Drug information of Methenamine

Mechanism of effect

Methenamine does not have antibacterial properties in an alkaline environment (pH≥6); however, in a more acidic environment (pH<6), methenamine is hydrolyzed to formaldehyde. Formaldehyde is considered to be highly bactericidal. Formaldehyde has nonspecific antibacterial activity and works by denaturing proteins and nucleic acid of bacteria.

 Certain bacteria such as Proteus sp. can alkalize urine, inhibiting the beneficial activity of formaldehyde.  The key role of the salt component of the drug, for example hippuric acid, is to maintain the acidic state of the urine.

Pharmacodynamic

Ingestion of a 1-gram dose of methenamine hippurate produces antibacterial activity in the urine within 1/2 hour. Administration of 1 g twice daily produces continuous antibacterial activity in the urine.

Pharmacokinetics

Half-life: 4.3 hr

Onset: 30 min

Peak plasma time: 3-8 hr

Vd: 0.56 L/kg

Excretion: urine; methenamine 90% excreted w/in 24 hr; mandelic or hippuric acid may accumulate with severe renal impairment

Drug indications

Urinary Tract Infections

Dosage

Adult

Urinary Tract Infection Prophylaxis

Indicated for prophylactic or suppressive treatment of frequently recurring urinary tract infections when long-term therapy is considered necessary

Methenamine hippurate: 1 g PO q12hr

Methenamine mandelate: 1 g PO q6hr

Pediatric

Urinary Tract Infection Prophylaxis

Indicated for prophylactic or suppressive treatment of frequently recurring urinary tract infections when long-term therapy is considered necessary

Methenamine hippurate

• <6 years: Safety and efficacy not established
• 6-12 years: 0.5-1 g PO q12hr
• >12 years: 1 g PO q12hr

Methenamine mandelate

• <6 years: 18.4 mg/kg PO q6hr
• 6-12 years: 0.5-1 g PO q6hr
• >12 years: 1 g PO q6hr

Drug 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

Side effects

Dysuria, Gastric upset, Nausea, Rash

Interactions

acetazolamide, cholera vaccine, sulfadiazine, sulfamethoxazole, sulfisoxazole

Alerts

Large doses may cause bladder irritation, urinary frequency, albuminuria, hematuria

Maintain acidic pH of urine, especially when treating urea-splitting organisms, e.g. Proteus, Pseudomonas

Monitor LFTs, especially in patients with history of liver impairment

Safe use not established during pregnancy, especially 1st and 2nd trimester

May precipitate uric acid stones in patients with gout

Points of recommendation

• Have blood work checked as you have been told by the doctor. Talk with the doctor.
• If you are allergic to tartrazine, talk with your doctor. Some products have tartrazine.
• You may need to check the acid content (pH) of your urine using a dipstick test.
• Talk with your doctor before you take aspirin, antacids, or drugs that raise the pH of the urine while on methenamine.
• Talk with your doctor before you eat citrus fruits, dairy products, and some veggies while taking methenamine.
• This medicine may affect certain lab tests. Tell all of your health care providers and lab workers that you take methenamine.
• Tell your doctor if you are pregnant or plan on getting pregnant. You will need to talk about the benefits and risks of using methenamine while you are pregnant.
• Tell your doctor if you are breast-feeding. You will need to talk about any risks to your baby.

• To gain the most benefit, do not miss doses.
• Keep using methenamine as you have been told by your doctor or other health care provider, even if you feel well.
• Take with or without food.

• Take a missed dose as soon as you think about it.
• If it is close to the time for your next dose, skip the missed dose and go back to your normal time.
• Do not take 2 doses at the same time or extra doses.

Pregnancy level

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