Pyrimethamine Oral: Dosing and Precautions
Pyrimethamine guidance for toxoplasmosis and isosporiasis in immunodeficient patients, with folate-deficiency precautions.
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Guide category
Oral drugs
Safety boundary
Prescription medicine information for clinician-guided use.
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Therapeutic action
Antiprotozoal.
Indications
- Treatment and secondary prophylaxis of toxoplasmosis in immunodeficient patients, in combination with sulfadiazine or clindamycin.
- Primary prophylaxis of toxoplasmosis in immunodeficient patients, in combination with dapsone only if co-trimoxazole cannot be used.
- Second-line treatment of isosporiasis in immunodeficient patients only if co-trimoxazole cannot be used.
Forms and strengths
- 25 mg tablet.
Dose and duration
Treatment of toxoplasmosis
- Adult: 2 doses of 100 mg on D1, then 75 to 100 mg once daily for at least 6 weeks.
Secondary prophylaxis of toxoplasmosis
- Adult: 25 to 50 mg once daily, as long as necessary.
Primary prophylaxis of toxoplasmosis
- Adult: 50 to 75 mg once weekly, as long as necessary.
Treatment of isosporiasis
- Adult: 50 to 75 mg once daily for 10 days.
Contra-indications, adverse effects, precautions
- Do not administer to patients with severe renal or hepatic impairment.
- May cause gastrointestinal disturbances, seizures, leucopenia, thrombocytopenia, and megaloblastic anaemia due to folic acid deficiency.
- Administer calcium folinate to prevent folic acid deficiency.
- Avoid if possible combination with other folate antagonists such as co-trimoxazole and methotrexate.
- Monitor combination with zidovudine because of the increased risk of zidovudine-associated haematotoxicity.
- Pregnancy: contraindicated during the first trimester.
- Breast-feeding: no contra-indication; however avoid concomitant administration of other folate antagonists.
Source
MSF Essential drugs practical guidelines (January 2026)
This page combines source-derived medicine reference sections with Kenza editorial context, while leaving out the Storage and Remarks sections from the source batch.
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