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pyrimethamine (peer-i-meth-a-meen)



Therapeutic: antimalarials, antiprotozoals

Pharmacologic: folic acid antagonist


Used in combination with other antimalarials in the treatment of chloroquine-resistant malaria. Used in combination with a sulfonamide in the treatment of toxoplasmosis. Unlabeled Use: Used in combination with other agents (sulfonamides, dapsone) in the treatment of Pneumocystis carinii pneumonia.


Binds to an enzyme in the protozoa, which results in depletion of folic acid. Therapeutic Effects: Death and arrested growth of susceptible organisms (protozoa).

Adverse Reactions/Side Effects

CNS: SEIZURES (HIGH DOSES), headache, insomnia, lightheadedness, malaise, mental depression. Resp: dry throat, pulmonary eosinophilia. CV: ARRHYTHMIAS (LARGE DOSES). GI: atrophic glossitis (high doses), anorexia, diarrhea, nausea. GU: hematuria. Derm: abnormal pigmentation, dermatitis. Hemat: megaloblastic anemia (high doses), pancytopenia, thrombocytopenia. Misc: fever.


Examination and Evaluation

  • Be alert for new seizures or increased seizure activity, especially at the onset of drug treatment. Document the number, duration, and severity of seizures, and report these findings immediately to the physician.

  • Assess heart rate, ECG, and heart sounds, especially during exercise (See Appendices G, H). Report any rhythm disturbances or symptoms of cardiac dysfunction, including palpitations, chest discomfort, shortness of breath, fainting, and fatigue/weakness.

  • Monitor signs of thrombocytopenia (bruising, nose bleeds, bleeding gums) or unusual weakness and fatigue that might be due to megaloblastic anemia or other blood dyscrasias. Report these signs to the physician.

  • Assess any breathing problems, and report difficult, labored breathing or abnormal breath sounds (rales, crackles). Notify physician because these may be signs of pulmonary eosinophilia.

  • Monitor severe or prolonged malaise, mental depression, or other changes in mood or behavior. Notify physician if these changes become problematic.

  • If treating malaria, monitor any changes in symptoms (decreased fever, chills, sweating) to help document whether antimalarial drug therapy is successful.


  • Because of the risk of arrhythmias, use caution during aerobic exercise and other forms of therapeutic exercise. Assess exercise tolerance frequently (blood pressure, heart rate, fatigue levels), and terminate exercise immediately if any untoward responses occur (See Appendix L).

Patient/Client-Related Instruction

  • Remind patient to take this drug as directed when treating malaria even if patient is asymptomatic.

  • Instruct patient to report other untoward side effects such as severe or prolonged headache, fever, sleep loss, throat irritation, bloody urine, skin reactions (hyperpigmentation, dermatitis), GI problems (nausea, diarrhea, loss of appetite), or changes in the tongue resulting in a smooth and tender/painful tongue surface (atrophic glossitis).


Absorption: Well absorbed after oral administration.

Distribution: Widely distributed with high concentrations achieved in blood cells, kidneys, lungs, liver, and spleen. ...

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