Pyrimethamine
Also known as: Daraprim, Malocide, Pirimecidan
> **Important:** This medication is used to treat serious parasitic infections and requires careful monitoring. - **Give exactly as directed:** It is often given with other antibiotics (like sulfa drugs) for maximum effect. Do not skip doses. - **Watch for side effects:** Contact your veterinarian immediately if your pet becomes lethargic, stops eating, vomits, or shows signs of unusual bleeding or bruising (such as pale gums or red spots on the skin). - **Dosing challenges in cats:** The commercial tablets are 25 mg, which makes accurate dosing for cats very difficult. The tablets also taste bad to cats. Your veterinarian will likely recommend a specially compounded liquid or capsule to ensure accurate and easier dosing. - **Pregnancy warning:** Do not handle this medication if you are pregnant, and do not use it in pregnant pets unless explicitly directed by your veterinarian.
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
- Toxoplasmosis
- Toxoplasmosis (enteroepithelial cycle)
- Toxoplasmosis (extraintestinal cycle)
- Protozoal myocarditis
- Toxoplasmosis
- Equine protozoal myeloencephalitis
- Coccidian organisms in raptors (Toxoplasmosis, Atoxoplasmosis, Sarcocystis)
- Toxoplasmosis
- Toxoplasmosis
- Neospora
- Hepatazoon canis
- Hepatazoon americanum
- Hepatazoon americanum
- Neosporosis
How it is given
Possible side effects
- Small animals: Anorexia, malaise, vomiting, depression
- Small animals: Bone marrow depression (anemia, thrombocytopenia, leukopenia)
- Cats: Highly susceptible to adverse reactions (noted 4-6 days after starting combination therapy)
- Horses: Leukopenias, thrombocytopenia, and anemias
- Bone marrow suppression (anemia, leukopenia, thrombocytopenia)
- Anorexia
- Vomiting
- Lethargy
- Depression
Cautions & contraindications
- Known hypersensitivity to pyrimethamine
- Preexisting hematologic disorders (use with extreme caution)
- Pregnancy (potentially teratogenic, weigh risks vs. benefits)
- Pregnancy (teratogenic)
- Pre-existing bone marrow suppression
- Hypersensitivity to pyrimethamine
- **Hematologic Toxicity:** Hematologic effects can develop rapidly. Frequent monitoring is recommended, particularly if therapy persists longer than 2 weeks. Oral administration of folinic acid, folic acid, or Brewer's yeast has been suggested to alleviate adverse effects. **Feline Sensitivity:** Some clinicians recommend avoiding its use in cats entirely because of its adverse effect profile. **Pregnancy:** Pyrimethamine is teratogenic in rats and fetal abnormalities have been seen in foals. FDA Category C. Weigh risks against benefits. Excreted in maternal milk; consider using milk replacer.
Drug interactions
- p-Aminobenzoic acid (PABA): Reportedly antagonistic towards the activity of pyrimethamine; clinical significance is unclear.
- Sulfonamides: Synergistic with pyrimethamine in activity against toxoplasmosis and malaria.
- 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.
- Phenobarbital: May increase folate deficiency
- Phenytoin: May increase folate deficiency
Frequently asked questions
What is Pyrimethamine used for in pets?
> **Important:** This medication is used to treat serious parasitic infections and requires careful monitoring. - **Give exactly as directed:** It is often given with other antibiotics (like sulfa drugs) for maximum effect. Do not skip doses. - **Watch for side effects:** Contact your veterinarian immediately if your pet becomes lethargic, stops eating, vomits, or shows signs of unusual bleeding or bruising (such as pale gums or red spots on the skin). - **Dosing challenges in cats:** The commercial tablets are 25 mg, which makes accurate dosing for cats very difficult. The tablets also taste bad to cats. Your veterinarian will likely recommend a specially compounded liquid or capsule to ensure accurate and easier dosing. - **Pregnancy warning:** Do not handle this medication if you are pregnant, and do not use it in pregnant pets unless explicitly directed by your veterinarian.
Is Pyrimethamine 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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