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Folate antagonist (antimalarial/antiprotozoal)

Pyrimethamine

Brand names: Daraprim

Pyrimethamine is an antiprotozoal folate-antagonist used, usually in combination, for the treatment and prophylaxis of toxoplasmosis and as part of historical antimalarial regimens.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

Clinical monograph

How it works

It selectively inhibits protozoal dihydrofolate reductase, depleting reduced folates needed for nucleic acid synthesis and thereby halting parasite replication.

Prescribing in practice

  • It suppresses bone marrow by interfering with folate metabolism, so folinic acid (calcium folinate) is co-administered to protect haematopoiesis and full blood counts must be monitored.
  • It is typically combined with sulfadiazine for toxoplasmosis, exploiting sequential blockade of the folate pathway.
  • Caution and folate supplementation are needed in pregnancy and in patients with existing haematological or folate deficiency.

Monitoring

Monitor full blood count regularly during treatment because of the risk of megaloblastic anaemia, leucopenia and thrombocytopenia.

Counselling the patient

  • You will usually be given folinic acid alongside this medicine to protect your blood counts.
  • Report sore throat, mouth ulcers, unusual bruising or bleeding.
  • Attend for the blood tests arranged to monitor treatment.

Evidence & guidelines

Pyrimethamine with sulfadiazine and folinic acid is the established standard regimen for toxoplasmosis encephalitis.

Reference: BHIVA; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

Pathways