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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Urinary antiseptics

Antibacterials that reach useful levels only in the urine: nitrofurantoin, fosfomycin and methenamine, for cystitis and its prevention.

Updated 2026-11-01

## What they are

A practical group of unrelated drugs whose activity is concentrated in the bladder. They treat or prevent lower urinary tract infection and are not suitable for pyelonephritis or bloodstream infection, because tissue and blood levels stay low.

## Mechanism of action

Each works differently: nitrofurantoin is reduced by bacterial flavoproteins to reactive products that damage ribosomes and other macromolecules; fosfomycin irreversibly blocks MurA, an early step of cell-wall synthesis; methenamine releases formaldehyde in acid urine, which denatures bacterial proteins.

Target: Different for each drug (bacterial flavoproteins, MurA, formaldehyde in acid urine)

## Key points

- Used for cystitis, not for pyelonephritis or sepsis.
- Nitrofurantoin: chronic lung fibrosis, liver injury and neuropathy with long use; avoided in poor kidney function and at term.
- Fosfomycin is given as a single oral sachet for uncomplicated cystitis.
- Methenamine needs acid urine; it is a prevention drug, not a treatment for active infection.
- Nitrofurantoin is in the WHO «Access» group: a first-line choice that spares broader antibiotics.

## Drugs in this class

- [**Nitrofurantoin** A first-line oral drug for uncomplicated cystitis that acts only in the bladder. Long courses can harm the lungs… Bacterial ribosomal proteins and other macromolecules (after reduction by bacterial flavoproteins)](https://durgical.com/en/pharmacology/nitrofurantoin/)
- [**Fosfomycin** A small antibiotic that blocks the first step of cell-wall building. One oral sachet treats uncomplicated cystitis; an… UDP-N-acetylglucosamine enolpyruvyl transferase (MurA)](https://durgical.com/en/pharmacology/fosfomycin/)
- [**Methenamine** A urinary antiseptic that turns into formaldehyde in acid urine. Used long-term to prevent recurrent cystitis; it does… Bacterial proteins and nucleic acids in the urine (via formaldehyde)](https://durgical.com/en/pharmacology/methenamine/)

## Others in the class

- nalidixic acid (older quinolone)

## Test yourself

Think of the answer, then tap the question to see it.

## Why are urinary antiseptics not used in pyelonephritis?

They reach adequate levels only in the bladder urine, not in kidney tissue or blood.

## What does methenamine need to work?

Acid urine, where it is hydrolysed to formaldehyde.

## Which enzyme does fosfomycin block?

MurA (enolpyruvyl transferase), an early step of peptidoglycan synthesis.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - NITROFURANTOIN- MONOHYDRATE/MACROCRYSTALS capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=35c0a6d8-ff85-4d9b-9ad3-ae20808119dd) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - FOSFOMYCIN TROMETHAMINE granule, for solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=36bca22f-49ea-42ce-926b-035c42eb6038) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - METHENAMINE HIPPURATE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a3eeacd4-1983-49ec-a60d-3abaf505ce57) (DailyMed (U.S. National Library of Medicine), 2026); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/?query=nitrofurantoin) (World Health Organization, 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/classes/urinary-antiseptics/