Description

Phenobarbital is a barbiturate that occurs in a crystalline state.  It is most commonly formulated as a tablet or injectable solution.

Usage

Phenobarbital is used as an anti-seizure medication or as a sedative for anesthesia. While not as commonly used in horses as in dogs, cats, and humans, it may be used in conjunction with other drugs to break or control seizure activity, particularly in foals.

Dosage and Administration

Phenobarbital
Method	Dosage (click row for calculator)	Concentration	Period	Duration
IV (Adult Horse)	16-20 mg/kg	Multiple available	Loading dose over 20 min.	Once
Oral (Adult Horse)	1-5 mg/kg	Multiple available	Twice daily over 20 min	Maintenance
IV (Foal)	16-20 mg/kg	Multiple available	Loading dose over 20 min	Once
Oral (Foal)	100-500mg (total dose)	Multiple available	Twice daily over 20 min	Maintenance
Notes: Federal law restricts this drug to use by or on the order of a licensed veterinarian. This is an Association of Racing Commissioners International (ARCI) Uniform Classifications for Foreign Substances (UCFS) Class 2 Drug: These are “Drugs that have a high potential to affect performance, but less of a potential than Class 1. These drugs are 1) not generally accepted as therapeutic agents in racing horses, or 2) they are therapeutic agents that have a high potential for abuse.” Phenobarbital is a human-labeled drug.  Use in animals is considered off-label, and may only be instituted under proper veterinary supervision in accordance with the Animal Medicinal Drug Use Clarification Act (AMDUCA) guidelines. Calculator is for educational purposes only. Follow your veterinarian's instructions regarding use of this, or any medication.

Side Effects

Phenobarbital can cause tiredness, anxiety, and agitation at beginning of treatment.

Precautions

Phenobarbital may potentiate (make more powerful) or decrease the effect of a wide variety of other drugs.  Consultation with your veterinarian is imperative!

Phenobarbital can cause liver damage in higher dosages and may stimulate the production of certain liver enzymes requiring monitoring by a veterinarian.

Sedation and/or ataxia often become significant concerns as serum levels reach the higher ends of the therapeutic range.

Interactions

Phenobarbital has been shown to interact in various ways with a large number of drugs.  While little work has been done to determine adverse effects or interactions in the horse, interactions may mirror those found in other animals. 

A complete list of interactions is beyond the scope of this article, however, the following drugs have been shown to have adverse effects when paired with phenobarbital:

• Acetaminophen
• Carprofen
• Monamine Oxidase (MAO) Inhibitors (e.g., amitraz, possibly selegiline)
• Phenytoin
• Rifampin

Overdose

As with drug interactions, adverse effects and toxicity signs are not as well documented in horses as in other species. However, adverse reactions may include: nervous system/behavioral changes such as agitation or increased lethargy, increases in feed and water intake and in urination, liver damage.

Overdose/toxicity signs tend to relate to the central nervous system and present as uncoordination, profound sedation, coma, and potentially death.

Treatment of phenobarbital overdose consists of removal of ingested product from the gut if appropriate and offering respiratory and cardiovascular support. Activated charcoal has been demonstrated to be of considerable benefit in enhancing the clearance of phenobarbital, even when the drug was administered parenterally. Charcoal acts as a “sink” for the drug to diffuse from the vasculature back into the gut.