METHENAMINE HIPPURATE
| METHENAMINE HIPPURATE (meth-en'a-meen hip'yoo-rate) Hiprex, Urex METHENAMINE MANDELATE Mandelamine Classifications: urinary tract antiinfective; Therapeutic: 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
Action
Tertiary amine liberates formaldehyde in an acid medium. Nonspecific antibiotic agent with bactericidal activity.
Therapeutic Effect
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; pregnancy (category C). Safety during lactation is not established.
Cautious Use
Oral suspension for patients susceptible to lipoid pneumonia (e.g., older adults, debilitated patients); gout.
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- 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.
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 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: In urine. Half-Life: 4 h.Nursing Implications
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).
Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; 
Canadian drug name; 
Prototype drug