazole for bacterial and protozoal infections, but it has been banned for use in food-producing animals.19,20 This has left the poultry industry without an effective treatment for histomoniasis (blackhead disease), which re-emerged with up to 100% mortality in turkey flocks.16,19
Medicated Feed
Food-producing animals may be treated individually, but group dosing via medicated water or feed is often preferred.3 Dosing the herd or flock in this manner minimizes handling stress and allows more efficient drug administration during a disease outbreak. Unlike individual animal dosing, medicated feed does not ensure that a specific dose of the drug reaches each animal. Rather, the goal is to manage the health of the entire herd or flock. Drugs mixed into animal feeds thus represent a unique kind of drug formulation.3
Drugs mixed into animal feeds represent a unique kind of drug formulation.
In 1959, the Food Additive Amendments Act gave the FDA authority to regulate medicated animal feeds and drug residues in animal-derived foods.3
Because of concerns about bacterial resistance, the FDA’s Veterinary Feed Directive stipulates how medically important antibiotics may be legally incorporated into the feed or water given to food-producing animals.21 One objective of the Directive was to eliminate the use of anti-microbials for promoting growth. To achieve this goal, licensed veterinarians must now prescribe the drugs incorporated in feed and water and ensure that the medicated feed is used only for specific therapeutic purposes.3,5,21 The Directive also recommends that pharmaceutical manufacturers include a defined duration of use on the labels of veterinary drugs used to treat food-producing animals, to minimize the chance of developing resistant microbial strains.17
Withdrawal Time Considerations
For both performance and food-producing animals, drug pharmacokinetics is a critical factor in determining treatment regimens, but for different reasons. For performance animals (that is, horses), the main concern is avoiding drugs during competition that mask injury, unfairly enhance performance, or both. For food-producing animals, the main concern is drug residues that remain in foods derived from the animals. In both cases, veterinarians are expected to employ an appropriate withdrawal time for each drug they prescribe.
Withdrawal time is based on a drug’s pharmacokinetics and is defined as the length of time that should elapse following the last dose in order to avoid a detectable (or unallowed) concentration of the drug in the animal (or in food derived from the animal).3 Calculating withdrawal times is challenging and complex for veterinarians, because the pharmacokinetics and toxicity profiles of drugs vary widely between species and breeds, which have diverse sizes, behaviors, metabolic rates, and lifespans.3
Until the 1970s, veterinarians typically based the dose on the animal’s body weight. This was because analytical methods were largely unavailable, and few pharmacokinetics studies had been performed in large domesticated animals.16
Then, in 1972, Lloyd Davis and colleagues at the University of Missouri conducted a pivotal study.22 Using salicylate as a representative drug, they found marked differences in the plasma elimination rates in five domesticated species. The observed half-lives of salicylate were 48 minutes in goats, 1 hour in horses, 5.9 hours in pigs, 8.6 hours in dogs, and 37.6 hours in cats.22 Clearly, the previous method of basing a drug dose solely on body weight was flawed.
Davis’s report has been called “the most game changing publication in veterinary pharmacology to that date (and even now).”16 Subsequently, pharmacokinetics became the cornerstone of veterinary pharmacology.
To assist equine veterinarians, various organizations have developed guidelines for drug withdrawal times in horses. The objective of these guidelines is to ensure that horses receive proper medical treatment but are not given drugs that unfairly enhance their performance and/or risk injuries. The Racing Medication and Testing Consortium (RMTC) has developed a comprehensive Laboratory Code of Standards including withdrawal guidelines for many therapeutic drugs administered to racehorses. Similarly, the US Equestrian Federation (USEF) has published recommended withdrawal times for drugs commonly given to horses that compete in non-racing equestrian events.3
Pharmacokinetics became the cornerstone of veterinary pharmacology.
But for many drugs, setting a specific withdrawal time is not feasible. For example, in May 2021, Medina Spirit won the Kentucky Derby. The post-race drug test revealed that the corticosteroid betamethasone was found in his system.2 Betamethasone’s pharmacokinetics varies, depending on the route of administration, formulation, and frequency of dosing. Because of these variables, the withdrawal time ranges from 3 to 21 days. Rather than specifying a withdrawal time, the oversight organizations rely on veterinarians to ensure that the administered drug dose has cleared the horse’s system at race time. The 2021 Kentucky Derby disqualification made headlines, because Medina Spirit was the fifth racehorse to fail a drug test that racing season.2
Drug Residues in Foods
For food-producing animals, the labeled withdrawal time aims to ensure that drug residues are eliminated (or at least minimized) prior to processing milk and eggs or slaughtering an animal for meat.21 The FDA and the USDA share regulatory oversight of food-producing animals in the US, and they work together to ensure compliance with drug residue regulations.
The FDA sets the acceptable level of drug residues in food-producing animals. The Food Animal Residue Avoidance Databank (FARAD), which is funded by USDA, recommends withdrawal times to achieve those drug residue targets.20 To assist veterinarians, the FARAD website provides a withdrawal calculator. For any given date of drug administration in a given species, the FARAD calculator determines the date that the drug will achieve an acceptably low residue level, based on the drug’s pharmacokinetics in that species.20
The USDA’s Food Safety and Inspection Service is responsible for analyzing drug residues in meat, dairy, and poultry products and reporting the results to FDA. FDA officials then determine which USDA-reported residues warrant follow-up, and if so, they investigate to identify the source and circumstances of the drug residue. Often, the residue can be traced to a producer who did not allow a sufficient withdrawal time after drug administration. If there is a lack of voluntary corrective actions, the FDA may take steps to ensure food safety and hold those accountable for causing drug residue violations.
For example, in September 2010, tests conducted by USDA found 0.162 ppm (parts per million) of flunixin in the liver of a cow that was slaughtered for food by American Beef Packers, Inc. Pastime Lakes Dairy in Lakeview, CA, had sold the cow to American Beef Packers.23 FDA officials cited Pastime Lakes Dairy for not following the label instructions for flunixin, a veterinary NSAID drug. The flunixin label states that “cattle must not be slaughtered for human consumption within 4 days of the last treatment,” and the level of flunixin in edible tissues of the animal must not exceed 0.125 ppm.23
Drug Regulation
The FDA has approved over 20,000 drugs for human use but only about 1600 veterinary drugs.24 The FDA’s Center for Veterinary Medicine is the primary regulatory body for veterinary drugs sold in the US. Most of those veterinary drugs derive from either the human pharmaceutical industry (antibiotics, functional drugs, etc.) or the agrochemical industry (insecticides, anti-parasitic drugs, etc.)3,16
Approval and marketing of a new veterinary drug parallels that of human pharmaceuticals. But veterinary drug development encompasses complexities that are not faced in human drug development, such as human food safety and related public health concerns.3
The FDA ensures that veterinary drugs given to food-producing animals not only maintain the health of the animals but also avoid adversely impacting human health. Due to close monitoring by both the FDA and USDA, food products derived from animals in the US are rarely tainted with pathogens or violate drug residue regulations. When inspectors spot a violation, they are quick to take action: citing the producer for the violation (including prosecution if the producer is uncooperative), notifying distributors and consumers, and recalling the tainted product from the market.
FDA ensures that veterinary drugs given to food-producing animals not only maintain the health of the animals but also avoid adversely impacting human health.
Extralabel Drugs
Another complication for veterinarians is that, like human pharmaceuticals, the drug label restricts a drug’s use to the listed therapeutic indication(s), dose(s), and (in the case of veterinary products) the species. Until 1994, the relative lack of FDA-approved veterinary drugs often put veterinarians in an untenable position.3 They frequently resorted to administering a drug for a disease, at a dose, by a route of administration, and/or in a species that were not indicated on the FDA-approved label. In many cases, the drug had been approved only for use in humans.
Recognizing this dilemma, the US Congress passed the Animal Medical Drug Use Clarification Act in 1994.3 This permitted veterinarians to use a drug off-label when there was no approved drug for the intended use, the labeled dose was not effective for that use, or the approved formulation was not appropriate. Treating animals under these conditions is referred to as “extralabel” drug use.3
Of particular concern has been the critical lack of FDA-approved analgesics and anesthetics for food-producing animals.3 Lidocaine is allowed for extralabel use and is often the drug of choice. For anesthesia in food-producing animals, extralabel use of xylazine, ketamine, and acepromazine are permitted. NSAIDs such as aspirin and meloxicam are used extralabel for pain in food-producing animals, but because these drugs have a long half-life, their use is discouraged.3,20
Despite granting greater latitude, the extralabel regulations still impose some limits. Veterinarians may use an extralabel drug only if the health of the animal is threatened, or the animal is suffering and might die for lack of treatment. FDA also bans the use of extralabel drugs in animal feeds, because of the difficulties in controlling drug residues.3
Allometric Scaling
When veterinarians are faced with using an extralabel drug, they employ allometric scaling to calculate an appropriate dose or dosage schedule. Allometric scaling attempts to extrapolate doses between species empirically by factoring the animals’ body surface area and existing pharmacokinetic data in the species.25
Online algorithms such as the Javascript calculator are now available to assist veterinarians in scaling inter-species doses. These calculators take into account various pharmacokinetic parameters including metabolic rate, renal clearance, and volume
of distribution.26
Veterinarians remain responsible for managing the presence of residues from extralabel drug use. For horses, the main concern is doping. Current regulations ban extralabel drugs that are given specifically as doping agents to enhance a horse’s performance. Extralabel drugs may be given to horses to treat medical conditions, but veterinarians must ensure that residues of those drugs are minimized during a race or equestrian event.
For food-producing animals, veterinarians are responsible for ensuring that administered extralabel drugs will not result in unacceptable residues in milk, meat, eggs or other edible products derived from the animal.3 They are encouraged to implement a substantially longer withdrawal period as an added safeguard. The FARAD website also assists veterinarians by recommending withdrawal times for selected extralabel drugs.20 Those FARAD recommendations are based on pharmacokinetics data and tissue residue studies.
FARAD also maintains a list of drugs for which extralabel use in food-producing animals is specifically prohibited.20 For example, phenylbutazone has an elimination half-life in dairy cattle of about 40 hours and an estimated withdrawal time of 120 hours.27 Because of this long half-life, as well as evidence that phenylbutazone is actively transported into milk, phenylbutazone is prohibited from extralabel use in dairy cows 20 months of age or older.20,27
Analytical Sensitivity
The sensitivity of analytical lab methods has improved so much that it is now possible to detect some drugs days, weeks, or even months after administration.3 This is primarily a concern in the treatment of performance animals. In cases where treatment during training is medically appropriate, residues of those drugs may sometimes be detected long after they cease to have any demonstrable pharmacological effect.3,28 The difficulty is differentiating between a pharmacologically insignificant residue and one that may be associated with a beneficial effect on performance, even if that effect is small.3
The difficulty is differentiating between a pharmacologically insignificant residue and one that may be associated with a beneficial effect on performance, even if that effect is small.
Another consequence of increased analytical sensitivity is detection of substances that are components of the animal’s feed and fodder. Small naturally occurring amounts of prohibited substances in the feedstock can result in a positive drug test. For example, coca husks that are mixed into some feeds as filler contain theobromine, a xanthine related to caffeine and considered a prohibited substance in racehorses.3,12
To address this, some regulatory authorities and equine organizations have determined an acceptable limiting concentration for certain drugs, below which the analytical laboratory will not report the presence of the drug as a violation. This reportable concentration is variously called the threshold, reporting level, or cutoff.3 The RMTC has established reporting levels for a list of therapeutic drugs given to racehorses. These include the anti-inflammatory drugs dexamethasone and firocoxib and the muscle relaxant methocarbamol. Although RMTC is an industry-aligned consortium and has no regulatory authority, it wields a great deal of influence at racetracks across the US.3
Gaming the System
Until recently, creative drug doping, combined with lax state regulations, made American racetracks among the deadliest in the world.29 In 2018, an average of 10 horses a week died at US racetracks, according to the Jockey Club’s Equine Injury Database.9,29 The following year, the euthanized horses at the Santa Anita racetrack raised additional concerns about how horses were being treated.
In March 2020, US federal prosecutors indicted 27 trainers, veterinarians, and drug distributors for doping horses. The charges alleged the use of “drugs designed to secretly and dangerously enhance the racing performance of horses beyond their natural ability, a dishonest practice that places the lives of affected animals at risk.”2
The 27 individuals had manufactured and distributed adulterated and misbranded drugs and secretly administered them to racehorses.29 The horses were injected and force-fed all manner of illegal and experimental drugs, which masked pain and allowed the horses to run unnaturally fast. Some horses suffered injuries and others died. To avoid detection, the defendants routinely defrauded and misled federal and state regulators. It was the most far-reaching prosecution of racehorse doping in the history of the US Justice Department.29
In 2022, to address concerns about the health and safety of racehorses, the US Congress passed a bill authorizing the Horseracing Integrity and Safety Authority (HISA). A private self-regulatory organization, HISA developed rules related to medication control, anti-doping, and racetrack safety. Those rules now ensure consistency in the treatment of racehorses and racetrack conditions across the country.30
HISA’s database adds over 5,000 veterinary records daily. Ongoing monitoring of that data allows HISA to identify trends and potential problem areas before catastrophic injuries occur.31
HISA’s Anti-Doping and Medication Control program took effect on May 22, 2023, and is designed to enhance the safety and wellbeing of both horses and riders.30 Under this program, HISA maintains a list of “controlled” drugs, along with their corresponding drug cutoff levels, withdrawal times, and recommended dosing schedules. HISA also publishes a list of drugs that are banned outright in horses during a race.12
HISA’s anti-doping program and its rules for racetrack safety have helped to reduce the risk of racehorses suffering catastrophic injuries that require euthanasia.31 Largely due to HISA, the fatality rate at US racetracks has steadily declined from an average 1.89 per 1000 racing starts in the early 2010s to 0.90 per 1000 racing starts in 2024.9,30 The 2024 results represent the lowest fatality rate since the industry began tracking such data.31
For non-racing equestrian events in the US, the largest governing body is the US Equestrian Federation (USEF), a private, non-government organization. USEF’s Therapeutic Substance Provisions allow certain therapeutic medications to be present in clinically significant concentrations during competition, while banning the presence of others outright. Like HISA, USEF publishes recommended withdrawal times, residual thresholds, and maximum doses for medications commonly given to horses competing in equestrian events.3
HISA’s rules now ensure consistency in the treatment of racehorses and racetrack conditions across the nation.
Santa Anita Lessons
At the end of 2019, California prosecutors found no evidence of animal cruelty or other crimes to account for the spike in horse injuries and deaths at Santa Anita Park. But they did find that those deaths occurred at a higher rate (2.04 deaths per 1000 racing starts) than the national average.32
In response, California legislators passed SB 469, which grants the California Horse Racing Board the authority to immediately suspend horse racing licenses in situations that threaten the health and safety of horses and riders.33 The owners of the Santa Anita racetrack also announced they would improve the Park’s facilities and track conditions to better protect the horses.
But despite greater compliance and the marked decline in injury rates, animal welfare groups, such as the American Society for the Prevention of Cruelty to Animals, are still advocating stricter rules and greater regulatory oversight to protect horses.33
