Pyrimethamine/sulfadiazine Sodium/folic Acid

Combination antiprotozoal for treatment of equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona; pyrimethamine and sulfadiazine act synergistically to inhibit folate synthesis in the parasite. Sequential antifolate combination: pyrimethamine inhibits dihydrofolate reductase (DHFR); sulfadiazine inhibits dihydropteroate synthase (DHPS) → blocks two sequential steps in folate synthesis → bactericidal/antiprotozoal synergy. Active against Sarcocystis neurona (EPM) and Toxoplasma at lower concentrations than either drug alone. Key information Antiprotozoal combination (EPM in horses). Sulfonamide component: ensure hydration. Folate antagonist — monitor CBC; supplement with folinic acid if prolonged use. Not for use in pregnant mares.

Oral tablets: Administer orally. May be given with food to improve compliance and reduce GI upset. Administer with food to reduce GI adverse effects. Tablets may be crushed (wear gloves — teratogenic). Administer daily for full prescribed course. Supplement folic acid concurrently. Store at room temperature; protect from light. If a dose is missed Administer as soon as you remember. If it is almost time for the next scheduled dose, skip the missed dose and resume the normal schedule. Never double up doses to make up for a missed one. Do not skip doses — pyrimethamine is often used with a sulfonamide. Supplement with folinic acid as directed.

Bone marrow suppression (cumulative folate depletion from both components — CBC every 2 weeks; supplement folic acid). GI: vomiting, anorexia, diarrhea. Neurological: seizures (folate deficiency). Teratogenic — avoid in pregnancy. Crystalluria (sulfadiazine).

For animals Avoid in pregnant animals. Teratogenic. Supplement folic acid (5 mg/day) to prevent myelosuppression. Monitor CBC periodically with prolonged use. every 2 weeks. Drug interaction: trimethoprim (additive antifolate toxicity — use cautiously together). Reduce dose or extend interval in renal impairment. Use with caution in hepatic impairment. For people handling this medication Wear gloves when handling. Wash hands thoroughly with soap and water after handling. Antifolate combination — Keep out of reach of children. Pregnant women and women of childbearing age should avoid contact. Teratogenic. Wear gloves. Supplement folic acid if handling regularly. Keep out of reach of children.

Contact your veterinarian. Pyrimethamine overdose causes bone marrow suppression (folate antagonist), GI signs, and neurological effects. Folinic acid supplementation required. Contact your veterinarian, a veterinary emergency clinic, or the ASPCA Animal Poison Control Center (888-426-4435) immediately. Do not induce vomiting unless specifically directed by a veterinary professional.