Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
Pyrimethamine-Sulfadoxine vs ARALEN PHOSPHATE W/ PRIMAQUINE PHOSPHATE
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Pyrimethamine inhibits dihydrofolate reductase, blocking tetrahydrofolate synthesis. Sulfadoxine inhibits dihydropteroate synthase, blocking folate synthesis. Sequential blockade of folate metabolism.
Chloroquine and primaquine: Chloroquine inhibits heme polymerase in malaria parasites, preventing conversion of toxic heme to hemozoin; primaquine disrupts mitochondrial function and generates reactive oxygen species, targeting hypnozoites and gametocytes.
Treatment of chloroquine-resistant Plasmodium falciparum malaria,Prophylaxis of malaria in travelers to areas with chloroquine resistance,Treatment of toxoplasmosis (off-label, in combination with leucovorin)
Treatment of acute attacks of vivax malaria due to Plasmodium vivax,Radical cure of vivax malaria (elimination of hypnozoites),Suppression of malaria (prophylaxis) in areas with chloroquine-sensitive P. vivax
Pyrimethamine 25 mg plus sulfadoxine 500 mg per tablet; typical adult dose for acute uncomplicated malaria is 3 tablets (pyrimethamine 75 mg, sulfadoxine 1500 mg) orally as a single dose. For toxoplasmosis in immunocompromised patients: loading dose pyrimethamine 200 mg orally once, then pyrimethamine 50-75 mg orally once daily plus sulfadoxine 1000-1500 mg orally once daily (dosing based on sulfadoxine component) for 4-6 weeks, then reduce to half.
Chloroquine phosphate 600 mg base (1 g salt) orally once daily for 2 days, then 300 mg base (500 mg salt) once daily for at least 2 weeks; plus primaquine phosphate 30 mg base orally once daily for 14 days.
Pyrimethamine: ~80-120 hours; Sulfadoxine: ~100-200 hours. Long half-lives allow single-dose therapy for malaria.
Chloroquine: 40-60 days (terminal); Primaquine: 6-8 hours (terminal). Clinical context: chloroquine accumulates extensively, requiring prolonged monitoring for toxicity; primaquine, shorter half-life, once-daily dosing.
Pyrimethamine is metabolized in the liver via CYP450 isoenzymes (minor). Sulfadoxine is primarily excreted unchanged in urine, with some hepatic metabolism (acetylation).
Chloroquine: hepatic metabolism via CYP2C8 and CYP3A4; primaquine: hepatic metabolism via CYP2D6 and other enzymes.
Renal: ~60% unchanged sulfadoxine, ~5% unchanged pyrimethamine; fecal: ~10% pyrimethamine. Biliary excretion minimal.
Renal: 70% (chloroquine as unchanged drug and metabolites), 20% (primaquine as metabolites); Fecal: ~10% (chloroquine); Biliary: minor for both.
Pyrimethamine: ~87% (albumin); Sulfadoxine: ~90% (albumin).
Chloroquine: 50-65% bound to albumin; Primaquine: ~20% bound to albumin.
Pyrimethamine: 2.3-3.6 L/kg (wide tissue distribution, crosses blood-brain barrier); Sulfadoxine: 0.14-0.2 L/kg (limited to plasma and interstitial fluid).
Chloroquine: Vd 100-200 L/kg (extensive tissue distribution); Primaquine: Vd 3-5 L/kg (moderate distribution). Clinical meaning: large Vd of chloroquine indicates deep tissue compartments with slow release.
Oral: ~90-100% for both components, with high and consistent absorption.
Both: Oral bioavailability ~80-90% for chloroquine; ~90% for primaquine. No parenteral form for this combination.
Contraindicated in severe renal impairment (GFR <30 m L/min). For GFR 30-50 m L/min: reduce dose by 50% or extend interval to every 48-72 hours. For GFR >50 m L/min: no adjustment needed.
For chloroquine: GFR 10-50: 50% dose; GFR <10: 25% dose. For primaquine: No adjustment required, but monitor for hemolysis in GFR <10 due to accumulation.
Contraindicated in severe hepatic impairment (Child-Pugh class C). For Child-Pugh class B: reduce dose by 50% and monitor liver function. For Child-Pugh class A: no adjustment typically required, but caution advised.
For chloroquine: Child-Pugh A/B: no adjustment; Child-Pugh C: reduce dose by 50% or avoid. For primaquine: Child-Pugh A/B: no data, use with caution; Child-Pugh C: contraindicated due to risk of hemolysis in glucose-6-phosphate dehydrogenase (G6PD) deficiency and impaired clearance.
For acute uncomplicated malaria: weight-based single dose as follows: 5-6 kg: 125 mg sulfadoxine/6.25 mg pyrimethamine (¼ tablet); 7-10 kg: 250 mg/12.5 mg (½ tablet); 11-20 kg: 500 mg/25 mg (1 tablet); 21-30 kg: 750 mg/37.5 mg (1.5 tablets); 31-45 kg: 1000 mg/50 mg (2 tablets); >45 kg: adult dose (3 tablets). For congenital toxoplasmosis: pyrimethamine 1-2 mg/kg/day plus sulfadoxine 50-100 mg/kg/day orally divided every 12-24 hours for 6-12 months.
Chloroquine: 10 mg base/kg orally once daily for 2 days, then 5 mg base/kg once daily (max 300 mg base/day) for 2 weeks. Primaquine: 0.5 mg base/kg orally once daily for 14 days (max 30 mg base/day). Ensure G6PD screening before use.
Use with caution due to increased risk of sulfonamide adverse reactions (e.g., severe cutaneous reactions, renal toxicity). Dose adjustment per renal function; consider lower end of dosing range. Monitor renal function and electrolytes. Avoid in patients with significant hepatic impairment or glucose-6-phosphate dehydrogenase deficiency.
Use lower end of adult dose for chloroquine due to reduced renal function; adjust according to Cr Cl. For primaquine, monitor for G6PD deficiency and hemolysis; dose as per adult. Consider increased risk of QT prolongation with chloroquine.
Fatalities due to severe adverse reactions (e.g., Stevens-Johnson syndrome, toxic epidermal necrolysis) have been reported with sulfonamide-containing products. Discontinue at first sign of rash or any sign of adverse reaction.
Primaquine may cause hemolytic anemia in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency. Test for G6PD deficiency before starting therapy.
Hypersensitivity reactions (including severe cutaneous adverse reactions), hematologic toxicity (folate deficiency, megaloblastic anemia), sulfonamide hypersensitivity (including anaphylaxis), glucose-6-phosphate dehydrogenase deficiency (hemolysis risk), photosensitivity, renal impairment (accumulation), hepatic impairment, caution in pregnancy (antifolate effect - use with leucovorin when indicated), caution in lactation.
Hemolytic anemia (especially G6PD deficiency), bone marrow suppression, prolonged QT interval, visual disturbances (retinopathy with chloroquine), methemoglobinemia, and severe hypersensitivity reactions.
Hypersensitivity to pyrimethamine, sulfadoxine, or any sulfonamide; severe hepatic or renal impairment; megaloblastic anemia due to folate deficiency; concurrent use with methotrexate; infants <2 months (sulfadoxine contraindicated in neonates due to bilirubin displacement and kernicterus risk); pregnancy (except when benefit outweighs risk, with leucovorin).
G6PD deficiency (primaquine), known hypersensitivity to chloroquine or primaquine, porphyria, concurrent use of drugs with known hemolytic potential, pregnancy (based on risk-benefit), and severe liver or kidney disease.
No significant food interactions. May be taken with or without food. Maintain adequate hydration to prevent crystalluria.
No clinically significant food interactions reported. However, antacids containing magnesium or aluminum can reduce chloroquine absorption; separate administration by at least 4 hours. Grapefruit juice may increase chloroquine levels via CYP3A4 inhibition; avoid concurrent use.
First trimester: Avoid due to antifolate effect; sulfonamides may cause kernicterus if used near term. FDA Category C; pyrimethamine is teratogenic in animal studies. Second/third trimester: Use only if benefit outweighs risk; monitor for neonatal hyperbilirubinemia.
In first trimester, chloroquine is generally considered low risk for major malformations, but primaquine is contraindicated due to risk of hemolytic anemia in G6PD-deficient fetuses. Second and third trimesters: chloroquine is safe, but primaquine should be avoided as fetal G6PD status is unknown.
Both components excreted in breast milk. M/P ratio: pyrimethamine ~0.4, sulfadoxine ~0.5. Avoid in nursing infants with G6PD deficiency or hyperbilirubinemia. Use caution in infants <2 months due to sulfonamide displacement of bilirubin.
Chloroquine is excreted into breast milk in low concentrations; M/P ratio is approximately 0.5-0.6. Primaquine is excreted in breast milk; M/P ratio not well established. Breastfeeding is generally considered safe if infant is G6PD normal, but caution is advised due to potential for hemolysis in G6PD-deficient infants.
No standard dose adjustment required; however, increased clearance in pregnancy may necessitate monitoring of therapeutic effect. Consider prophylactic folate (5 mg/day) to reduce risk of folate deficiency.
Chloroquine: No dose adjustment required; pharmacokinetics are not significantly altered. Primaquine: Contraindicated in pregnancy due to risk of hemolytic anemia in the fetus; no dose adjustment is applicable as it is not recommended.
Always administer with folinic acid (leucovorin) to reduce hematologic toxicity. Monitor for severe cutaneous adverse reactions (e.g., Stevens-Johnson syndrome) especially in first 8 weeks. Contraindicated in patients with sulfonamide allergy. Not recommended for prophylaxis in pregnancy; alternative antimalarials preferred.
Combination of chloroquine and primaquine is used for radical cure of P. vivax and P. ovale malaria. Chloroquine is effective against blood-stage parasites; primaquine eradicates hypnozoites in the liver. Screen for G6PD deficiency before initiating primaquine to prevent hemolytic anemia. Concurrent use with hematotoxic drugs (e.g., dapsone) increases hemolysis risk. Contraindicated in G6PD-deficient patients, pregnancy, and breastfeeding unless no alternative. Monitor for QT prolongation, especially with electrolyte abnormalities or concurrent QT-prolonging agents.
Take exactly as prescribed; do not skip doses.,Report any skin rash, blistering, or mouth sores immediately (signs of severe allergic reaction).,Complete full course even if feeling better.,Avoid alcohol during treatment.,Use effective contraception during and for 1 month after treatment if female of childbearing potential.
Take with food or milk to reduce gastrointestinal upset.,Complete full course regardless of symptom resolution to prevent relapse.,Avoid alcohol during treatment due to risk of disulfiram-like reaction.,Report signs of hemolysis: dark urine, jaundice, pallor, fatigue (especially if G6PD deficient).,Do not take antacids containing magnesium or aluminum within 4 hours of chloroquine as they reduce absorption.,Seek medical attention for visual disturbances, QT prolongation symptoms (palpitations, syncope), or severe GI distress.,Use effective contraception during and for 1 month after treatment due to potential fetal harm from primaquine.
"Metoprolol, a beta-1 selective adrenergic blocker, may inhibit the metabolism of pyrimethamine via competitive interaction with cytochrome P450 enzymes, particularly CYP2D6, potentially leading to increased plasma concentrations of pyrimethamine. Elevated pyrimethamine levels are associated with dose-related toxicity, including myelosuppression (e.g., megaloblastic anemia, leukopenia, thrombocytopenia) and gastrointestinal disturbances. This interaction is clinically significant in patients requiring prolonged pyrimethamine therapy, such as for toxoplasmosis or malaria prophylaxis."
"Zidovudine inhibits the hepatic metabolism of pyrimethamine, likely via competitive inhibition of CYP450 enzymes, leading to increased pyrimethamine plasma concentrations. This can potentiate pyrimethamine's antifolate effects, elevating the risk of dose-dependent toxicities such as megaloblastic anemia, leukopenia, and thrombocytopenia. Clinically, patients may present with worsening anemia or pancytopenia, requiring close hematologic monitoring."
"Ondansetron, a 5-HT3 receptor antagonist, is primarily metabolized by hepatic cytochrome P450 enzymes, including CYP3A4, CYP1A2, and CYP2D6. Pyrimethamine is also metabolized by CYP enzymes, particularly CYP3A4 and CYP2C8. Concomitant use of ondansetron may competitively inhibit the metabolism of pyrimethamine, leading to increased plasma concentrations of pyrimethamine. This can potentiate the risk of pyrimethamine-related toxicity, including megaloblastic anemia, thrombocytopenia, and neurological adverse effects such as seizures."
"Alimemazine, a phenothiazine derivative with antihistaminergic and anticholinergic properties, may inhibit the metabolism of Primaquine, an antimalarial agent primarily metabolized by cytochrome P450 enzymes including CYP2D6 and CYP3A4. This interaction can lead to increased plasma concentrations of Primaquine, heightening the risk of dose-dependent adverse effects such as hemolytic anemia in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency and methemoglobinemia. Clinically, patients may present with signs of oxidant stress, including hemoglobinuria and jaundice."
"Eliglustat, a CYP2D6 substrate and inhibitor, can increase the systemic exposure of primaquine, which is primarily metabolized by CYP2D6. This elevation in primaquine concentration may potentiate its QTc-prolonging effects, leading to an increased risk of torsades de pointes and other ventricular arrhythmias. Caution is advised, especially in patients with pre-existing cardiac conditions or electrolyte abnormalities."
"Primaquine, an antimalarial agent, can inhibit the cardiac potassium channel encoded by the hERG gene, leading to prolongation of the QTc interval. Ivabradine, a funny current (If) inhibitor used for chronic heart failure, also possesses a mild QTc-prolonging effect. Concomitant use increases the risk of excessive QTc prolongation, which may precipitate torsade de pointes and other ventricular arrhythmias, particularly in patients with underlying risk factors such as electrolyte disturbances or bradycardia."
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about Pyrimethamine-Sulfadoxine vs ARALEN PHOSPHATE W/ PRIMAQUINE PHOSPHATE, answered by our medical review team.
Pyrimethamine-Sulfadoxine is a Antimalarial that works by Pyrimethamine inhibits dihydrofolate reductase, blocking tetrahydrofolate synthesis. Sulfadoxine inhibits dihydropteroate synthase, blocking folate synthesis. Sequential blockade of folate metabolism.. ARALEN PHOSPHATE W/ PRIMAQUINE PHOSPHATE is a Antimalarial that works by Chloroquine and primaquine: Chloroquine inhibits heme polymerase in malaria parasites, preventing conversion of toxic heme to hemozoin; primaquine disrupts mitochondrial function and generates reactive oxygen species, targeting hypnozoites and gametocytes.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between Pyrimethamine-Sulfadoxine and ARALEN PHOSPHATE W/ PRIMAQUINE PHOSPHATE depend on the specific clinical indication. These are both Antimalarial agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of Pyrimethamine-Sulfadoxine is: Pyrimethamine 25 mg plus sulfadoxine 500 mg per tablet; typical adult dose for acute uncomplicated malaria is 3 tablets (pyrimethamine 75 mg, sulfadoxine 1500 mg) orally as a single dose. For toxoplasmosis in immunocompromised patients: loading dose pyrimethamine 200 mg orally once, then pyrimethamine 50-75 mg orally once daily plus sulfadoxine 1000-1500 mg orally once daily (dosing based on sulfadoxine component) for 4-6 weeks, then reduce to half.. The standard adult dose of ARALEN PHOSPHATE W/ PRIMAQUINE PHOSPHATE is: Chloroquine phosphate 600 mg base (1 g salt) orally once daily for 2 days, then 300 mg base (500 mg salt) once daily for at least 2 weeks; plus primaquine phosphate 30 mg base orally once daily for 14 days.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between Pyrimethamine-Sulfadoxine and ARALEN PHOSPHATE W/ PRIMAQUINE PHOSPHATE in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. Pyrimethamine-Sulfadoxine is classified as Category C. First trimester: Avoid due to antifolate effect; sulfonamides may cause kernicterus if used near term. FDA Category C; pyrimethamine is teratogenic in animal studies. Second/third . ARALEN PHOSPHATE W/ PRIMAQUINE PHOSPHATE is classified as Category D/X. In first trimester, chloroquine is generally considered low risk for major malformations, but primaquine is contraindicated due to risk of hemolytic anemia in G6PD-deficient fetuse. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.