Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
SULFAPYRIDINE vs BACTRIM
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Sulfapyridine is a sulfonamide antibiotic that inhibits bacterial dihydropteroate synthase, blocking folate synthesis and thereby nucleic acid production. It also has anti-inflammatory and immunomodulatory effects in dermatologic conditions through unknown mechanisms.
BACTRIM (sulfamethoxazole/trimethoprim) inhibits bacterial folate synthesis. Sulfamethoxazole, a sulfonamide, inhibits dihydropteroate synthase, blocking PABA incorporation into dihydrofolic acid. Trimethoprim inhibits dihydrofolate reductase, blocking conversion of dihydrofolic acid to tetrahydrofolic acid. Sequential blockade leads to bactericidal effect.
FDA-approved for dermatitis herpetiformis,Off-label: rheumatoid arthritis, inflammatory bowel disease, and other inflammatory dermatoses
Urinary tract infections,Acute otitis media,Acute exacerbations of chronic bronchitis,Traveler's diarrhea,Shigellosis,Pneumocystis jirovecii pneumonia (treatment and prophylaxis),Toxoplasmosis (prophylaxis in immunocompromised),Nocardia infections,Methicillin-resistant Staphylococcus aureus (MRSA) infections (off-label)
500 mg orally four times daily for initial treatment of dermatitis herpetiformis; maintenance dose 500 mg daily to 1.5 g daily in divided doses.
1 DS tablet (160 mg TMP/800 mg SMX) orally every 12 hours for 10-14 days.
Terminal elimination half-life: 6–10 hours (prolonged in renal impairment or slow acetylators); clinical context: requires dosing adjustment in renal insufficiency.
Sulfamethoxazole: 9-12 hours (prolonged in renal impairment); Trimethoprim: 8-10 hours (prolonged in renal impairment).
Primarily hepatic via N-acetylation (N-acetyltransferase 2, NAT2) and glucuronidation; also undergoes hydroxylation. Excreted renally.
Sulfamethoxazole is metabolized primarily via N-acetylation in the liver (N-acetyltransferase-2, NAT2). Trimethoprim is metabolized via O-demethylation and alpha-hydroxylation by cytochrome P450 (CYP) enzymes, mainly CYP3A4, with minor contribution from CYP1A2 and CYP2C9.
Renal: approximately 70–80% (30% as unchanged drug, remainder as metabolites, primarily N4-acetylsulfapyridine); biliary/fecal: minor (<5%).
Renal: sulfamethoxazole 20-30% unchanged, trimethoprim 40-70% unchanged; biliary/fecal: minimal (<10%) for both components.
Approximately 50–70% bound to albumin.
Sulfamethoxazole: 70% bound to albumin; Trimethoprim: 30-40% bound to albumin.
Vd: 0.25–0.35 L/kg; clinical meaning: indicates distribution primarily into extracellular fluid, with limited tissue penetration.
Sulfamethoxazole: 0.21 L/kg; Trimethoprim: 1.8 L/kg (high tissue penetration including lung, kidney, and CSF).
Oral: 85–100% (well absorbed from gastrointestinal tract).
Oral: 100% for both components (well absorbed).
Cr Cl 10-50 m L/min: administer every 8-12 hours. Cr Cl <10 m L/min: administer every 12-24 hours. Avoid use in severe renal impairment.
Cr Cl >30 m L/min: No adjustment. Cr Cl 15-30 m L/min: 50% of standard dose. Cr Cl <15 m L/min: Contraindicated.
Child-Pugh Class A: no adjustment. Child-Pugh Class B or C: avoid use due to potential accumulation and hepatotoxicity.
Child-Pugh Class A: No adjustment. Child-Pugh Class B: Use with caution, monitor for toxicity; consider dose reduction. Child-Pugh Class C: Avoid use.
Not recommended for children due to risk of kernicterus and adverse effects; safety not established.
8 mg/kg/day TMP / 40 mg/kg/day SMX in two divided doses every 12 hours (max 320 mg TMP/1600 mg SMX per day). For PCP treatment: 15-20 mg/kg/day TMP / 75-100 mg/kg/day SMX in 3-4 divided doses.
Start at lower end of dosing range; monitor renal function and for adverse effects; increased risk of sulfonamide-induced reactions.
Initiate at lower doses; monitor renal function closely; contraindicated if Cr Cl <15 m L/min; adjust based on Cr Cl (see renal adjustment).
None.
BACTRIM may cause life-threatening severe adverse reactions including: Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), fulminant hepatic necrosis, agranulocytosis, aplastic anemia, and other blood dyscrasias. Discontinue at first sign of skin rash or any sign of adverse reaction. Hypersensitivity reactions can occur even with re-challenge of the same or other sulfonamides.
Severe hypersensitivity reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis), hematologic toxicity (agranulocytosis, hemolytic anemia in G6PD deficiency), hepatotoxicity, renal toxicity. Discontinue if rash or signs of hypersensitivity.
Fatal hypersensitivity reactions including SJS/TEN,Hepatotoxicity and hepatic failure,Blood dyscrasias (leukopenia, thrombocytopenia, agranulocytosis),Clostridioides difficile-associated diarrhea,Renal impairment: risk of crystalluria; maintain adequate fluid intake,Hyperkalemia in patients with renal disease or on potassium-sparing drugs,Megaloblastic anemia in folate-deficient patients,Elderly patients at increased risk of severe adverse reactions,Pregnancy: avoid near term due to risk of kernicterus (sulfonamide displaces bilirubin),Lactation: caution; sulfonamides excreted in breast milk,Photosensitivity
Hypersensitivity to sulfonamides, porphyria, severe hepatic or renal impairment, pregnancy (especially near term) and lactation, infants <2 months (except for congenital toxoplasmosis).
Hypersensitivity to sulfonamides, trimethoprim, or any component,History of drug-induced immune thrombocytopenia with sulfonamides or trimethoprim,Megaloblastic anemia due to folate deficiency,Severe hepatic or renal impairment (Cr Cl <15 m L/min),Pregnancy at term and during breastfeeding,Infants less than 2 months of age,Combination with dofetilide (increased risk of torsades de pointes)
No specific food interactions. Avoid alcohol as it may increase risk of adverse effects like disulfiram-like reaction. Ensure adequate hydration with water; acidic foods do not significantly affect absorption.
Avoid high-potassium foods (bananas, oranges, potatoes) if hyperkalemia is a concern. No specific food interactions; however, maintain adequate fluid intake to prevent crystalluria.
First trimester: Sulfapyridine, a sulfonamide, crosses the placenta. There is a potential risk of neural tube defects and other malformations based on animal studies, but human data are limited. Second and third trimesters: Sulfonamides compete with bilirubin for albumin binding, increasing the risk of kernicterus in the neonate if administered near term. Use is generally avoided after 32 weeks gestation.
Pregnancy Category D. First trimester: Folate antagonist; associated with neural tube defects, cardiovascular malformations, and cleft palate. Second and third trimesters: Risk of kernicterus in neonates due to displacement of bilirubin from albumin; may cause hemolytic anemia in G6PD-deficient fetuses. Avoid use, especially near term.
Sulfapyridine is excreted into breast milk. The milk-to-plasma (M/P) ratio is approximately 0.45. Low levels are unlikely to cause adverse effects in healthy term infants, but caution is advised in premature, ill, or G6PD-deficient infants due to potential for hemolysis or kernicterus.
Both trimethoprim and sulfamethoxazole are excreted into breast milk. M/P ratio not well defined. Potential for kernicterus in jaundiced or G6PD-deficient infants; may interfere with folate metabolism. Caution advised; consider alternative therapy.
No specific dose adjustments are recommended, but pharmacokinetic changes in pregnancy (increased volume of distribution and renal clearance) may reduce drug levels. Monitor therapeutic response and consider adjusting dose based on clinical indication and serum levels if available.
Trimethoprim-sulfamethoxazole dose generally unchanged but avoid in first trimester and near term. If unavoidable, consider increased folate supplementation. No specific pharmacokinetic-driven dose adjustment established; monitor clinical response and adjust based on renal function.
Sulfapyridine is primarily used for dermatitis herpetiformis (DH). Dose adjustments needed in renal impairment. Monitor for hypersensitivity reactions, hemolytic anemia in G6PD deficiency, and crystalluria. Increase fluid intake to 2-3 L/day to prevent renal toxicity. Not first-line for other infections due to resistance.
Bactrim is contraindicated in G6PD deficiency due to risk of hemolytic anemia. Monitor renal function and potassium levels, especially in elderly patients, as sulfamethoxazole can cause hyperkalemia. Use with caution in patients with folic acid deficiency or megaloblastic anemia. Avoid in pregnancy at term and in lactating women due to risk of kernicterus. For PCP treatment, high doses may require leucovorin rescue to prevent bone marrow suppression.
Take with a full glass of water and maintain high fluid intake to prevent kidney stones.,Avoid prolonged sun exposure and use sunscreen, as sulfonamides can cause photosensitivity.,Report any skin rash, fever, sore throat, or unusual bleeding immediately.,Complete full course as prescribed, but do not use for viral infections.,Inform doctor if pregnant, breastfeeding, or have glucose-6-phosphate dehydrogenase deficiency.
Take with a full glass of water and stay well-hydrated to prevent crystalluria.,Complete the full course even if symptoms improve.,Report any signs of allergic reaction (rash, fever, sore throat) or severe skin reactions (blistering, peeling).,Avoid prolonged sun exposure; use sunscreen as photosensitivity may occur.,Do not take if you have a history of sulfa allergy or are pregnant/nursing without consulting doctor.
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about SULFAPYRIDINE vs BACTRIM, answered by our medical review team.
SULFAPYRIDINE is a Sulfonamide Antibiotic that works by Sulfapyridine is a sulfonamide antibiotic that inhibits bacterial dihydropteroate synthase, blocking folate synthesis and thereby nucleic acid production. It also has anti-inflammatory and immunomodulatory effects in dermatologic conditions through unknown mechanisms.. BACTRIM is a Sulfonamide Antibiotic Combination that works by BACTRIM (sulfamethoxazole/trimethoprim) inhibits bacterial folate synthesis. Sulfamethoxazole, a sulfonamide, inhibits dihydropteroate synthase, blocking PABA incorporation into dihydrofolic acid. Trimethoprim inhibits dihydrofolate reductase, blocking conversion of dihydrofolic acid to tetrahydrofolic acid. Sequential blockade leads to bactericidal effect.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between SULFAPYRIDINE and BACTRIM depend on the specific clinical indication. These are agents from distinct pharmacological classes and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of SULFAPYRIDINE is: 500 mg orally four times daily for initial treatment of dermatitis herpetiformis; maintenance dose 500 mg daily to 1.5 g daily in divided doses.. The standard adult dose of BACTRIM is: 1 DS tablet (160 mg TMP/800 mg SMX) orally every 12 hours for 10-14 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 SULFAPYRIDINE and BACTRIM 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. SULFAPYRIDINE is classified as Category C. First trimester: Sulfapyridine, a sulfonamide, crosses the placenta. There is a potential risk of neural tube defects and other malformations based on animal studies, but human dat. BACTRIM is classified as Category C. Pregnancy Category D. First trimester: Folate antagonist; associated with neural tube defects, cardiovascular malformations, and cleft palate. Second and third trimesters: Risk of . Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.