Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
AZO GANTRISIN vs BACTRIM DS
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
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.
BACTRIM DS is a combination of sulfamethoxazole and trimethoprim. Sulfamethoxazole inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid (PABA), while trimethoprim inhibits dihydrofolate reductase, blocking reduction of dihydrofolate to tetrahydrofolate. This sequential blockade of folic acid synthesis leads to bactericidal action.
Urinary tract infections,Pain relief associated with lower urinary tract irritation,Pyelonephritis
FDA-approved: Urinary tract infections, acute otitis media, acute exacerbations of chronic bronchitis, traveler's diarrhea, shigellosis, Pneumocystis jirovecii pneumonia (PCP), toxoplasmosis,Off-label: Methicillin-resistant Staphylococcus aureus (MRSA) infections, Stenotrophomonas maltophilia infections, nocardiosis, Wegener's granulomatosis (as second-line therapy), inflammatory bowel disease
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.
One double-strength tablet (trimethoprim 160 mg-sulfamethoxazole 800 mg) orally every 12 hours.
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: 8-10 hours; Trimethoprim: 8-12 hours; prolonged in renal impairment (creatinine clearance <30 m L/min: up to 24-48 hours).
Sulfamethoxazole is metabolized primarily via N-acetylation in the liver; phenazopyridine undergoes hepatic metabolism.
Sulfamethoxazole is metabolized primarily by N-acetylation and glucuronidation; trimethoprim is metabolized by O-demethylation and N-oxidation. Both are eliminated renally via glomerular filtration and tubular secretion.
Renal: 70-100% (sulfamethoxazole and metabolites; 15-30% as unchanged drug; remainder as acetylated and glucuronide conjugates). Biliary/fecal: <3%.
Renal: 50-70% as sulfamethoxazole (unchanged and acetylated metabolite), 40-60% as trimethoprim (unchanged); biliary: <10% for both; fecal: <4%.
Sulfamethoxazole: 65-70% bound to albumin; trimethoprim: 40-45% bound to albumin.
Sulfamethoxazole: 68% bound (albumin); Trimethoprim: 44% bound (albumin, alpha-1-acid glycoprotein).
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.
Sulfamethoxazole: 0.21 L/kg; Trimethoprim: 1.3-1.8 L/kg (wide distribution, higher in tissues than plasma).
Oral: 85-95% for both components (tablets); suspension: ~90%.
Oral: >90% for both components; IV: 100%.
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.
Cr Cl >30 m L/min: No adjustment; Cr Cl 15-30 m L/min: 50% of usual dose every 12 hours; Cr Cl <15 m L/min: Not recommended.
Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 50% or extend interval; Child-Pugh C: contraindicated.
Child-Pugh Class A: No adjustment; Child-Pugh Class B: Use with caution, no specific dose recommendation; Child-Pugh Class C: Contraindicated.
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.
Based on trimethoprim component: 8 mg/kg/day of trimethoprim divided every 12 hours. For severe infections, up to 20 mg/kg/day of trimethoprim divided every 6 hours.
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.
Monitor renal function; adjust dose based on Cr Cl. Increased risk of hyperkalemia and folate deficiency; consider folate supplementation.
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.
BACTRIM DS carries a black box warning for severe hypersensitivity reactions including Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), and fulminant hepatic necrosis. Also warns about fatal reactions such as agranulocytosis, aplastic anemia, and other blood dyscrasias. Additionally, use in pregnancy at term may cause kernicterus in the newborn.
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 reactions: risk of SJS/TEN, especially in patients with HIV, folate deficiency, or genetic susceptibility (e.g., HLA-B*1502, HLA-A*3101). Discontinue at first sign of rash.,Hematologic toxicity: monitor CBCs; caution in patients with folate deficiency, renal impairment, or prolonged therapy.,Hepatic toxicity: can cause cholestatic jaundice, hepatic necrosis; avoid in hepatic impairment.,Renal toxicity: maintain adequate hydration to prevent crystalluria; adjust dose in renal impairment.,Hyperkalemia: risk with high-dose trimethoprim; monitor potassium, especially in patients with renal dysfunction or on potassium-sparing diuretics.,Hypoglycemia: risk in patients with renal impairment or malnutrition; caution with sulfonylureas.,Photosensitivity: avoid excessive sun exposure.,Pregnancy: avoid at term due to risk of kernicterus; use only if benefit outweighs risk.,Lactation: caution due to potential for kernicterus in infants with G6PD deficiency.
Hypersensitivity to sulfonamides or phenazopyridine; severe hepatic or renal impairment; porphyria; G6PD deficiency; pregnancy at term; lactation; children < 12 years (due to phenazopyridine component).
Hypersensitivity to sulfamethoxazole, trimethoprim, or any component.,History of drug-induced immune thrombocytopenia with sulfonamides or trimethoprim.,Severe hepatic disease (e.g., acute hepatitis, cirrhosis with jaundice).,Severe renal impairment (Cr Cl <15 m L/min) unless dialysis is available.,Megaloblastic anemia due to folate deficiency.,Pregnancy at term and nursing mothers (due to risk of kernicterus).,Concurrent use with dofetilide (increased risk of arrhythmias).,Infants <2 months of age (sulfonamides can cause kernicterus).
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.
Avoid high-potassium foods (e.g., bananas, oranges, potatoes) as trimethoprim can increase serum potassium. Avoid alcohol, which may cause disulfiram-like reaction (flushing, nausea, tachycardia). No significant food-drug interactions beyond potassium and alcohol.
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.
First trimester: Folate antagonist; associated with neural tube defects, cardiovascular malformations, and cleft palate. Second trimester: Growth restriction, preterm birth. Third trimester: Kernicterus risk due to bilirubin displacement from albumin. Avoid during entire pregnancy.
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.
Breastfeeding safety: Both trimethoprim and sulfamethoxazole are excreted into breast milk; M/P ratio for trimethoprim ~1.25, sulfamethoxazole ~0.15. Caution in infants under 2 months or with G6PD deficiency; theoretical risk of kernicterus.
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.
No standard dose adjustment recommended; avoid use if possible. If necessary, ensure adequate folic acid intake; may need to increase dose due to increased clearance in pregnancy, but specific data lacking.
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.
Bactrim DS (sulfamethoxazole/trimethoprim) is contraindicated in G6PD deficiency due to risk of hemolytic anemia. Monitor for hyperkalemia, especially in elderly or those with renal impairment. Caution with warfarin (potentiates anticoagulation). Avoid in pregnancy (teratogenic) and lactation. Use with caution in folate deficiency; supplement folate if needed.
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.
Take with a full glass of water and stay well-hydrated to prevent crystalluria.,Avoid prolonged sun exposure; use sunscreen as this drug may cause photosensitivity.,Complete the full course even if you feel better to prevent antibiotic resistance.,Report any skin rash, sore throat, fever, or unusual bleeding immediately.,Do not take if you are pregnant, planning to become pregnant, or breastfeeding.,Inform your doctor if you have kidney disease, G6PD deficiency, or are on blood thinners.
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 AZO GANTRISIN vs BACTRIM DS, answered by our medical review team.
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.. BACTRIM DS is a Sulfonamide Antibiotic Combination that works by BACTRIM DS is a combination of sulfamethoxazole and trimethoprim. Sulfamethoxazole inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid (PABA), while trimethoprim inhibits dihydrofolate reductase, blocking reduction of dihydrofolate to tetrahydrofolate. This sequential blockade of folic acid synthesis leads to bactericidal action.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between AZO GANTRISIN and BACTRIM DS 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 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.. The standard adult dose of BACTRIM DS is: One double-strength tablet (trimethoprim 160 mg-sulfamethoxazole 800 mg) orally every 12 hours.. 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 AZO GANTRISIN and BACTRIM DS 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. 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. BACTRIM DS is classified as Category C. First trimester: Folate antagonist; associated with neural tube defects, cardiovascular malformations, and cleft palate. Second trimester: Growth restriction, preterm birth. Third . Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.