Urinary Tract Antiseptics : Uses, Side Effects, Interactions, & Contraindications

Urinary Tract Antiseptics : Definition, Dose, Classification, Mechanism of Action, Antibacterial Spectrum, Therapeutic Uses, Clinical Uses, Side Effects, Drug Interactions, & Contraindications


  • Urinary Tract Antiseptics are anti microbial agents that do not achieve antibacterial levels in the circulation, but get concentrated in the urine only (a form of local therapy) with little or no systemic antibacterial effect.
  • Like many other drugs, Urinary Tract Antiseptics are concentrated in the kidney tubules, and are useful mainly in lower urinary tract infection.


  • Methenamine
  • Nitrofurantoin
  • Nalidixic acid can also be considered to be a urinary antiseptic.

Mechanism of action

  • Methenamine is inactive as such. It decomposes slowly at an acidic pH of 5.5 or less (acidic) in urine to release formaldehyde which is toxic to bacteria & inhibits all bacteria.
  • Nitrofurantoin sensitive bacteria reduce the drug to a highly active intermediate that inhibits various enzymes and damages bacterial DNA.

Antibacterial spectrum

  • Urea-splitting bacteria that alkalinize the urine, such as Proteus species, are usually resistant to the action of methenamine. It is primarily bacteriostatic, but may be cidal at higher concentrations and in acidic urine.
  • Nitrofurantoin inhibits many gram-negative bacteria but due to development of resistance, activity is now restricted largely to E. coli. Gram-positive cocci (for example, S. saprophyticus) are typically susceptible.

Therapeutic uses

Nitrofurantoin is indicated for uncomplicated lower urinary tract infection oniy but it is infrequently used now.

  • Acute infections due to E. coli can be treated with 50-100 mg TDS, given for 5-10 days. These doses should not be used for > 2 weeks at a time.
  • Suppressive long-term treatment has been successful with 50 mg BD.
  • It is also employed for prophylaxis of urinary tract infection when catheterization or instrumentation of the lower urinary tract is performed .

Methenamine is primarily used for chronic suppressive therapy(chronic, resistant type of urinary tract infections, not involving kidney substance) to reduce the frequency of UTIs.

  • It is not a good drug for acute urinary tract infections or for catheterization prophylaxis.
  • Routine use in patients with chronic urinary catheterization to reduce catheter associated bacteriuria or catheter-associated UTI is not generally recommended.
  • It should not be used to treat upper UTIs (for example, pyelonephritis).


  • Bacteria do not develop resistance to formaldehyde, which is an advantage of this drug.

Side effects

Nitrofurantoin may cause following side effects :

  • Gastrointestinal intolerance – nausea, epigastric pain and diarrhoea.
  • Haemolytic anaemia is rare, except in patients with G-6-PD deficiency.
  • An acute reaction with chills, fever and leucopenia occurs occasionally.
  • Neurologic problems like eripheral neuritis with long-term use.
  • Interstitial pulmonary fibrosis with chronic use.
  • Acute pneumonitis, Liver damage.
  • Urine of patients taking nitrofurantoin turns dark brown on exposure to air

Methenamine may cause following side effects :

  • Gastrointestinal distress
  • Gastritis (due to release of formaldehyde in stomach-patient compliance is often poor due to this).
  • Rashes
  • Chemical cystitis, haematuria & albuminuria may develop with high doses given for long periods.
  • CNS symptoms are produced occasionally.

Drug Interactions

  • Methenamine Χ Sulfonamides such as cotrimoxazole :  Increases the risk of crystalluria and mutual antagonism.
  • Nitrofurantoin Χ Nalidixic acid : Antagonizes the bactericidal action of nalidixic acid.
  • Nitrofurantoin Χ Probenecid : Probenecid inhibits its tubular secretion and reduces the concentration attained in urine -may interfere with its urinary antiseptic action.


  • Contraindicated in renal failure.

For detailed query or in case of uncertainty, Always consult your doctor or pharmacist.

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