Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
BACTRIM vs AZO GANTRISIN
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
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.
Sulfamethoxazole is a competitive inhibitor of dihydropteroate synthase, blocking bacterial folic acid synthesis. Phenazopyridine is an azo dye with local analgesic effects on urinary tract mucosa.
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)
Urinary tract infections,Pain relief associated with lower urinary tract irritation,Pyelonephritis
1 DS tablet (160 mg TMP/800 mg SMX) orally every 12 hours for 10-14 days.
AZO GANTRISIN (phenazopyridine 100 mg / sulfisoxazole 500 mg): 2 tablets orally 4 times daily for 2 days, then 1 tablet 4 times daily for up to 5 days.
Sulfamethoxazole: 9-12 hours (prolonged in renal impairment); Trimethoprim: 8-10 hours (prolonged in renal impairment).
Sulfamethoxazole: 9-12 hours (adults with normal renal function), prolonged to 20-50 hours in renal impairment; trimethoprim component: 8-11 hours. Clinical context: dosing interval adjusted based on Cr Cl.
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.
Sulfamethoxazole is metabolized primarily via N-acetylation in the liver; phenazopyridine undergoes hepatic metabolism.
Renal: sulfamethoxazole 20-30% unchanged, trimethoprim 40-70% unchanged; biliary/fecal: minimal (<10%) for both components.
Renal: 70-100% (sulfamethoxazole and metabolites; 15-30% as unchanged drug; remainder as acetylated and glucuronide conjugates). Biliary/fecal: <3%.
Sulfamethoxazole: 70% bound to albumin; Trimethoprim: 30-40% bound to albumin.
Sulfamethoxazole: 65-70% bound to albumin; trimethoprim: 40-45% bound to albumin.
Sulfamethoxazole: 0.21 L/kg; Trimethoprim: 1.8 L/kg (high tissue penetration including lung, kidney, and CSF).
Sulfamethoxazole: 0.2-0.3 L/kg (reflects distribution into extracellular fluid, not extensively tissue-bound); trimethoprim: 1-2 L/kg (higher due to lipophilicity, penetrates tissues including prostate and CSF). Clinical meaning: higher Vd of trimethoprim contributes to effective tissue concentrations.
Oral: 100% for both components (well absorbed).
Oral: 85-95% for both components (tablets); suspension: ~90%.
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.
Cr Cl 50-80 m L/min: 1 tablet 3-4 times daily; Cr Cl 10-49 m L/min: 1 tablet 2-3 times daily; Cr Cl <10 m L/min: contraindicated.
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.
Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 50% or extend interval; Child-Pugh C: contraindicated.
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.
Children 6-12 years: 0.5-1.5 teaspoons (2.5-7.5 m L) of suspension (equivalent to 75-225 mg sulfisoxazole and 15-45 mg phenazopyridine) orally 4 times daily; children >12 years: adult dose.
Initiate at lower doses; monitor renal function closely; contraindicated if Cr Cl <15 m L/min; adjust based on Cr Cl (see renal adjustment).
Initiate at lower doses (e.g., 1 tablet 3 times daily) and monitor for renal function and CNS side effects; contraindicated if Cr Cl <50 m L/min.
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.
Sulfonamides have been associated with severe reactions such as Stevens-Johnson syndrome, toxic epidermal necrolysis, agranulocytosis, aplastic anemia, and other blood dyscrasias. Fatalities have occurred.
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
Risk of severe hypersensitivity reactions, blood dyscrasias, hepatotoxicity, and renal impairment. Use caution in patients with G6PD deficiency, hepatic impairment, or renal insufficiency. Phenazopyridine may cause orange-red discoloration of urine.
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)
Hypersensitivity to sulfonamides or phenazopyridine; severe hepatic or renal impairment; porphyria; G6PD deficiency; pregnancy at term; lactation; children < 12 years (due to phenazopyridine component).
Avoid high-potassium foods (bananas, oranges, potatoes) if hyperkalemia is a concern. No specific food interactions; however, maintain adequate fluid intake to prevent crystalluria.
Avoid acidic foods and beverages (e.g., citrus fruits, tomatoes, cola) as they may decrease the efficacy of sulfisoxazole by increasing urine acidity, which can reduce solubility and increase risk of crystalluria. Maintain adequate fluid intake; avoid alcohol. No other significant food interactions.
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.
Pregnancy Category D. First trimester: Associated with neural tube defects, cardiovascular anomalies, and oral clefts due to antifolate effect of trimethoprim. Second and third trimesters: Risk of kernicterus in newborn due to sulfonamide displacement of bilirubin from albumin, especially near term. Avoid use during pregnancy unless benefit outweighs risk.
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.
Sulfamethoxazole and trimethoprim are excreted into breast milk; M/P ratio not established. Avoid in nursing mothers with infants under 2 months of age due to risk of kernicterus. In older infants, caution if infant has G6PD deficiency or hyperbilirubinemia.
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.
No standard dose adjustment recommended for pregnancy; however, caution due to increased volume of distribution and renal clearance. Monitor for therapeutic efficacy and toxicity. Consider folate supplementation (5 mg folic acid daily) to mitigate antifolate effects.
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.
AZO GANTRISIN combines phenazopyridine (urinary analgesic) and sulfisoxazole (sulfonamide antibiotic). Phenazopyridine imparts a red-orange color to urine and may stain contact lenses. Sulfisoxazole is contraindicated in infants <2 months due to risk of kernicterus. Use with caution in patients with G6PD deficiency, sulfonamide allergy, or renal impairment. Monitor for crystalluria; ensure adequate hydration. Avoid concurrent use with methenamine due to increased risk of crystalluria.
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.
Take this medication with a full glass of water and drink plenty of fluids throughout the day to prevent kidney stones.,Your urine may turn red-orange; this is harmless but may stain clothing or contact lenses.,Do not use for longer than 2 days unless directed by your doctor, as it only treats symptoms of UTI, not the infection.,Complete the full course of the sulfisoxazole component even if you feel better.,Avoid prolonged sun exposure; sulfonamides may cause photosensitivity. Use sunscreen.,Seek immediate medical attention if you develop skin rash, sore throat, fever, unusual bleeding, or bruising.
No interactions on record
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about BACTRIM vs AZO GANTRISIN, answered by our medical review team.
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.. AZO GANTRISIN is a Sulfonamide Antibiotic that works by Sulfamethoxazole is a competitive inhibitor of dihydropteroate synthase, blocking bacterial folic acid synthesis. Phenazopyridine is an azo dye with local analgesic effects on urinary tract mucosa.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between BACTRIM and AZO GANTRISIN 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 BACTRIM is: 1 DS tablet (160 mg TMP/800 mg SMX) orally every 12 hours for 10-14 days.. The standard adult dose of AZO GANTRISIN is: AZO GANTRISIN (phenazopyridine 100 mg / sulfisoxazole 500 mg): 2 tablets orally 4 times daily for 2 days, then 1 tablet 4 times daily for up to 5 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 BACTRIM and AZO GANTRISIN 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. 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 . AZO GANTRISIN is classified as Category C. Pregnancy Category D. First trimester: Associated with neural tube defects, cardiovascular anomalies, and oral clefts due to antifolate effect of trimethoprim. Second and third tri. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.