KEY POINTS
- Methenamine (Hiprex, Urex) is a urinary tract antiseptic. It is hydrolyzed to formaldehyde and ammonia in acidic urine, which produces antibacterial effects.
- The urine pH needs to be acidic (pH below 5.5 is a common goal) for it to convert to formaldehyde.
- Ascorbic acid (vitamin C) is a common drug used to acidify the urine, but it can have unfavorable gastrointestinal effects
- If urine acidification is not possible, use of methenamine should be discouraged
- Bacteria are not thought to develop resistance to formaldehyde, making methenamine an appealing option for long-term suppression therapy
- The urine pH needs to be acidic (pH below 5.5 is a common goal) for it to convert to formaldehyde.
- Only indicated for the prophylaxis or suppression of urinary tract infection
- Patients must retain urine for methenamine to allow the drug to produce it’s antibacterial effect in the bladder
- Patients who are chronically catheterized and do not retain urine are poor candidates for methenamine therapy
- Avoid concomitant use with sulfonamides
- Avoid use in patients with renal or hepatic dysfunction
- Avoid in patients with gout
- Is generally well tolerated, but beware AST/ALT increase, gastrointestinal effects, pruritus and bladder irritation
- Available as hippurate (Urex) or mandelate (Hiprex) formulation
- Typically dosed 1gm twice daily for hippurate
- Typically dosed 1gm every 6 hours for mandelate
RESOURCES
- Methenamine hippurate (Urex) Package Insert
- Methenamine hippurate (Hiprex) Package Insert
- IDSA Practice Guidelines
- Methenamine for urinary tract infection prophylaxis: A systematic review (2023)
- Alternative to prophylactic antibiotics for the treatment of recurrent urinary tract infections in women: multicentre, open label, randomised, non-inferiority trial
Methenamine is an old drug that is not frequently used. If you have a good resource on this drug, please share it with me! –> IDstewardship@gmail.com
