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Methenamine (Monograph)

Brand names: Hiprex, Mandelamine, Urex
Drug class: Urinary Anti-infectives
VA class: AM550
CAS number: 100-97-0

Introduction

Antibacterial.

Uses for Methenamine

Urinary Tract Infections (UTIs)

Prophylaxis or suppression of recurrent UTIs (bacteriuria), especially when long-term therapy is considered necessary.

Use only after UTI has been eradicated by other appropriate anti-infectives.

Not effective in systemic bacterial infections and has no effect on bacteria in blood or tissues outside the urinary tract. Do not use alone in the treatment of acute parenchymal infections causing systemic symptoms (e.g., chills, fever).

Monitor efficacy by periodic urine cultures.

Antibacterial effects are maximal when urine pH is ≤5.5. Monitor urinary pH during therapy; use supplementary acidification, if required. Supplementary acidification may be achieved by dietary regulation and/or concomitant administration of acidifying agents (e.g., ammonium chloride, ascorbic acid, methionine). This is particularly important when the causative organisms are urea-splitting strains of Proteus or Pseudomonas which increase urinary pH.

Methenamine Dosage and Administration

General

Administration

Oral Administration

Administer orally.

Dosage

Available as methenamine hippurate and methenamine mandelate; dosage expressed in terms of the salt.

Pediatric Patients

Prophylaxis or Suppression of Urinary Tract Infections (UTIs)
Methenamine Hippurate
Oral

Children <6 years of age: Dosage not established.

Children 6–12 years of age: 500 mg to 1 g twice daily (morning and night).

Children >12 years of age: 1 g twice daily (morning and night).

Methenamine Mandelate
Oral

Children <6 years of age: 18.4 mg/kg 4 times daily, (after meals and at bedtime).

Children 6–12 years of age: 500 mg 4 times daily (after meals and at bedtime).

Children >12 years of age: 1 g 4 times daily (after meals and at bedtime).

Alternatively, some clinicians recommend 50 mg/kg daily in 3 divided doses for children.

Adults

Prophylaxis or Suppression of Urinary Tract Infections (UTIs)
Methenamine Hippurate
Oral

1 g twice daily (morning and night).

Methenamine Mandelate
Oral

1 g 4 times daily (after meals and at bedtime).

Special Populations

No special population dosage recommendations at this time.

Cautions for Methenamine

Contraindications

Warnings/Precautions

Warnings

Dysuria

Large doses (8 g daily for 3–4 weeks) have caused bladder irritation, painful and frequent micturition, albuminuria, and gross hematuria.

Dysuria may be controlled by reducing dosage and/or reducing urine acidification.

Hepatic Effects

Patients with preexisting hepatic insufficiency may have adverse effects from the small amounts of ammonia and formaldehyde that are produced following administration of methenamine. Acute hepatic failure may occur in some patients.

Transient elevations in serum AST and ALT concentrations have occurred in patients receiving methenamine hippurate.

Perform periodic liver function tests in patients receiving methenamine hippurate, especially in those with hepatic impairment. (See Hepatic Impairment under Cautions.)

Sensitivity Reactions

Tartrazine Sensitivity

Hiprex tablets contain tartrazine (FD&C yellow No. 5), which may cause allergic reactions, including bronchial asthma, in susceptible individuals. Incidence of tartrazine sensitivity is low, but it frequently occurs in patients who are sensitive to aspirin.

General Precautions

Acidic Urine

Ensure that urine is maintained at an acidic pH during treatment, especially when causative organisms are urea-splitting strains of Proteus or Pseudomonas. (See Dosage and Administration.)

Manufacturer of methenamine mandelate states the drug is not recommended if urine acidification is contraindicated or unattainable (e.g., when some urea-splitting bacteria are present).

Selection and Use of Anti-infectives

To reduce development of drug-resistant bacteria and maintain effectiveness of methenamine and other antibacterials, use only to prevent infections proven or strongly suspected to be caused by susceptible bacteria.

When selecting or modifying anti-infective therapy, use results of culture and in vitro susceptibility testing. In the absence of such data, consider local epidemiology and susceptibility patterns when selecting anti-infectives for empiric therapy.

Specific Populations

Pregnancy

Category C.

One manufacturer of methenamine hippurate states that safety during the last trimester is suggested, but not definitely proven.

Effects of methenamine during labor and delivery are unknown and there are no recognized uses for the drug during labor or delivery.

Lactation

Distributed into milk; discontinue nursing or the drug.

Pediatric Use

Has been used in children without unusual toxicity.

Geriatric Use

Insufficient experience in patients ≥65 years of age to determine whether they respond differently than younger adults; clinical experience has not identified differences.

Select dosage with caution, usually starting at the low end of the dosage range, because of possible age-related decreases in hepatic, renal, and/or cardiac function and concomitant disease and drug therapy.

Hepatic Impairment

Perform periodic liver function tests.

Methenamine hippurate is contraindicated in those with severe hepatic insufficiency.

Common Adverse Effects

GI disturbances (nausea, vomiting, diarrhea, abdominal cramps, anorexia), pruritus, rash, dysuria.

Drug Interactions

Specific Drugs and Laboratory Tests

Drug or Test

Interaction

Comments

Sulfonamides

Insoluble precipitate forms in urine between formaldehyde and some sulfonamides (e.g., sulfamethizole, sulfathiazole [not commercially available in the US])

Avoid concomitant use

Test for 17-hydroxycorticosteroid

Formaldehyde in urine causes falsely elevated concentrations when Porter-Silber method is used

Test for 5-hydroxyindoleacetic acid (5HIAA)

Formaldehyde in urine causes falsely decreased concentrations when nitrosonaphthol methods are used

Test for urinary catecholamines and vanillylmandelic acid (VMA)

Formaldehyde in urine causes erroneously high results when fluorometric procedures are used

Test for urine estriol

Formaldehyde in urine causes falsely decreased urine estriol concentrations when acid hydrolysis techniques are used

Enzymatic hydrolysis technique unaffected

Methenamine Pharmacokinetics

Absorption

Bioavailability

Readily absorbed from the GI tract. About 10–30% of oral dose is hydrolyzed by gastric acidity to formaldehyde and ammonia. Enteric coating of methenamine mandelate tablets reduces hydrolysis in the GI tract and rate of absorption.

Plasma Concentrations

Following oral administration of a usual single dose to healthy fasting adults, concentrations of methenamine and formaldehyde in plasma are generally very low and antibacterial activity in plasma is negligible.

Distribution

Extent

Crosses the placenta.

Distributed into milk.

Elimination

Elimination Route

Within 24 hours, ≥70–90% of a single oral dose is excreted intact in the urine by glomerular filtration and tubular secretion. When urine is acidic, methenamine is hydrolyzed to formaldehyde and ammonia; maximum hydrolysis occurs when urine pH is ≤5.5.

Mandelic acid (mandelate) and hippuric acid (hippurate) are excreted in urine by glomerular filtration and tubular excretion.

Stability

Storage

Oral

Tablets

15–30°C in tight, light resistant containers.

Actions and Spectrum

Advice to Patients

Preparations

Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.

Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.

* available from one or more manufacturer, distributor, and/or repackager by generic (nonproprietary) name

Methenamine Hippurate

Routes

Dosage Forms

Strengths

Brand Names

Manufacturer

Oral

Tablets

1 g*

Hiprex (with povidone and tartrazine; scored)

Sanofi-Aventis

Methenamine Hippurate Tablets

Corepharma

Urex (with povidone; scored)

Vatring

Methenamine Mandelate

Routes

Dosage Forms

Strengths

Brand Names

Manufacturer

Oral

Tablets, film-coated

500 mg

Mandelamine Hafgrams (with povidone and polyethylene glycol)

Warner Chilcott

1 g

Mandelamine (with povidone and polyethylene glycol)

Warner Chilcott

AHFS DI Essentials™. © Copyright 2024, Selected Revisions January 1, 2008. American Society of Health-System Pharmacists, Inc., 4500 East-West Highway, Suite 900, Bethesda, Maryland 20814.

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