Pyrimethamine and Sulfadiazine
Also known as: ReBalance, Fansidar (related compound)
### Important Administration Instructions * **Shake the bottle well** before every use to ensure the medication is properly mixed. * **Fasting is required:** Give this medication on an empty stomach, at least **1 hour before** feeding your horse hay or grain. Food can prevent the drug from being absorbed properly. ### What to Expect * **Treatment Crisis:** It is common for your horse's neurologic signs to temporarily **worsen** during the first few days to weeks of treatment. This is usually caused by inflammation as the parasites die off in the nervous system. Do not stop the medication, but do inform your veterinarian. * **Long-term Therapy:** Treatment for EPM is a marathon, not a sprint. Daily treatment is typically required for 3 to 9 months. ### When to Call the Vet Watch closely for signs of drug toxicity, which can affect the bone marrow. Contact your veterinarian immediately if you notice: * Severe depression or lethargy * Unexplained bleeding or bruising (e.g., bleeding gums) * Bloody diarrhea or severe loose stools * Complete loss of appetite
Dosage must be set by your veterinarian for your specific pet. Never give human medication or another pet’s prescription without veterinary guidance.
What it is used for
- Treatment of EPM
- Maintaining clinical remission (for horses that have had two relapses)
How it is given
Possible side effects
- Bone marrow suppression (anemia, leukopenia, neutropenia, thrombocytopenia)
- Reduced appetite or anorexia
- Loose stools or diarrhea
- Urticaria (hives)
- Treatment crisis (temporary worsening of neurologic signs due to parasite die-off)
- CNS effects (seizures, depression - though often secondary to the underlying EPM)
Cautions & contraindications
- Hypersensitivity to pyrimethamine or sulfadiazine
- Horses intended for human consumption
- Use with extreme caution in horses with **preexisting hematologic abnormalities** or those receiving other bone marrow-suppressing drugs. **Pregnancy/Nursing:** Pyrimethamine is teratogenic in rats, and fetal abnormalities have been seen in foals. Sulfas cross the placenta and are distributed into milk. Use in pregnant animals only when benefits clearly outweigh risks. Consider using milk replacer for nursing offspring. **Treatment Crisis:** Neurologic signs may worsen during the first few days to 5 weeks of treatment due to an inflammatory reaction secondary to dying parasites in the CNS.
Drug interactions
- Antacids: May decrease the bioavailability of sulfonamides if administered concurrently.
- Highly protein-bound drugs (e.g., methotrexate, phenylbutazone, thiazide diuretics, salicylates, probenecid, phenytoin, warfarin): Sulfonamides may displace other highly bound drugs, potentially increasing their free (active) plasma concentrations and risk of toxicity.
- p-Aminobenzoic acid (PABA): Reportedly antagonistic towards the activity of pyrimethamine; clinical significance is unclear.
- Trimethoprim: Use with pyrimethamine/sulfa is not recommended in humans as adverse effects may be additive; however, this combination has been used clinically in horses.
Frequently asked questions
What is Pyrimethamine and Sulfadiazine used for in pets?
### Important Administration Instructions * **Shake the bottle well** before every use to ensure the medication is properly mixed. * **Fasting is required:** Give this medication on an empty stomach, at least **1 hour before** feeding your horse hay or grain. Food can prevent the drug from being absorbed properly. ### What to Expect * **Treatment Crisis:** It is common for your horse's neurologic signs to temporarily **worsen** during the first few days to weeks of treatment. This is usually caused by inflammation as the parasites die off in the nervous system. Do not stop the medication, but do inform your veterinarian. * **Long-term Therapy:** Treatment for EPM is a marathon, not a sprint. Daily treatment is typically required for 3 to 9 months. ### When to Call the Vet Watch closely for signs of drug toxicity, which can affect the bone marrow. Contact your veterinarian immediately if you notice: * Severe depression or lethargy * Unexplained bleeding or bruising (e.g., bleeding gums) * Bloody diarrhea or severe loose stools * Complete loss of appetite
Is Pyrimethamine and Sulfadiazine safe for my pet?
Dosage must be set by your veterinarian for your specific pet. Never give human medication or another pet’s prescription without veterinary guidance.
This article is for general education and is not a substitute for professional veterinary advice. If your pet is unwell, please consult a veterinarian.
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