Classifications: urinary tract antiinfective
Pregnancy Category: C
Methenamine Hippurate 1 g tablets
Methenamine Mandelate 0.5 g, 1 g tablets; 0.5 g/5 mL suspension
Tertiary amine liberates formaldehyde in an acid medium. Nonspecific antibiotic agent with bactericidal activity.
Most bacteria and fungi are susceptible to formaldehyde; however, bacteria that are urease-positive (e.g., Proteus sp) convert urea to ammonium hydroxide, which prevents the generation of formaldehyde from methenamine.
Prophylactic treatment of recurrent urinary tract infections (UTIs). Also long-term prophylaxis when residual urine is present
(e.g., neurogenic bladder).
Renal insufficiency; liver disease; gout; severe dehydration; combined therapy with sulfonamides. Safety during pregnancy
(category C) or lactation is not established.
Oral suspension for patients susceptible to lipoid pneumonia (e.g., older adults, debilitated patients).
Adult: PO (Hippurate) 1 g b.i.d.; (Mandelate) 1 g q.i.d.
Child: PO 6 y, (Mandelate) 18.4 mg/kg q.i.d.; 612 y, (Hippurate) 0.51 g b.i.d.; (Mandelate) 500 mg q.i.d. or 50 mg/kg/d in 3 divided doses
GI: Nausea, vomiting, diarrhea, abdominal cramps, anorexia. Renal: Bladder irritation, dysuria, frequency, albuminuria, hematuria, crystalluria.
- Give after meals and at bedtime to minimize gastric distress.
- Give oral suspension with caution to older adult or debilitated patients because of the possibility of lipid (aspiration)
pneumonia; it contains a vegetable oil base.
- Store at 15°30° C (59°86° F) in tightly closed container; protect from excessive heat.
Methenamine (formaldehyde) may produce falsely elevated values for urinary catecholamines and urinary steroids
(17-hydroxycorticosteroids) (by Reddy method). Possibility of false urine glucose determinations with Benedict's test. Methenamine interferes with urobilinogen and possibly urinary VMA determinations.
Sulfamethoxazole forms insoluble precipitate in acid urine; acetazolamide,
sodium bicarbonate may prevent hydrolysis to formaldehyde.
Absorption: Readily absorbed from GI tract, although 1030% of dose is hydrolyzed to formaldehyde in stomach. Peak: 2 h. Duration: Up to 6 h or until patient voids. Distribution: Crosses placenta; distributed into breast milk. Metabolism: Hydrolyzed in acid pH to formaldehyde. Elimination: Excreted in urine. Half-Life: 4 h.
Assessment & Drug Effects
- Monitor urine pH; value of 5.5 or less is required for optimum drug action.
- Monitor I&O ratio and pattern; drug most effective when fluid intake is maintained at 1500 or 2000 mL/d.
- Do not force fluids with this drug; copious amounts may increase diuresis, elevate urine pH, and dilute formaldehyde concentration
to subinhibitory levels.
- Consult physician about changing to enteric-coated tablet if patient complains of gastric distress.
- Supplemental acidification to maintain pH of 5.5 or below required for drug action may be necessary. Accomplish by drugs (ascorbic
acid, ammonium chloride) or by foods.
Patient & Family Education
- Do not self-medicate with OTC antacids containing sodium bicarbonate or sodium carbonate (to prevent raising urine pH).
- Achieve supplementary acidification by limiting intake of foods that can increase urine pH [e.g., vegetables, fruits, and
fruit juice (except cranberry, plum, prune)] and increasing intake of foods that can decrease urine pH (e.g., proteins, cranberry
juice, plums, prunes).
- Do not breast feed while taking this drug without consulting physician.