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PRIM-ih-doan or PRY-meh-doan - Pronunciation guide

Brand Names

  • Neurosyn™ Tablets
  • PrimiTabs


Rx symbolPrimidone is a medication used in the management of grand mal, psychomotor, and focal seizures in horses and foals. Primidone is also used in horses with seizures induced by toxins or adverse drug effects.

In the body, primidone is converted into other chemicals, including phenobarbital, a type of barbiturate that can help control seizures by acting as an anticonvulsant. Primidone works by decreasing the excitability of the brain cells, thereby helping to prevent abnormal electrical activity from starting and helps keep such activity from spreading to other parts of the brain.

Primidone is less preferred than direct use of phenobarbital because most of its anticonvulsant activity is due to its metabolism of phenobarbital. It requires administration three times a day and it can be toxic to the liver.

One of the main advantages of primidone, unlike phenobarbital, is that it is not a controlled substance with special rules for prescribing and dispensing.


Primidone is used in the management of grand mal, psychomotor, and focal seizures, and sometimes to treat tremors.

Dosage and Administration

Prescription medicationPrimidone
Method Dosage
(click row for calculator)
Concentration Period Duration
Oral 55 mg/kg 1 250 mg/tablet Daily NA


  • 1When convulsions are frequent, the dosage should be divided and administered at intervals. When convulsions occur only every few days, or less often, daily dosage should be given at one time.
    Reduction in dosage should always be made gradually and should never be discontinued abruptly.
  • Federal law restricts this drug to use by or on the order of a licensed veterinarian.
  • Extra-label use of drugs in treating animals is allowable only by licensed veterinarians within the context of a valid veterinarian-client-patient relationship, and does not include drug use in treating animals by the layman (except under the supervision of a licensed veterinarian).
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your equine appears to feel better, the entire treatment plan should be completed to prevent relapse.
  • This medication may be available in forms and concentrations not noted in the above table. Always check the label and literature provided with the medication about the form and concentration and DO NOT USE the calculator if the information differs.
  • Calculator is for educational purposes only. Follow your veterinarian's instructions regarding use of this, or any medication.

Side Effects

Primidone may cause drowsiness and lethargy in horses


Careful monitoring of liver functions to prevent hepatotoxicity may be necessary.


No literature detailing interactions is available.


In case of overdose, an activated charcoal slurry introduced through a stomach tube will help rid the system of the drug.


Primidone TabletsPrimidone Tablets

Primitabs Primidone TabletsPrimitabs Primidone Tablets


About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.