# Pyrimethamine: veterinary dose and monograph

> Pyrimethamine is a veterinary antiparasitic. Dihydrofolate-reductase inhibitor used with a sulfonamide (sequential folate blockade) against Toxoplasma, Neospora, Hepatozoon and Sarcocystis neurona (EPM). Indications include toxoplasmosis (dogs, cats) with a sulfonamide; neosporosis and Hepatozoon infections (dogs, combination protocols); equine protozoal myeloencephalitis (with sulfadiazine). This monograph lists 6 reference doses for dogs, cats, horses and birds, each with route, frequency and source.

Source: https://vetmasterji.com/drugs/pyrimethamine · Last reviewed: 2026-10-01

## 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

*Pyrimethamine doses for dogs*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Toxoplasmosis (with a sulfonamide) | 0.25–1 mg/kg | PO | q24h | 14–28 days |
| Neosporosis (with TMP-sulfa) | 1 mg/kg | PO | q24h | 28 days |
| Hepatozoonosis (with TMP-sulfa + clindamycin) | 0.25–0.5 mg/kg | PO | q24h | 14 days (H. americanum) to 2–4 weeks |

- 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

*Pyrimethamine doses for cats*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Toxoplasmosis (with a sulfonamide) | 0.25–1 mg/kg | PO | q24h | 14–28 days (avoid >2 weeks continuous where possible) |

- 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

*Pyrimethamine doses for horses*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| EPM (with sulfadiazine) | 1 mg/kg | PO | q24h, 1 h before hay | 90–270 days (by response) |

- 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

*Pyrimethamine doses for birds*

| Indication | Dose | Route | Frequency | Duration |
| --- | --- | --- | --- | --- |
| Coccidian infections (raptors: Toxoplasma, Sarcocystis) | 0.5 mg/kg | PO | q12h | 14–28 days |

- 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

> **Caution:** 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.

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## Frequently asked questions

### What is the dose of Pyrimethamine for dogs?

Reference doses for Pyrimethamine in dogs: 0.25–1 mg/kg PO q24h for 14–28 days (Toxoplasmosis (with a sulfonamide)); 1 mg/kg PO q24h for 28 days (Neosporosis (with TMP-sulfa)); 0.25–0.5 mg/kg PO q24h for 14 days (H. americanum) to 2–4 weeks (Hepatozoonosis (with TMP-sulfa + clindamycin)). Confirm against the label and the patient before use.

### What is the dose of Pyrimethamine for cats?

Reference doses for Pyrimethamine in cats: 0.25–1 mg/kg PO q24h for 14–28 days (avoid >2 weeks continuous where possible) (Toxoplasmosis (with a sulfonamide)). Confirm against the label and the patient before use.

### What is the dose of Pyrimethamine for horses?

Reference doses for Pyrimethamine in horses: 1 mg/kg PO q24h, 1 h before hay for 90–270 days (by response) (EPM (with sulfadiazine)). Confirm against the label and the patient before use.

### When should Pyrimethamine not be used?

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.

### What are the side effects of Pyrimethamine in animals?

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

## Sources

- Plumb's Veterinary Drug Handbook
- Papich 5e — Pyrimethamine; Pyrimethamine and Sulfadiazine
- Papich, Papich Handbook of Veterinary Drugs, 5th edition

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VetMasterJi is a clinical decision-support and education platform for veterinary professionals and students. It does not provide veterinary advice, diagnosis or treatment for specific animals. All drug doses, calculators and differentials are references that must be independently verified before clinical use; the attending registered veterinarian remains solely responsible for every clinical decision.