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peye-ruh-METH-uh-meen - Pronunciation guide

Brand Names

  • Daraprim
  • Rebalance


Rx symbolPyrimethamine, along with a sulfa antibiotic, is used by veterinarians in the treatment of equine protozoal myeloencephalitis. The combination of drugs is effective in blocking the process of metabolism of the protozoa and a step in the protazoa's folic acid synthesis, creating a synergistic effect against Sarcocystis neurona. Although the combination of drugs does not kill the protozoa, it inhibits further growth or reproduction.


Pyrimethamine is used along with either sulfadiazine or sulfamethoxazole in the treatment of equine protozoal myeloencephalitis. An FDA-approved combination of sulfadiazine and pyrmethamine is now available.

Dosage and Administration

Prescription medicationPyrimethamine
Method Dosage
(click row for calculator)
Concentration Period Duration
Oral 1 mg/kg 1 25 mg/tablet Daily 30-180 days


  • 1Use in combination with 16.7 mg of sulfadiazine or sulfamethoxazole per kg of body weight every twelve hours.
  • 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

The combination of pyrimethamine and sulfa may cause anemia, decreased platelets, decreased white blood cell counts, and suppressed bone marrow. Loss of appetite, diarrhea, depression, or lethargy are other side effects. Temporary neurologic symptoms can occur in horses and is thought to be due to inflammation caused by dying parasites in the central nervous system.


Pyrimethamine should be used with extreme caution in horses with blood or bone marrow problems. The diagnosis and treatment of equine protozoal myeloencephalitis is a controversial area, with newer drugs that work by killing the protazoa now available. In all cases, a veterinarian should determine what is best for a particular horse.

Pyrimethamine is not FDA-approved as a drug for horses, but it's use is commonly accepted practice. It is a prescription drug restricted to use by or on the lawfully written or oral order of a licensed veterinarian.

It is unlikely that these drugs would be used in a horse that is actively competing, but would be prohibited in any drug-free competition. It is important to check with the individual regulatory organization.


No drug interactions are listed for horses.


No information is available.


Sulfaquinoxalina Pyrimethamine Solution  Sulfaquinoxalina Pyrimethamine Solution


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.