Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
ACEPHEN 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
ACEPHEN (acetaminophen) is a para-aminophenol derivative with analgesic and antipyretic activity. Its mechanism involves inhibition of cyclooxygenase (COX) enzymes in the central nervous system, particularly COX-2, reducing prostaglandin synthesis. It has weak peripheral COX inhibition and minimal anti-inflammatory 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.
Mild to moderate pain,Fever
Urinary tract infections,Pain relief associated with lower urinary tract irritation,Pyelonephritis
325-650 mg orally every 4-6 hours as needed; maximum 4 g/day.
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.
Terminal elimination half-life: 1.0-1.5 hours in adults with normal renal function. Prolonged to 2-5 hours in hepatic impairment or elderly; requires dose adjustment in severe hepatic disease.
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.
Acetaminophen is primarily metabolized in the liver via glucuronidation (UGT1A1, UGT1A6, UGT1A9) and sulfation (SULT1A1, SULT1A3). A minor fraction is oxidized by cytochrome P450 enzymes (CYP2E1, CYP1A2, CYP3A4) to a reactive toxic metabolite (NAPQI), which is normally detoxified by conjugation with glutathione.
Sulfamethoxazole is metabolized primarily via N-acetylation in the liver; phenazopyridine undergoes hepatic metabolism.
Renal: 90-95% as unchanged drug; tubular secretion and glomerular filtration. Biliary/fecal: <5%.
Renal: 70-100% (sulfamethoxazole and metabolites; 15-30% as unchanged drug; remainder as acetylated and glucuronide conjugates). Biliary/fecal: <3%.
Approximately 10-20% bound to serum albumin; extensive tissue binding.
Sulfamethoxazole: 65-70% bound to albumin; trimethoprim: 40-45% bound to albumin.
Apparent Vd: 0.5-0.7 L/kg (30-40 L in a 70 kg adult). Distributions into CSF and breast milk.
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: 85-90% (first-pass metabolism minimal). Rectal: approximately 70-80% of oral bioavailability.
Oral: 85-95% for both components (tablets); suspension: ~90%.
GFR 10-50 m L/min: 650 mg every 6 hours; GFR <10 m L/min: 650 mg every 8 hours.
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: maximum 2 g/day; Child-Pugh Class C: maximum 1 g/day.
Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 50% or extend interval; Child-Pugh C: contraindicated.
10-15 mg/kg/dose orally every 4-6 hours; maximum 75 mg/kg/day or 4 g/day, whichever is less.
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.
Start at lowest effective dose (325 mg every 6 hours); avoid exceeding 3 g/day unless closely monitored.
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.
Acetaminophen has been associated with cases of acute liver failure, at times resulting in liver transplant and death. Most of the cases of liver injury are associated with the use of acetaminophen at doses that exceed 4,000 milligrams per day, and often involve more than one acetaminophen-containing product.
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.
Risk of severe liver injury with doses >4000 mg/day; use caution with hepatic impairment, chronic alcoholism, malnutrition, or concomitant hepatotoxic drugs; avoid exceeding recommended dose; limit use to 10 days for pain or 3 days for fever unless directed by physician; serious skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis) have occurred.
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 acetaminophen or any component of the formulation; severe hepatic impairment or active liver disease.
Hypersensitivity to sulfonamides or phenazopyridine; severe hepatic or renal impairment; porphyria; G6PD deficiency; pregnancy at term; lactation; children < 12 years (due to phenazopyridine component).
Alcohol: increased risk of hepatotoxicity. Avoid concurrent use. Food: no significant interaction, but taking with food may reduce minor gastrointestinal irritation.
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 C. First trimester: potential risk of neural tube defects and orofacial clefts (limited human data, animal studies show embryotoxicity). Second and third trimesters: NSAID exposure associated with oligohydramnios, premature ductus arteriosus constriction, and fetal renal impairment. Avoid in third trimester.
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.
Excreted into breast milk in low concentrations (M/P ratio approximately 0.10). Considered compatible with breastfeeding; however, use lowest effective dose for shortest duration given potential for neonatal adverse effects (e.g., thrombocytopenia, renal dysfunction).
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.
No standard dose adjustments recommended; however, due to increased plasma volume and metabolism in pregnancy, higher doses may be required to achieve therapeutic effect. Avoid near term.
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.
ACEPHEN (acetaminophen) is commonly used for mild to moderate pain and fever. Avoid exceeding 4 g/day in adults to prevent hepatotoxicity. In patients with hepatic impairment, reduce maximum daily dose to 2 g. Consider acetylcysteine for overdose. Onset of action is 15-30 minutes orally.
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.
Do not exceed 4000 mg (4 grams) in 24 hours.,Avoid drinking alcohol while taking this medication.,Do not combine with other products containing acetaminophen.,Take with food if stomach upset occurs.,Seek immediate medical help if you experience symptoms of liver damage: yellowing of skin/eyes, dark urine, severe abdominal pain.
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 ACEPHEN vs AZO GANTRISIN, answered by our medical review team.
ACEPHEN is a Non-Opioid Analgesic that works by ACEPHEN (acetaminophen) is a para-aminophenol derivative with analgesic and antipyretic activity. Its mechanism involves inhibition of cyclooxygenase (COX) enzymes in the central nervous system, particularly COX-2, reducing prostaglandin synthesis. It has weak peripheral COX inhibition and minimal anti-inflammatory 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 ACEPHEN 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 ACEPHEN is: 325-650 mg orally every 4-6 hours as needed; maximum 4 g/day.. 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 ACEPHEN 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. ACEPHEN is classified as Category C. Pregnancy Category C. First trimester: potential risk of neural tube defects and orofacial clefts (limited human data, animal studies show embryotoxicity). Second and third trimest. 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.