Methenamine

Methenamine is a prescription medicine used for the prevention and treatment of urinary tract infections.

• Methenamine is available under the following different brand names: Hiprex, Urex

Adult and pediatric dosage

Oral solution

• 500 mg/5mL

Tablet, hippurate

• 1g

Tablet, mandelate

• 500mg
• 1g

Urinary Tract Infection Prophylaxis

Adult dosage

• Methenamine hippurate: 1 g orally every 12 hours
• Methenamine mandelate: 1 g orally every 6 hours

Pediatric dosage

Methenamine hippurate

• Children below 6 years: Safety and efficacy not established
• Children between 6 to 12 years: 0.5-1 g orally every 12 hours
• Children above 12 years: 1 g orally every 12 hours

Methenamine mandelate

• Children below 6 years: 18.4 mg/kg orally every 6 hours
• Children between 6 to 12 years: 0.5-1 g orally every 6 hours
• Children above 12 years: 1 g orally every 6 hours

Dosage Considerations – Should be Given as Follows: 

• See “Dosages”

Common side effects of Methenamine include:

• nausea,
• vomiting,
• upset stomach,
• diarrhea,
• abdominal cramps,
• painful or difficult urination,
• loss of appetite, and
• skin rash.

Serious side effects of Methenamine include:

• increased urination,
• painful urination,
• blood in urine,
• swelling, and
• shortness of breath.

Rare side effects of Methenamine include:

• none 

This is not a complete list of side effects and other serious side effects or health problems that may occur as a result of the use of this drug. Call your doctor for medical advice about serious side effects or adverse reactions. You may report side effects or health problems to FDA at 1-800-FDA-1088.

If your medical doctor is using this medicine to treat your pain, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider, or pharmacist first.

• Methenamine has severe interactions with no other drugs.
• Methenamine has serious interactions with the following drugs:
  • acetazolamide
  • cholera vaccine
  • sulfadiazine
  • sulfamethoxazole
  • sulfisoxazole
• Methenamine has moderate interactions with the following drugs:
  • pseudoephedrine
  • sodium picosulfate/magnesium oxide/anhydrous citric acid
• Methenamine has minor interactions with no other drugs. 

This information does not contain all possible interactions or adverse effects. Visit the RxList Drug Interaction Checker for any drug interactions. Therefore, before using this product, tell your doctor or pharmacist about all the products you use. Keep a list of all your medications with you and share this information with your doctor and pharmacist. Check with your health care professional or doctor for additional medical advice, or if you have health questions or concerns.

Contraindications

• Hypersensitivity to methenamine or tartrazine (FD&C Yellow No. 5)
• Severe renal or hepatic insufficiency
• Concurrent sulfonamides or acetazolamide may form insoluble precipitate in urine

Effects of drug abuse

• None

Short-Term Effects

• See “What Are Side Effects Associated with Using Methenamine?”

Long-Term Effects

• See “What Are Side Effects Associated with Using Methenamine?”

Cautions

• Patients with pre-existing hepatic insufficiency may suffer adverse effects from the small amounts of ammonia and formaldehyde that are produced; the classical syndrome of acute hepatic failure may be evoked in these patients
• Taking therapy in absence of proven or strongly suspected bacterial infection or a prophylactic indication is unlikely to provide benefit to the patient and increases the risk of development of drug-resistant bacteria
• Large doses of methenamine (8 g daily for 3 to 4 weeks) have caused bladder irritation, painful and frequent micturition, albuminuria, and gross hematuria
• Maintain acidic pH of urine, especially when treating urea-splitting organisms, e.g. Proteus, Pseudomonas
• Monitor LFTs, especially in patients with a history of liver impairment
• Safe use is not established during pregnancy, especially 1st and 2nd trimesters
• May precipitate uric acid stones in patients with gout

Pregnancy and Lactation

• Use with caution if the benefits outweigh the risks during pregnancy.
• Lactation
  • Excreted in breast milk; no adverse effects documented