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Primidone (Neurosyn®, Mylepsin®)

By: Dr. Dawn Ruben

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  • Seizure disorders or convulsions are the physical manifestations of uncontrolled electrical activity in the brain. Recurrent seizures are often classified as epilepsy (fits). While there are numerous causes of convulsions, treatments that control epileptic seizures are relatively limited.
  • Primidone belongs to the class of drugs known as anticonvulsants. Other related drugs include phenobarbital, diazepam (Valium®) and pentobarbital.
  • The exact action of primidone in the brain is unclear, but it is known to reduce the frequency and severity of seizures.
  • Primidone is broken down (metabolized) by the body to another anticonvulsant drug, phenobarbital, as well as other chemicals (metabolites).
  • Since primidone rapidly metabolizes to phenobarbital, it is uncertain what, if any, benefit is derived from treatment with primidone instead of the drug phenobarbital.
  • The frequency of serious liver side effects may be higher with primidone than with phenobarbital.
  • One advantage of primidone is that, unlike phenobarbital, primidone is not classified as a controlled substance by the Food and Drug Administration and the Drug Enforcement Administration.
  • Primidone is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.

    Brand Names and Other Names

  • This drug is registered for use in humans and animals.
  • Human formulations: Mysoline® (Wyeth-Ayerst), Myidone® (Major) and various generic preparations
  • Veterinary formulations: Neurosyn® (Techamerica), Mylepsin® (Fort Dodge) and various generic preparations

    Uses of Primidone

  • Primidone is used to reduce the frequency and severity of seizures and to control epilepsy in dogs. It is rarely used in cats.

    Precautions and Side Effects

  • While generally safe and effective when prescribed by a veterinarian, primidone can cause side effects in some animals.
  • Primidone should not be used in animals with known hypersensitivity or allergy to the drug.
  • Extreme care should be used if primidone is used in cats. veterinarians recommend that cats with seizures or epilepsy be treated with phenobarbital, not primidone.
  • Caution should be used if primidone is given to animals with anemia, heart or lung disease, liver disease or kidney impairment.
  • Primidone may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with primidone. Such drugs include narcotics, tranquilizers, barbiturates, antihistamines and corticosteroids.
  • Adverse effects include restlessness, anxiety, lethargy, increased appetite, thirst and liver problems. Liver injury can be severe and, in some dogs, fatal.

    How Primidone Is Supplied

  • Primidone is available in 50 mg and 250 mg tablets.
  • Primidone is also available in a 50 mg/ml suspension.

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • Initially, a dose of 7 to 10 mg per pound (15 to 20 mg/kg) two to three times daily is used to control convulsions.
  • This dose may be increased up to 40 mg per pound (80 mg/kg) per day if seizures continue.
  • Liver tests should be carefully monitored in dogs receiving this drug.
  • 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 pet feels better, the entire treatment plan should be completed to prevent relapse or prevent the development of resistance.

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