Phenobarbital for Horses: Uses, Side Effects & Status in Competition

Phenobarbital is a prescription anticonvulsant medication used to help control recurrent or severe seizures in horses. It may be administered intravenously when longer-lasting seizure control is needed or given orally as maintenance therapy to reduce the frequency and severity of future episodes.

Seizures can pose serious risks to horses, handlers, and riders. Because phenobarbital does not address the underlying cause, veterinary evaluation is essential to identify and manage any contributing disease or injury.

Horses vary in how they absorb, metabolize, and respond to phenobarbital. Consistent dosing, close observation, and periodic blood testing help maintain seizure control while limiting sedation and other adverse effects.

Keep reading to learn how phenobarbital works, when it may be used in horses and foals, and what owners should know about administration, monitoring, side effects, drug interactions, and competition regulations.

Phenobarbital for Horses

Phenobarbital is an anticonvulsant used to control seizures in horses and foals. It may be given intravenously when faster-acting medications do not provide adequate control or administered orally to reduce the frequency and severity of seizures.

Anticonvulsants may help stop an active seizure, prevent additional seizures, or make future episodes less severe. ^[1]

Phenobarbital is commonly used for long-term seizure control in horses. However, horses vary in how they respond to and eliminate the drug. Safe treatment requires consistent administration, close observation, and periodic measurement of blood phenobarbital concentrations. ^[2]

Phenobarbital is a long-acting barbiturate with anticonvulsant and sedative effects. It has been used for decades to manage recurrent seizures in people and animals and remains one of the most commonly used anticonvulsant medications in horses. ^[2]

Phenobarbital does not treat the underlying cause of seizures. Whenever possible, veterinarians investigate and treat the underlying disease or injury while controlling the seizures.

Available Forms

Phenobarbital is available in oral and injectable formulations. Oral products may include tablets, capsules, powders, solutions, or compounded preparations, depending on availability and the horse’s needs. Owners administering oral medication should follow the prescription label and their veterinarian’s instructions carefully.

Phenobarbital sodium is the water-soluble injectable form used for intravenous administration. Injectable phenobarbital must be given slowly because rapid administration or high doses can cause marked sedation and cardiovascular or respiratory depression. ^[3]

Phenobarbital is marketed under the following human and veterinary brand names:

• Luminal^®
• Luminal Sodium^®
• Solfoton^®
• Sezaby^®

Products used in horses may be labeled for people or another animal species, or they may be compounded under veterinary direction. Since there are no equine-specific formulations available, phenobarbital treatment in horses is extra-label, also known as off-label.

Extra-label use means that a veterinarian prescribes an approved drug in a manner that differs from its product label, such as for another species, dosage, route, or condition.

Drug Class: Barbiturates

Phenobarbital belongs to the barbiturate class of central nervous system depressants. Depending on the drug, dose, route of administration, and individual horse, barbiturates can produce effects ranging from mild sedation to general anesthesia, coma, and death. ^[3][4]

Barbiturates influence several neurotransmitter systems within the brain. Their most important anticonvulsant effects involve gamma-aminobutyric acid, commonly called GABA. GABA is an inhibitory neurotransmitter that reduces excessive neuronal activity. ^[3]

Other barbiturates include: ^[3]

• Pentobarbital
• Thiopental
• Thiamylal
• Methohexital

Not all barbiturates are equally useful as anticonvulsants. Pentobarbital, for example, may stop seizures by inducing profound central nervous system depression or anesthesia, but it is not considered a maintenance anticonvulsant. ^[3]

Mechanism of Action

A seizure occurs when groups of nerve cells in the brain fire excessively and simultaneously. This abnormal electrical activity may remain in one area or spread across the brain. ^[2]

Phenobarbital primarily works by strengthening the inhibitory effects of GABA, a chemical messenger in the brain. It allows more chloride to enter nerve cells, making them less likely to fire. This stabilizes the cells, raises the seizure threshold, and reduces the likelihood that abnormal electrical activity will spread. ^[2]

Phenobarbital may also: ^[3]

• Reduce stimulation caused by glutamate
• Block certain calcium channels involved in nerve signaling
• Reduce the release or effects of stimulating chemical messengers
• Lower brain activity during severe seizures

Together, these effects make nerve cells less excitable and help control seizure activity. ^[3]

Phenobarbital also slows activity in the central nervous system, which accounts for its sedative effects. These effects generally become stronger as the dose or blood concentration increases. ^[3][4]

Uses in Horses

Phenobarbital is used in horses to help control repeated or severe seizures and reduce the risk of recurrence during long-term treatment.

Acute Seizure Management

Benzodiazepines such as diazepam or midazolam are generally preferred for rapidly stopping an active seizure because they act more quickly. However, their effects are relatively brief, and repeated doses may be required. ^[3]

Phenobarbital may be given intravenously for acute seizures when: ^[3]

• Seizures continue despite benzodiazepine treatment
• Seizures repeatedly return after short-term control
• Longer-lasting anticonvulsant activity is required
• Benzodiazepines are unavailable
• A horse or foal requires a loading dose before maintenance treatment

A loading dose is a larger initial dose used to reach an effective blood concentration more quickly.

Intravenous phenobarbital can produce therapeutic blood concentrations within approximately 20 minutes, but its clinical effect may develop more slowly than that of benzodiazepines. It should be administered as a controlled infusion rather than a rapid injection. ^[5]

In foals, phenobarbital is commonly used for repeated seizures that continue despite benzodiazepine administration. However, its relatively slow onset makes it unsuitable as the only treatment for an ongoing episode of status epilepticus – a seizure lasting more than 5 minutes or repeated seizing without regaining consciousness. ^[3]

Maintenance Treatment

Phenobarbital is commonly considered the drug of choice for maintenance anticonvulsant therapy in horses. It is usually administered orally once the initial seizure episode has been controlled. ^[2]

Long-term anticonvulsant treatment may be considered when: ^[2][5]

• A horse has a diagnosed brain condition linked to seizures
• Seizures are becoming more frequent, lasting longer, or becoming more severe
• The horse has several seizures close together
• Generalized seizures cause the horse to fall, become injured, or put handlers at risk
• A foal needs repeated doses of short-acting medication
• Seizures continue to recur over several days
• The risk of another seizure outweighs the risks and practical demands of long-term treatment

The goal is not always to stop seizures completely. In some horses, treatment is considered successful if seizures happen less often, last for less time, or are less severe without causing unacceptable side effects. Treatment may continue for months, years, or possibly for the rest of the horse’s life. ^[5]

Affected horses may remain unsafe to ride or handle even when treatment reduces their seizures. Decisions about treatment and safe handling should be based on the individual horse, the cause, frequency, and severity of the seizures, and the response to medication. ^[5]

Administration Routes

Phenobarbital may be administered intravenously for acute seizure management or orally for maintenance treatment. The appropriate route depends on the horse’s condition and treatment needs.

Phenobarbital use in horses is generally extra-label, so there is no single established dosage. Dosing must be individualized based on the horse’s age, seizure severity, whether the drug is being used for emergency or long-term treatment, and the horse’s response.

Published equine protocols include intravenous loading and maintenance regimens for adult horses and foals, followed by oral maintenance treatment once seizures are controlled. However, these protocols are starting points for veterinary decision-making, not owner-directed dosing instructions. ^[2][3][4][5]

Intravenous Administration

When phenobarbital is administered intravenously, the veterinarian gives it in a controlled clinical setting with appropriate monitoring. Injectable phenobarbital may be diluted when indicated, delivered slowly through a secure intravenous catheter, and adjusted based on the horse’s response. ^[3]

During intravenous administration, the veterinary team monitors respiratory and cardiovascular function because rapid or excessive administration increases the risk of respiratory and cardiovascular complications, including respiratory depression, bradycardia, hypotension, hypoperfusion, sedation, and hypothermia. ^[3]

Oral Administration

Oral phenobarbital is used primarily for long-term seizure control and is well-absorbed in horses. ^[2]

Medication should be administered consistently at the prescribed times. Missed or irregular doses may allow blood concentrations to fall and reduce seizure control, particularly in horses that eliminate the drug rapidly. ^[2][6]

Storage

Phenobarbital products should be stored according to the manufacturer’s or dispensing pharmacy’s instructions. Tablets and oral preparations are generally kept in tightly closed, child-resistant containers at controlled room temperature and protected from moisture and excessive heat.

Injectable products should be protected from inappropriate temperature exposure and inspected for discoloration, particles, or damage before use.

Safety Warnings & Precautions

Intravenous loading should be performed in a setting where the horse can be closely observed and respiratory or cardiovascular support can be provided. Horses that are recumbent, have severe neurologic signs, or are actively seizing must also be protected from injury.

Phenobarbital is a controlled substance, so its use is subject to additional legal and handling requirements. Owners should give the medication only to the horse for whom it was prescribed, follow the prescription label exactly, and never share leftover medication with another animal. ^[4]

With repeated use, phenobarbital can cause the liver to break down the drug more quickly. As a result, blood concentrations may decrease over time, and the treating veterinarian may need to adjust the dose. Phenobarbital can also cause the body to process some other medications faster, potentially reducing their effects. ^[6]

Monitoring

Managing a horse with epilepsy or recurring seizures requires ongoing monitoring to help ensure seizures are well-controlled.

Owners should keep a detailed seizure and medication log that records: ^[2]

• Date and time of each seizure
• Approximate duration
• Type and severity of signs
• Recovery time
• Behavior between seizures
• Possible triggers
• Missed or delayed doses
• Observed medication side effects

Video recordings can help the veterinarian evaluate suspected seizures when they can be obtained safely. ^[2]

Serum phenobarbital concentrations are checked periodically because the drug’s metabolism varies among horses and may change during treatment. Peak and trough measurements may initially be taken several days after treatment begins, although the horse’s clinical response remains equally important. ^[2]

Additional monitoring may be appropriate in specific scenarios, such as:

• After treatment is initiated
• After any dosage change
• When seizures remain poorly controlled
• When side effects appear
• Periodically during long-term treatment
• When another interacting medication is added or discontinued

The goal is to use the lowest phenobarbital concentration that controls seizures without causing unacceptable side effects.

Contraindications

Contraindications refer to circumstances where a drug should be avoided or used with caution.

Phenobarbital should not be used in horses with a known hypersensitivity to barbiturates. It should be used cautiously in horses with: ^[4]

• Significant liver dysfunction
• Severe respiratory disease
• Heart disease
• Dehydration
• Shock, blood loss, or low blood pressure
• Anemia
• Kidney disease or reduced urine production
• Low body temperature
• Severe debilitation

Because phenobarbital is primarily metabolized by the liver, liver disease may cause the drug to remain in the body longer and increase the risk of toxicity. Kidney disease may also slow the removal of phenobarbital that has not been broken down by the body. ^[5]

The sedative and respiratory effects of phenobarbital are particularly important in neonatal foals. Treatment decisions must balance seizure control against the risks of respiratory depression and reduced nursing, as well as hypothermia and low blood pressure. ^[3]

Side Effects

Phenobarbital is generally well-tolerated in horses when appropriately dosed and monitored. Drowsiness or sedation is the most commonly reported side effect.

Other possible side effects of equine barbiturate use include: ^[2]

• Drowsiness
• Excessive sedation
• Ataxia or incoordination
• Weakness
• Reduced alertness
• Decreased nursing in foals
• Respiratory depression
• Bradycardia (low heart rate)
• Hypotension (low blood pressure)
• Hypothermia (low body temperature)

Sedation in neonatal foals may persist for several hours and interfere with nursing. Foals receiving phenobarbital should be monitored closely for changes in body temperature, blood pressure, respiratory rate, alertness, and ability to nurse. ^[2]

Sedation may decrease during the first few weeks of treatment, although this has not been established specifically in horses. Persistent or excessive sedation should be reported to the treating veterinarian, who may assess the dose and serum phenobarbital concentration. ^[1][5]

Acute Toxicity (Overdose)

Phenobarbital overdose produces excessive central nervous system depression. Signs may include: ^[3]

• Severe sedation
• Marked ataxia
• Recumbency
• Weak or absent responses
• Low body temperature
• Slow or shallow breathing
• Low blood pressure
• Coma

Severe toxicity can cause respiratory or cardiovascular collapse. Treatment is primarily supportive and may include airway management, oxygen, ventilation, intravenous fluids, temperature support, and cardiovascular monitoring. Measures that increase phenobarbital elimination may be considered in selected cases. ^[4]

Drug Interactions

Phenobarbital can interact with other medications by changing how quickly the liver processes them or by increasing their effects on the brain and nervous system.

Medications That May Increase Phenobarbital’s Effects

Tetracyclines and chloramphenicol may slow the liver’s breakdown of phenobarbital. This can prolong its effects and increase the risk of excessive sedation or toxicity. Monoamine oxidase inhibitors (MAOIs) may also prolong phenobarbital’s effects. ^[2][4]

Drugs that also depress the central nervous system may have additive effects when used with phenobarbital. This can increase drowsiness, incoordination, or respiratory depression. Examples include:

• Sedatives
• Anesthetics
• Opioids
• Antihistamines
• Phenothiazines
• Other barbiturates

Medications That May Become Less Effective

Phenobarbital can speed up the liver’s processing of certain medications, which may lower their blood concentrations and reduce their effects. These may include:

• Certain antimicrobials
• Corticosteroids
• Anticoagulants
• Heart medications
• Hormones
• Other anticonvulsants

Rifampin may also speed up phenobarbital metabolism and shorten its effects. ^[3][4]

Other Important Drug Interactions

Phenytoin and phenobarbital may alter each other’s blood concentrations, so additional monitoring may be needed. ^[4]

Caution is also advised when using ivermectin in horses receiving anticonvulsants because loss of seizure control has been reported. ^[2][5]

Regulatory Status & Legal Considerations

Phenobarbital is a prescription medication and controlled substance. It may only be obtained, stored, and administered under the direction of a licensed veterinarian. Because it has sedative effects and potential for misuse, phenobarbital is subject to additional legal controls beyond those that apply to many other prescription medications. ^[4]

Phenobarbital use in horses is generally extra-label. This means a veterinarian prescribes an approved medication in a way that differs from the product label, such as for another species, route, dose, or condition.

The veterinarian is responsible for determining whether phenobarbital is appropriate for the individual horse, providing administration instructions, monitoring response to treatment, and adjusting the dose when needed.

Owners should follow the prescription label exactly and should not share phenobarbital between horses, use leftover medication, change the dose, or stop treatment without veterinary direction. Abrupt discontinuation can trigger severe or recurrent seizures.

Status in Competition

Phenobarbital is classified as a banned substance under FEI rules. Banned substances are not permitted in competition horses at any time, whether in or out of competition. A positive test may result in elimination, sanctions, loss of ranking, or public notification. ^[7]

Because phenobarbital can cause sedation and affect performance, horses receiving ongoing treatment may be unsuitable for FEI competition. ^[7] No published detection time or recommended withdrawal period is available for phenobarbital. ^[8]

A withdrawal period does not automatically make a horse eligible to compete after receiving a banned substance. Owners and veterinarians should check the current FEI database or applicable competition rules before entering a treated horse. ^[8]

Frequently Asked Questions

Here are some frequently asked questions about phenobarbital use in horses:

Summary

Phenobarbital is a long-acting anticonvulsant used to manage recurrent or severe seizures in horses and foals. Treatment requires veterinary supervision, consistent administration, and ongoing monitoring to balance seizure control with potential adverse effects.

• Veterinarians may use IV phenobarbital for acute control or oral phenobarbital for maintenance therapy
• Common side effects include sedation, drowsiness, ataxia, weakness, and reduced alertness
• Foals require close monitoring because sedation may interfere with nursing
• Serum testing helps guide dosing because metabolism and blood concentrations vary between horses
• Phenobarbital can interact with other drugs and must be tapered gradually, not stopped abruptly
• Horses receiving phenobarbital may be unsafe to ride and are not eligible for FEI competition