HIPREX
Clinical safety rating: caution
Comprehensive clinical and safety monograph for HIPREX (HIPREX).
Hippuric acid, the active metabolite of methenamine, acidifies urine and releases formaldehyde, which denatures bacterial proteins and nucleic acids, bactericidal activity requires acidic urine (pH < 5.5).
| Metabolism | Methenamine is absorbed in the gastrointestinal tract and partially hydrolyzed in acidic urine to ammonia and formaldehyde; approximately 70-80% of the dose is excreted unchanged by the kidneys within 24 hours. |
| Excretion | Renal excretion: >90% as unchanged drug (methenamine) and formaldehyde; biliary/fecal: <5%. |
| Half-life | 3-6 hours (methenamine); clinical context: prolonged in renal impairment, requiring dose adjustment. |
| Protein binding | Methenamine: negligible (<10%); formaldehyde: negligible (<10%). |
| Volume of Distribution | Methenamine: 1.6 L/kg; distributes into total body water. |
| Bioavailability | Oral: 70-90% (methenamine hippurate). |
| Onset of Action | Oral: within 1-2 hours (bacteriostatic effect due to formaldehyde release in acidic urine). |
| Duration of Action | 6-12 hours (bacteriostatic effect persists while urine pH is ≤5.5); clinical note: requires continuous acidic urine for activity. |
1 gram orally twice daily (every 12 hours) with meals
| Dosage form | TABLET |
| Renal impairment | CrCl < 10 mL/min: avoid use; CrCl 10-29 mL/min: 1 gram every 24 hours; CrCl 30-49 mL/min: 1 gram every 18 hours; CrCl ≥ 50 mL/min: no adjustment |
| Liver impairment | No adjustment required for mild to moderate hepatic impairment (Child-Pugh A or B); safety and efficacy not established in severe hepatic impairment (Child-Pugh C), use with caution |
| Pediatric use | Children ≥ 6 years and ≥ 22 kg: 1 gram twice daily with meals; For children < 6 years or < 22 kg: not recommended due to lack of safety data |
| Geriatric use | No specific adjustment; monitor renal function and consider age-related decline in CrCl; if GFR < 50 mL/min, adjust per renal impairment dosing |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for HIPREX (HIPREX).
| Breastfeeding | Methenamine is excreted into breast milk in low concentrations. M/P ratio not determined. Due to low oral bioavailability and minimal systemic absorption, it is considered compatible with breastfeeding. Use caution in infants with G6PD deficiency. |
| Teratogenic Risk | FDA Pregnancy Category B. Animal studies show no fetal risk, but no adequate human studies in first trimester. Second and third trimester use is not associated with teratogenicity. Methenamine (active component) has low systemic absorption; however, avoid in renal impairment or dehydration due to risk of formaldehyde toxicity. |
■ FDA Black Box Warning
None
| Serious Effects |
Severe hepatic impairment (hepatotoxicity risk); severe renal insufficiency (creatinine clearance < 10 mL/min); concomitant therapy with sulfonamides; hypersensitivity to methenamine or any component; dehydration; history of hemolytic anemia (associated with formaldehyde metabolite).
| Precautions | Avoid concomitant use with sulfonamides (formaldehyde combines with sulfonamides to form insoluble precipitates in urine); use with caution in patients with hepatic impairment or severe dehydration; monitor urine pH to ensure acidity (pH < 5.5) for efficacy; may cause dysuria, hematuria, or bladder irritation at high doses; use caution in renal impairment. |
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| Fetal Monitoring | Monitor renal function (serum creatinine, BUN) and urine pH (maintain acidic pH <5.5 for efficacy). Assess for signs of hemolytic anemia or urinary tract infection recurrence. |
| Fertility Effects | No known adverse effects on fertility. Methenamine does not affect hormonal balance or reproductive function. |