Methenamine

Background

• Indicated for the prevention of urinary tract infections

Mechanism of Action

• The prodrug is hydrolysed into formaldehyde and ammonia in acidic urine

Spectrum of Activity

• Active against essentially all bacteria and some fungi
• Urease-positive organisms such as Proteus can inhibit its action by alkalinizing the urine and preventing the production of formaldehyde

Indications

• Prophylaxis against urinary tract infection, especially in people without underlying urinary tract abnormalities
• Levels of formaldehyde achieved in urine probably aren't high enough to treat active infection
• No systemic antimicrobial activity

Dosing

• Methenamine mandelate 1 g po QID (max 6 g daily)
  • Add urine acidifying drugs like ammonium chloride, ascorbic acid, or methionine if urine pH is greater than 5.5
• Methenamine hippurate 1 g PO bid
  • Does not need urinary acidification

Adverse Drug Reactions

• Nausea, vomiting, diarrhea
• Pruritis and rash
• Urinary tract irritation from formaldehyde, including dysuria, albuminuria, and hematuria
• Gout flares
• Rare elevations in liver enzymes