Folate antagonist (antimalarial/antiprotozoal) Pregnancy: Teratogenic in animals; no adequate and well-controlled studies in pregnant women. Use during pregnancy only if the potential benefit justifies the potential risk to the fetus. Concurrent folinic acid strongly recommended when used in pregnancy.

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.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 50 to 75 mg daily (starting dose, for toxoplasmosis)

Route: Oral

Frequency: Once daily

For treatment of toxoplasmosis, given together with 1-4 g daily of a sulfonamide of the sulfapyrimidine type (e.g. sulfadoxine); concurrent folinic acid is strongly recommended in all patients. The starting dose is ordinarily continued for 1-3 weeks depending on response and tolerance, then reduced to about one half of that previously given for each drug and continued for a further 4-5 weeks. Dosage must be carefully adjusted for maximum effect and minimum side effects; there is marked variation in tolerance and young patients may tolerate higher doses than older individuals. US labelling (Teva) — verify against UK SPC.

Paediatric dose

Dose: 1 mg/kg

Route: Oral

Frequency: Divided into 2 equal daily doses

Max: Not stated

For toxoplasmosis. After 2-4 days this dose may be reduced to one half and continued for approximately 1 month; the usual paediatric sulfonamide dosage is used in conjunction. US labelling (Teva) — verify against UK SPC and a children's formulary.

Dose auto-extracted from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Paediatric weight-based calculator

Patient weight (kg)

Verify in a children's formulary

Contraindications

Known hypersensitivity to pyrimethamine or to any component of the formulation
Documented megaloblastic anaemia due to folate deficiency

Side effects

Hypersensitivity reactions, occasionally severe (Stevens-Johnson syndrome, toxic epidermal necrolysis, erythema multiforme, anaphylaxis)
Anorexia and vomiting (with toxoplasmosis doses; minimised by giving with meals)
Megaloblastic anaemia, leukopenia, thrombocytopenia, pancytopenia, neutropenia
Atrophic glossitis, haematuria, disorders of cardiac rhythm
Pulmonary eosinophilia (rare)

Interactions

Other antifolic drugs or agents associated with myelosuppression (sulfonamides, trimethoprim-sulfamethoxazole, proguanil, zidovudine, cytostatic agents e.g. methotrexate): may increase the risk of bone marrow suppression
Lorazepam: mild hepatotoxicity reported with concomitant use
May be used with sulfonamides, quinine and other antimalarials, and with other antibiotics

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. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Pathways