METHENAMINE HIPPURATE
(meth-en'a-meen hip'yoo-rate)
Hiprex, Urex
METHENAMINE MANDELATE
Mandelamine, Mandameth
Classifications: urinary tract antiinfective
Prototype: Trimethoprim
Pregnancy Category: C

Availability

Methenamine Hippurate 1 g tablets

Methenamine Mandelate 0.5 g, 1 g tablets; 0.5 g/5 mL suspension

Actions

Tertiary amine liberates formaldehyde in an acid medium. Nonspecific antibiotic agent with bactericidal activity.

Therapeutic Effects

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.

Uses

Prophylactic treatment of recurrent urinary tract infections (UTIs). Also long-term prophylaxis when residual urine is present (e.g., neurogenic bladder).

Contraindications

Renal insufficiency; liver disease; gout; severe dehydration; combined therapy with sulfonamides. Safety during pregnancy (category C) or lactation is not established.

Cautious Use

Oral suspension for patients susceptible to lipoid pneumonia (e.g., older adults, debilitated patients).

Route & Dosage

UTI Prophylaxis
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.; 6–12 y, (Hippurate) 0.5–1 g b.i.d.; (Mandelate) 500 mg q.i.d. or 50 mg/kg/d in 3 divided doses

Administration

Oral

Adverse Effects (1%)

GI: Nausea, vomiting, diarrhea, abdominal cramps, anorexia. Renal: Bladder irritation, dysuria, frequency, albuminuria, hematuria, crystalluria.

Diagnostic Test Interference

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.

Interactions

Drug: Sulfamethoxazole forms insoluble precipitate in acid urine; acetazolamide, sodium bicarbonate may prevent hydrolysis to formaldehyde.

Pharmacokinetics

Absorption: Readily absorbed from GI tract, although 10–30% 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.

Nursing Implications

Assessment & Drug Effects

Patient & Family Education


Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug