Pyrimethamine: veterinary dose and monograph

At a glance

Drug class

Antiparasitic

Also known as

Daraprim

Species with doses

Dogs, Cats, Horses and Birds

General dose range

0.25–1 mg/kg PO q24h — check the species tables for the indication

Routes

PO

Last reviewed

1 October 2026

Indications

• Toxoplasmosis (dogs, cats) with a sulfonamide
• Neosporosis and Hepatozoon infections (dogs, combination protocols)
• Equine protozoal myeloencephalitis (with sulfadiazine)
• Coccidian infections in raptors

Pyrimethamine dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Dogs

• Toxoplasmosis (with a sulfonamide): Papich 5e: 1 mg/kg q24h PO for 14–21 days. Plumb’s p.1031: 0.5–1 mg/kg once daily for 2 days, then 0.25 mg/kg daily for 2 weeks, with sulfadiazine 30–50 mg/kg PO divided q6–12h (Murtaugh & Ross); 0.25–0.5 mg/kg daily for 28 days (Lappin). Monitor CBC.
• Neosporosis (with TMP-sulfa): Plumb’s p.1031: 1 mg/kg once daily for 28 days with trimethoprim-sulfa 15–30 mg/kg PO q12h (Lappin; Blagburn). Papich 5e lists 5 days for Neospora caninum.
• Hepatozoonosis (with TMP-sulfa + clindamycin): Plumb’s p.1031: H. americanum — pyrimethamine 0.25 mg/kg q24h + TMP-sulfa 15 mg/kg q12h + clindamycin 10 mg/kg q8h for 14 days, then decoquinate for maintenance (Baneth); H. canis 0.25–0.5 mg/kg daily for 2–4 weeks (Lappin).

Cats

• Toxoplasmosis (with a sulfonamide): Papich 5e: 0.5–1 mg/kg q24h PO for 14–28 days. Plumb’s p.1031: 0.5–1 mg/kg daily for 2 days then 0.25 mg/kg daily for 2 weeks with sulfadiazine (Murtaugh); 0.5 mg/kg daily with sulfadiazine 30 mg/kg q12h for 7–10 days, not continuously for more than 2 weeks — folinic acid 1 mg/kg/day or folic acid 5 mg/day may reduce toxicity (Swango). Cats are prone to marrow toxicity.

Horses

• EPM (with sulfadiazine): Papich 5e and Plumb’s p.1031/1033: 1 mg/kg pyrimethamine with sulfadiazine 20 mg/kg once daily (4 mL of the combination suspension per 50 kg), 1 h before feeding hay or grain; monitor CBC; relapsing horses may be maintained twice weekly (MacKay).

Birds

• Coccidian infections (raptors: Toxoplasma, Sarcocystis): Plumb’s p.1031 (Jones 2007): raptors 0.5 mg/kg PO twice daily for 14–28 days — especially effective against Toxoplasma, Atoxoplasma and Sarcocystis.

Contraindications

Pregnancy (teratogenic). Pre-existing anaemia, leukopenia or thrombocytopenia. Use with caution in cats (more prone to toxicity) — Plumb’s advises limiting continuous use to about 2 weeks in cats.

Adverse effects

• Anorexia, vomiting, depression
• Bone marrow suppression (anaemia, neutropenia, thrombocytopenia) — folate related
• Teratogenicity

Monitoring

• CBC every 1–2 weeks during treatment (anaemia, neutropenia, thrombocytopenia)
• Clinical/neurological response

Clinical pearls

• Always pair with a sulfonamide — the combination is synergistic
• Give folinic acid (leucovorin), not folic acid, if cytopenias develop

Formulations and products

• 25 mg tablet

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

Other antiparasitics

• Piperazine
• Ponazuril
• Praziquantel
• Pyrantel pamoate
• Rafoxanide
• Ronidazole
• Sarolaner
• Selamectin

Frequently asked questions