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a folic acid antagonist used as an antimalarial agent, especially for suppressive prophylaxis, and also used concomitantly with a sulfonamide in treatment of toxoplasmosis.



Pharmacologic class: Folic acid antagonist

Therapeutic class: Antiprotozoal, antimalarial

Pregnancy risk category C


Inhibits reduction of dihydrofolic acid to tetrahydrofolic acid (folinic acid) by binding to and reversibly inhibiting dihydrofolate reductase


Tablets: 25 mg

Indications and dosages

To control plasmodia transmission and suppress susceptible strains Adults and children ages 10 and older: 25 mg P.O. daily for 2 days, given with a sulfonamide


Adults: Initially, 50 to 75 mg P.O. daily for 1 to 3 weeks, given with a sulfonamide. Depending on response and tolerance, reduce dosages of both drugs by 50% and continue therapy for 4 to 5 more weeks.

Children: 1 mg/kg P.O. daily in two equally divided doses for 2 to 4 days, then reduced to 0.5 mg/kg/day for approximately 1 month. Alternatively, 2 mg/kg (up to 100 mg) P.O. daily in two equally divided doses for 3 days, then 1 mg/kg (up to 25 mg) in two equally divided doses for 4 weeks, given with sulfadiazine for 4 weeks.

Prophylaxis of malaria caused by susceptible plasmodia strains

Adults and children older than age 10: 25 mg P.O. weekly

Children ages 4 to 10: 12.5 mg P.O. weekly

Infants and children younger than age 4: 6.25 mg P.O. weekly

Off-label uses

• Isosporiasis
• Prophylaxis of Pneumocystis jiroveci pneumonia


• Hypersensitivity to drug
• Megaloblastic anemia caused by folate deficiency
• Concurrent folate antagonist therapy


Use cautiously in:
• anemia, bone marrow depression, hepatic or renal impairment, G6PD deficiency
• history of seizures
• patients more than 16 weeks pregnant
• breastfeeding patients.


• Administer with meals.
• When giving tablets to young children, crush them and administer as oral suspension in water, cherry syrup, or sweetened solution.
• Know that because of worldwide resistance to pyrimethamine, its use alone to prevent or treat acute malaria is no longer recommended.
• Be aware that fixed combination of pyrimethamine and sulfadoxine is available and has been used for uncomplicated mild to moderate malaria caused by chloroquineresistant Plasmodium falciparum and for presumptive self-treatment by travelers.

Adverse reactions

CNS: headache, light-headedness, insomnia, malaise, depression, seizures

CV: arrhythmias

EENT: dry throat

GI: nausea, vomiting, diarrhea, anorexia, atrophic glossitis

GU: hematuria

Hematologic: megaloblastic anemia, leukopenia, pancytopenia, thrombocytopenia

Metabolic: hyperphenylalaninemia

Respiratory: pulmonary eosinophilia

Skin: pigmentation changes, dermatitis, erythema multiforme, toxic epidermal necrolysis, Stevens-Johnson syndrome

Other: fever, anaphylaxis


Drug-drug. Lorazepam: hepatotoxicity Myelosuppressants (including antineoplastics): increased risk of bone marrow depression

Drug-diagnostic tests. Platelets, white blood cells: decreased counts

Patient monitoring

• Monitor CBC. Watch for evidence of blood dyscrasias.
• Assess for signs and symptoms of folic acid deficiency.
• Closely monitor neurologic and cardiovascular status. Stay alert for seizures and arrhythmias.

Watch for evidence of erythema multiforme, including sore throat, cough, mouth sores, rash, iritic lesions, and fever. Report early signs before condition can progress to Stevens-Johnson syndrome.

Patient teaching

• Advise patient to take with meals.

Tell patient to discontinue drug and contact prescriber at first sign of rash.
• Caution patient to avoid driving and other hazardous activities until he knows how drug affects concentration and alertness.
• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs and tests mentioned above.


/pyr·i·meth·amine/ (pir″ĭ-meth´ah-mēn) a folic acid antagonist, used in the treatment of malaria and of toxoplasmosis.


(pī′rə-mĕth′ə-mēn′, -mĭn)
A folic acid antagonist, C12H13ClN4, often administered in combination with a sulfonamide, used as an antimalarial and to treat toxoplasmosis.


an antimalarial.
indications It is prescribed in the treatment of malaria and toxoplasmosis.
contraindications Use is contraindicated in chloroguanide-resistant malaria, in patients with megaloblastic anemia resulting from folate deficiency, and in patients who are hypersensitive to the drug. Caution is recommended in use of the drug to treat toxoplasmosis because dosages needed may be at a toxic level.
adverse effects Among the more serious adverse effects, primarily with large doses, are megaloblastic anemia, atrophic glossitis, leukopenia, and convulsions.


Parasitology An anti-protozoal combined with sulfadiazine for managing toxoplasmosis Adverse effects BM damage, rashes; folic acid deficiency can cause a painful burning tongue, loss of taste, anemia. See Toxoplasmosis.


A drug used in the treatment of MALARIA and TOXOPLASMOSIS. The drug is on the WHO official list. Brand names are Daraprim and, with other antimalarial drugs, Fansidar and Maloprim.

pyrimethamine (pir´imeth´əmēn),

n a folic acid inhibitor that acts against
T. gondii and
P. spp., which cause toxoplasmosis and malaria.


a folic acid antagonist used in combination with sulfonamides in the treatment of toxoplasmosis and avian coccidiosis.
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As per the Centers for Disease Control and Prevention, pyrimethamine works to prevent folic acid synthesis in the parasite T.
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falciparum and the loci associated with pyrimethamine resistance.
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Others received pills combining sulfadoxine and pyrimethamine, an old, inexpensive therapy still used extensively in Africa (SN: 11/11/06, p.
Systemic disease responds to sulfonamide and pyrimethamine.