Sedation in Horses: Medications, Uses & Risks – [Medication Guide]

Equine sedation is a critical aspect of veterinary care, playing a vital role in ensuring the safety and well-being of both horses and handlers during various medical procedures.

The horse is a large, powerful animal with a strong flight response, which can make certain veterinary procedures difficult or dangerous to perform. Sedation allows veterinarians to perform necessary tasks with minimal stress and discomfort to the animal. Veterinarians commonly use sedation during routine dental work, minor surgeries, and diagnostics.

Sedation is safe for horses and has very few side effects. Equine veterinarians strongly prefer sedation over general anesthesia for procedures, as anesthesia has a high mortality rate in horses due to their large size and unpredictable nature when stressed.

It’s helpful for owners and caretakers to familiarize themselves with how equine sedation procedures work, the common sedation medications and their side effects, and what to look out for after sedation so they are prepared when the time comes.

Common Sedation Medications for Horses

There are several options for sedation medications in horses. Most sedation protocols are a combination of an alpha-2 agonist and an opioid. ^[1] One study showed that 64% of veterinarians use a combination of these medications as their primary sedation protocol. ^[2]

Acepromazine, ketamine, and benzodiazepines are other common medications that veterinarians may use as part of a sedation protocol. ^[1]

Alpha-2 Agonists

Alpha-2 agonists are a class of drug that act on alpha-2 receptors. ^[1] These receptors are found throughout the body, including in the brain, brainstem, and spinal cord. ^[1]

When alpha-2 agonists bind to these receptors, they block the release of calcium from nerves, resulting in reduced nerve firing. ^[1] This results in muscle relaxation, reduced blood pressure, and sedation. ^[1][3]

The most common alpha-2 agonists used for sedating horses are: ^[3][4]

• Xylazine (Rompun®)
• Detomidine (Dormosedan®)
• Romifidine
• Dexmedetomidine

Side effects of alpha-2 agonists include: ^[3][5]

• Increased urination
• Sweating
• Poor thermoregulation, potentially leading to overheating
• Slow heart rate
• Reduced intestinal motility

Rarely, some horses receiving xylazine sedation become aggressive when sedated (“Rompun® Rage”). ^[1]

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Opioids

Opioids are primarily a pain control medication, however they also have sedative effects. ^[1] These medications act on opioid receptors in the horse’s brain and spinal cord. ^[1]

Binding of opioids to opioid receptors blocks calcium channels, similar to alpha-2 agonists. ^[1] This results in reduced activity of excitatory nerves, nerves that stimulate increased activity. ^[1] Blocking these nerves produces sedation and reduced response to painful stimuli. ^[1]

Since opioids have a similar function as alpha-2 agonists, they produce a synergistic effect when used together. ^[4] Combining the two types of drugs allows veterinarians to use less of both types of medication while still seeing the same sedative effect. ^[4]

Common opioids used for sedation of horses include: ^[5]

• Butorphanol (Torbugesic®)
• Methadone
• Buprenorphine
• Morphine
• Meperidine
• Hydromorphone

Side effects of opioids include: ^[5][6]

• Slow heart rate
• Twitching of the facial muscles
• Reduced intestinal motility

Acepromazine

Acepromazine is a phenothiazine medication commonly used for mild sedation in horses. ^[3]

Acepromazine acts on dopamine receptors in the part of the brain responsible for motor control. ^[1] Blocking these receptors reduces the stimuli the brain produces to trigger muscle movement, resulting in sedation. ^[1]

On its own, acepromazine only has a mild sedative effect. ^[5] Common uses for acepromazine include reducing anxiety for non-invasive and non-medical procedures including: ^[5]

• Clipping
• Farriery
• Trailering

Some veterinarians use this drug as a premedication (medication given before sedation) in sedation protocols to reduce anxiety and stress during procedure preparation. ^[7]

Ketamine

Ketamine is most commonly used as an anesthetic agent in horses, however it can also be a component of sedation protocols at low doses. ^[1] Ketamine blocks the function of NMDA receptors, resulting in reduced excitatory nerve stimulation. ^[1]

Most veterinarians using ketamine in sedation protocols combine the drug with an alpha-2 agonist. ^[1] Usually this is in the form of a “ketamine stun“, where the veterinarian administers a single dose of ketamine to prevent an already sedated horse from waking up if they are showing signs of arousal. ^[4]

Ketamine is also particularly useful for horses experiencing chronic pain, as it can block pain at the level of the spinal cord. ^[5] Horses requiring sedation for inflammatory diseases, such as laminitis, may benefit from ketamine in their sedation protocols to reduce their pain level. ^[4]

Benzodiazepines

Like ketamine, benzodiazepines are most commonly used for anesthesia in horses. ^[1] These medications are not used for sedation in adult horses, as they cause profound muscle relaxation and incoordination, frequently causing horses to lay or fall down. ^[1]

However, they can be an effective sedative for foals, who often lay down during sedation regardless of the medication used. ^[1] These medications increase the effect of GABA in the brain, a neurotransmitter that produces a calming effect. ^[1] The two main benzodiazepines used for sedation in foals are diazepam and midazolam. ^[1]

Uses for Sedation

Standing sedation is common in equine veterinary medicine, as it allows veterinarians to safely work on horses who may be fractious, scared, or stressed.

Whenever possible, veterinarians prefer to use standing sedation over general anesthesia, as general anesthesia can have mortality rates of 1 – 2% in horses. ^[4]

Common uses for sedation in equine medicine include: ^[1][5]

• Thorough assessment of anxious, fractious, or relatively unhandled horses
• Dental procedures, including floatation
• Ocular procedures, such as eye examinations, minor surgeries, or administering medication
• Diagnostic imaging procedures, such as MRI or nuclear scintigraphy (“bone scan”)
• Treating wounds, including debridement (removal of dead tissue) and suturing
• Surgeries on the head, limbs, perineal or genital areas
• Skin tumour removals

Sedation Procedure

The goal of sedation is to calm the horse and make them unresponsive to any veterinary procedures taking place. ^[5] Horses who are excited, nervous, anxious, or extremely painful may not respond to sedative drugs appropriately, due to continued stimulation. ^[1][5] Therefore, the first step in sedation is moving the horse to a quiet location with few distractions. ^[5]

Once the horse settles into the environment where the procedure will take place, the veterinarian can proceed with sedation. The route of administration for the medication depends on the veterinarian’s preference, the horse’s demeanor, type of medication used, and other factors.

Common methods for administering sedatives to horses include: ^[5]

• Intravenously: Into the bloodstream
• Transmucosal: Placing the medication on the horse’s gums for absorption
• Intramuscular: Injection into the muscle

Intravenous administration is most common, as it has the fastest onset time. ^[5]

After administration, the environment should be kept quiet and free from distractions until the horse is fully sedate. ^[5] The time to full sedation depends on the medication and administration route, but general guidelines are: ^[5]

• Intravenous: 5 – 10 minutes
• Transmucosal: 45 minutes
• Intramuscular: 30 minutes

Once the horse is fully sedate, the procedure can begin. Depending on the horse’s response, additional sedative medications may be necessary to keep the horse sedated and unresponsive to the procedure. Signs of a well-sedated horse include: ^[8][9]

• Relaxed facial muscles, including drooping lips, ears hanging to the side, and partially closed eyelids
• Unresponsive to stimuli, such as touching the ears, loud noises, and suddenly appearing or moving objects
• Unresponsive to the procedure occurring
• Signs of ataxia (incoordination), such as swaying when standing and wide stance with the front and/or back limbs

Recovery

When the procedure is finished, the horse is allowed to wake up on their own under supervision. Most horses start to show signs of awareness around 30 minutes after their last sedation administration. However, it typically takes 1 – 3 hours for the horse to become fully aware.

During the recovery period, sedate horses are unable to eat, move around safely, or interact with other horses normally. For this reason, they should be kept in an individual stall, pen, or paddock with no feed sources.

If no safe enclosures are available, the horse should be held on a leadrope until they are awake, interacting with their environment, and coordinated when moving around. Once they are behaving normally, they can return to their typical environment.

Adverse Events

Sedation occurs daily in equine veterinary practice and is considered very safe with a low risk of side effects. Adverse events that may occur from any sedation protocol include:

• Intracarotid injection
• Allergic reactions
• Priapism
• Paradoxical excitement
• Collapse

Intracarotid Injection

Veterinarians typically administer sedation drugs into the horse’s jugular vein, the large vein running along the horse’s neck near their windpipe. ^[10] This vein sits directly over top of the carotid artery, the major artery supplying blood to the brain. ^[10] Therefore, there is a risk of injecting into the carotid artery when administering medication into the jugular vein.

Drugs accidentally injected into the carotid artery travel directly to the brain, potentially causing tissue damage, blood clots, and spasm of blood vessel walls. ^[10] Depending on the drug administered, permanent or temporary brain damage or sudden death can occur. ^[10]

Intracarotid injection of sedatives typically results in seizures, with no cases of death reported in one analysis of 15 cases. ^[10] The most common complication in the short and long-term was musculoskeletal injuries incurred during the horse’s seizure event. ^[10]

Prior to seizure activity, horses typically show signs such as flared nostrils, muscle tremors, and sweating, indicating intracarotid injection. ^[10] If these signs are noticed, any personnel near the horse should immediately clear the area to avoid injury. ^[10]

To prevent intracarotid injection, veterinarians typically draw back on the syringe prior to injection. This draws blood into the syringe for examination. The color of the blood can provide an indication of whether the needle is in an artery or a vein.

Some veterinarians also place the needle without an attached syringe for this reason, so that they can assess whether the blood flow through the needle is low pressure or high pressure. High pressure flow indicates arterial blood, indicating the needle placement needs adjustment.

Allergic Reactions

Any drug administered to a horse has the potential to cause an allergic reaction. The two most common types of allergic reactions are anaphylactic shock and hives. ^[11]

Anaphylactic Shock

Anaphylactic shock is an immediately life-threatening reaction to the administered medication. Symptoms typically develop within seconds to minutes after injection. ^[11]

Signs of anaphylactic shock include: ^[11]

• Difficulty breathing
• Seizures
• Pale gums
• Fast heart rate with weak pulses

Anaphylaxis is an emergency. ^[11] Veterinarians treat this condition by administering epinephrine, which counteracts the allergic reaction. ^[11] Other interventions, including ensuring airway patency (i.e. emergency tracheotomy), may also be necessary. ^[11]

Hives

Hives are areas of skin swelling and can result from exposure to a drug. ^[11] They are a very common allergic reaction in horses. ^[11]

Hives typically develop within 20 minutes of exposure. ^[11] They are often itchy, causing horses to become restless and uncomfortable. ^[11] Hives typically disappear on their own after 24 hours. ^[11] In severe cases, your veterinarian may prescribe corticosteroids to reduce the immune response and resolve the hives faster. ^[11]

Priapism

Priapism, the inability to retract the penis, is a rare adverse event associated with administration of acepromazine. ^[12] In some cases, priapism lasts 12 or more hours. ^[13] Left untreated, priapism can cause irreparable damage to the penis and affect the horse’s future reproductive function. ^[13]

Although many veterinarians use acepromazine cautiously in male horses for this reason, studies suggest that the risk of priapism is extremely low. ^[13] One study estimated that irreparable damage occurred in only 1 in 10,000 cases. ^[13]

Current recommendations suggest that acepromazine is safe to use in male horses, however the horses should be monitored closely to ensure penile retraction within 1 hour after sedation. ^[14]

Paradoxical Excitement

Sedation protocols can occasionally cause “paradoxical excitement” in some horses. These horses have an extreme reaction to the sedation medication, causing them to shake their heads, pace continuously, and show other signs of agitation and frenzy. ^[4]

Severely affected horses may violently thrash, fall down, or lunge forward. ^[15] Horses may injure themselves during this excitement period. Excitement usually develops rapidly after injection and can last for up to 5 minutes. ^[15]

Opioid medications are more likely to trigger paradoxical excitement than other sedatives. ^[1] Administering opioids in combination with an alpha-2 agonist reduces the risk of paradoxical excitement. ^[1]

Collapse

Heavy sedation protocols may cause a sedated horse to collapse if they become unsteady on their feet. Horses may injure themselves or personnel around them when falling. Anyone near a sedated horse should carefully watch the horse for signs of

• Swaying
• Unsteadiness
• Knuckling of the limbs

Affected horses will not be able to stand until their sedation wears off sufficiently.

Acepromazine can trigger sudden collapse in some horses after administration. This medication may cause rapid blood vessel dilation after injection, resulting in a sudden drop in blood pressure that causes the horse to fall. ^[4] This event is extremely rare, but horses who are excited or already have a low blood pressure from dehydration may have a higher risk. ^[4]

Summary

Sedation involves administering drugs to have a calming, sleep-inducing effect on a horse so that they will tolerate medical procedures.

• Sedation is critical to the safety of horses, handlers, and veterinarians
• Veterinarians use sedation daily to perform procedures such as dental work, minor surgeries, and diagnostics
• Sedation is safe for horses and has few side effects
• Veterinarians prefer sedation over general anesthesia, as anesthesia has a high rate of complications