(meth-en'a-meen hip'yoo-rate)
Hiprex, Urex
Classifications: urinary tract antiinfective;
Therapeutic: urinary tract antiinfective

Prototype: Trimethoprim
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.

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.


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; 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: PO6 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


  • 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.


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


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

© 2006-2022 Last Updated On: 11/23/2022 (0)
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