Skip to Content

Pyrimethamine Dosage

Medically reviewed on August 10, 2017.

Applies to the following strengths: 25 mg

Usual Adult Dose for Malaria Prophylaxis

25 mg orally once a week. Prophylaxis should begin one week prior to departure and continue for at least 6 to 10 weeks following exposure.

Usual Adult Dose for Toxoplasmosis

Initially: 50 to 75 mg orally once a day with 1 to 4 g of a sulfonamide (e.g., sulfadoxine, sulfadiazine). Continue for 1 to 3 weeks, depending on response and tolerance. Dosage for each drug may then be reduced by one-half and continued for an additional 4 or 5 weeks. Patients receiving pyrimethamine should also receive folinic acid.

Usual Adult Dose for Toxoplasmosis - Prophylaxis

1 mg/kg or 15 mg/m2 (max 25 mg) orally every day plus folinic acid (leucovorin) 5 mg orally every 3 days plus sulfadiazine 85 to 120 mg/kg/day divided into 2 to 4 oral doses. Clindamycin 20 to 30 mg/kg/day may be used in place of sulfadiazine as an alternative regimen.

Usual Adult Dose for Pneumocystis Pneumonia Prophylaxis

50 to 75 mg orally once a week. Pyrimethamine is used in combination with dapsone and leucovorin. This is considered an alternative regimen for patients who do not tolerate trimethoprim-sulfamethoxazole.

Usual Pediatric Dose for Malaria Prophylaxis

Less than 4 years: 6.25 mg orally once a week.

4 to 10 years: 12.5 mg orally once a week.

Usual Pediatric Dose for Toxoplasmosis

Newborns and infants:
Initial: 2 mg/kg/day orally divided every 12 hours for 2 days, then 1 mg/kg/day once daily given with sulfadiazine for the first 6 months; next 6 months: 1 mg/kg/day 3 times per week with sulfadiazine; oral folinic acid 5 to 10 mg 3 times per week should be administered to prevent hematological toxicity.

1 to 12 years: 2 mg/kg/day divided every 12 hours for 3 days followed by 1 mg/kg/day (maximum 25 mg/day) once daily or divided twice daily for 4 weeks given with sulfadiazine; oral folinic acid 5 to 10 mg 3 times per week should be administered to prevent hematological toxicity.

Renal Dose Adjustments

Data not available

Liver Dose Adjustments

Data not available


Pyrimethamine is a folate antagonist and is contraindicated in patients with megaloblastic anemia caused by folate deficiency.

Patients being treated for toxoplasmosis with high doses of pyrimethamine should be monitored for signs or symptoms and laboratory abnormalities suggestive of folate deficiency. If folate deficiency occurs, the dosage should be reduced or the drug discontinued, depending on the response of the patient. Folinic acid (leucovorin) should be administered until normal hematopoiesis is restored.

Pyrimethamine may be carcinogenic. Pyrimethamine therapy has been reported to produce a significant increase in the number of lung tumors in mice when given intraperitoneally at doses of 25 mg/kg.

The recommended dosage of chemoprophylaxis of malaria should not be exceeded. A small "starting" dose for toxoplasmosis is advised in patients with convulsive disorders to avoid the potential nervous system toxicity of pyrimethamine. Pyrimethamine should be administered with caution in patients with impaired renal or hepatic function or in patients with possible folate deficiency, such as individuals with malabsorption syndrome, alcoholism, or pregnancy.

Semiweekly blood and platelet counts are recommended for patients being treated for toxoplasmosis.

Patients should be warned to discontinue pyrimethamine therapy if a rash develops and to promptly seek medical attention. Additionally, patients should be advised to contact their physician if sore throat, pallor, purpura, and/or glossitis develop.

The drug may be taken with meals to minimize anorexia and vomiting.

Pyrimethamine therapy should be kept out of the reach of infants and children as they are extremely susceptible to adverse effects from an overdose. Deaths in pediatric patients have been associated after accidental ingestion.

Clinical studies of pyrimethamine therapy did not include sufficient numbers of patients aged 65 and over to determine whether they respond differently from younger patients. Other observed clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for elderly patients should be cautious, usually starting at low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.


Data not available

Other Comments

Pyrimethamine may be taken with meals.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.