Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
Pyrimethamine-Sulfadoxine vs ARALEN
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, a 4-aminoquinoline, accumulates in acidic organelles such as food vacuoles of malaria parasites, inhibiting heme polymerase and preventing the conversion of toxic heme to hemozoin. It also interferes with DNA synthesis and repair by intercalating into DNA. Additionally, it has immunomodulatory effects via inhibition of Toll-like receptors and cytokine production.
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 uncomplicated malaria caused by susceptible strains of Plasmodium vivax, P. malariae, P. ovale, and P. falciparum,Prophylaxis of malaria in areas with chloroquine-sensitive P. falciparum,Treatment of extraintestinal amebiasis (as amebicide) and giardiasis (off-label),Disease-modifying antirheumatic drug (DMARD) for rheumatoid arthritis and lupus erythematosus (off-label)
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.
Adults: 500 mg (300 mg base) orally once weekly on the same day each week for prophylaxis of malaria; 1 g (600 mg base) orally initially, followed by 500 mg (300 mg base) at 6, 24, and 48 hours for treatment of acute malaria.
Pyrimethamine: ~80-120 hours; Sulfadoxine: ~100-200 hours. Long half-lives allow single-dose therapy for malaria.
Terminal elimination half-life ranges from 30 to 60 days (mean ~45 days) due to extensive tissue binding; clinical context: prolonged half-life allows weekly dosing for malaria prophylaxis.
Pyrimethamine is metabolized in the liver via CYP450 isoenzymes (minor). Sulfadoxine is primarily excreted unchanged in urine, with some hepatic metabolism (acetylation).
Chloroquine is extensively metabolized in the liver via cytochrome P450 enzymes, primarily CYP2C8 and CYP3A4, to active metabolites such as desethylchloroquine. It has a long elimination half-life of approximately 1-2 months.
Renal: ~60% unchanged sulfadoxine, ~5% unchanged pyrimethamine; fecal: ~10% pyrimethamine. Biliary excretion minimal.
Primarily renal (approximately 70% as unchanged drug); minor biliary/fecal (about 10-20%).
Pyrimethamine: ~87% (albumin); Sulfadoxine: ~90% (albumin).
Approximately 50-60% bound; primarily to albumin and alpha-1-acid glycoprotein.
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).
Very large, 100-200 L/kg; extensive tissue distribution (liver, spleen, kidney, lungs, melanin-containing tissues).
Oral: ~90-100% for both components, with high and consistent absorption.
Oral: 80-90%.
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 malaria prophylaxis: No adjustment necessary. For treatment: If Cr Cl < 10 m L/min, reduce dose by 50%.
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.
No formal guidelines; use caution in severe hepatic impairment due to potential accumulation. Consider dose reduction in Child-Pugh class C.
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.
Prophylaxis: 5 mg/kg base (8.3 mg/kg salt) orally once weekly, max 300 mg base. Treatment: 10 mg/kg base (16.7 mg/kg salt) orally initially, followed by 5 mg/kg base at 6, 24, and 48 hours, max 600 mg base on day 1.
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.
No specific adjustments; consider age-related renal impairment and potential increased risk of QT prolongation. Monitor for cardiac effects.
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.
Retinopathy: Irreversible retinal damage including retinopathy and visual disturbances; risk increases with cumulative dose and duration of use; contraindicated in patients with pre-existing retinopathy; baseline and periodic ophthalmologic exams required.
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.
Retinopathy risk with prolonged use; cardiac effects including conduction disorders (e.g., QT prolongation) and cardiomyopathy; exacerbation of psoriasis and porphyria; neuropsychiatric effects (e.g., psychosis, seizures); hematologic toxicity (eg, agranulocytosis, aplastic anemia); hypoglycemia; myopathy; ototoxicity. Use with caution in hepatic or renal impairment, G6PD deficiency, and pregnancy (benefit vs risk).
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).
Hypersensitivity to chloroquine or 4-aminoquinolines; pre-existing retinopathy of any etiology; concurrent use with other agents causing retinal toxicity (e.g., hydroxychloroquine, tamoxifen); porphyria; psoriasis (relative, may exacerbate); neuromyopathy (relative); severe hepatic or renal impairment (relative).
No significant food interactions. May be taken with or without food. Maintain adequate hydration to prevent crystalluria.
Avoid grapefruit juice as it may increase chloroquine levels. No other significant food interactions.
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.
Pregnancy category C. First trimester: No conclusive evidence of major malformations in human studies, but animal studies show embryotoxicity and fetotoxicity. Second and third trimesters: Risk of sensorineural hearing loss, vestibular damage, and retinal toxicity in the fetus if used for prolonged periods or at high doses; accumulation in fetal ocular tissues reported.
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.
Excreted in breast milk in small amounts (M/P ratio approximately 0.44). American Academy of Pediatrics considers compatible with breastfeeding, but caution is advised in infants with glucose-6-phosphate dehydrogenase deficiency or hemolytic disease. Monitor infant for rash, retinal changes, and hemolysis.
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.
No specific dose adjustment recommended for pregnancy; pharmacokinetic changes (increased volume of distribution, decreased plasma concentrations) may require therapeutic drug monitoring, but empirical dose adjustments are not established. Use lowest effective dose and shortest duration.
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.
Chloroquine (Aralen) can cause retinal toxicity; cumulative dose should not exceed 200g. Use with caution in G6PD deficiency. Can prolong QTc interval; avoid with other QTc-prolonging drugs.
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 to reduce gastrointestinal upset.,Do not exceed prescribed dose; overdose can be fatal.,Report any vision changes immediately; regular eye exams are required.,Avoid alcohol as it may increase risk of liver toxicity.,Inform your doctor if you have a history of heart rhythm problems.
"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."
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about Pyrimethamine-Sulfadoxine vs ARALEN, 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 is a Antimalarial that works by Chloroquine, a 4-aminoquinoline, accumulates in acidic organelles such as food vacuoles of malaria parasites, inhibiting heme polymerase and preventing the conversion of toxic heme to hemozoin. It also interferes with DNA synthesis and repair by intercalating into DNA. Additionally, it has immunomodulatory effects via inhibition of Toll-like receptors and cytokine production.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between Pyrimethamine-Sulfadoxine and ARALEN 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 is: Adults: 500 mg (300 mg base) orally once weekly on the same day each week for prophylaxis of malaria; 1 g (600 mg base) orally initially, followed by 500 mg (300 mg base) at 6, 24, and 48 hours for treatment of acute malaria.. 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 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 is classified as Category C. Pregnancy category C. First trimester: No conclusive evidence of major malformations in human studies, but animal studies show embryotoxicity and fetotoxicity. Second and third tri. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.