Urinary antibacterial (nitrofuran)
Pregnancy: Safe in first and second trimester. AVOID at 36 weeks and beyond (neonatal haemolytic anaemia).
Nitrofurantoin
Brand names: Macrobid, Macrodantin, Nitrofurantoin MR
Adult dose
Dose: Treatment: 100 mg MR twice daily for 3–7 days (women); 7 days (men); Prophylaxis: 50–100 mg at night
Route: Oral (take with food to reduce GI side effects and improve absorption)
Frequency: Twice daily (MR); four times daily (standard release); once nightly (prophylaxis)
Max: 400 mg/day (standard release); 200 mg/day (MR)
Uncomplicated UTI (women): 100 mg MR BD for 3 days (NICE NG112). Complicated UTI or males: 7 days. Prophylaxis (recurrent UTI): 50 mg or 100 mg at night. Do not use if eGFR <45 (inadequate urinary levels and toxicity).
Paediatric dose
Dose: 1 mg/kg
Route: Oral
Frequency: Four times daily
Max: 100 mg/dose
Concentration: 25 mg/ml
≥3 months: 1 mg/kg 4 times daily (max 100 mg QDS) for 7 days. Prophylaxis: 1 mg/kg at night (max 100 mg). Not recommended in infants <3 months. Contraindicated at term and in neonates (haemolytic anaemia risk).
Dose adjustments
Renal
AVOID if eGFR <45 mL/min (inadequate urinary concentrations; risk of peripheral neuropathy and pulmonary toxicity).
Hepatic
Use with caution in hepatic impairment; monitor LFTs.
Paediatric weight-based calculator
≥3 months: 1 mg/kg 4 times daily (max 100 mg QDS) for 7 days. Prophylaxis: 1 mg/kg at night (max 100 mg). Not recommended in infants <3 months. Contraindicated at term and in neonates (haemolytic anaemia risk).
Clinical pearls
- Do not use for pyelonephritis or upper UTI — does not achieve adequate tissue levels
- Only for uncomplicated lower UTI (cystitis)
- eGFR <45: use trimethoprim or cefalexin instead
- Pulmonary reactions with long-term use — check CXR if new respiratory symptoms
- Urine turns dark yellow/brown — warn patient (harmless)
Contraindications
- eGFR <45 mL/min
- Pregnancy at term (36 weeks and beyond) — neonatal haemolytic anaemia
- Neonates <3 months
- G6PD deficiency (haemolysis)
Side effects
- GI upset and nausea (take with food)
- Pulmonary reactions (acute: pneumonitis; chronic: pulmonary fibrosis with long-term use)
- Peripheral neuropathy (long-term use or renal impairment)
- Hepatotoxicity
- Haemolytic anaemia (G6PD deficiency)
- Dark urine (harmless)
Interactions
- Antacids containing magnesium trisalicate — reduce nitrofurantoin absorption
- Quinolones — antagonistic effect
- Probenecid — increases nitrofurantoin levels and toxicity
Monitoring
- Renal function (eGFR before and periodically)
- LFTs (prolonged prophylaxis)
- Respiratory symptoms (long-term)
Reference: BNFc; BNF; NICE NG112 UTIs; PHE Antimicrobial Guidelines; BNFc. Verify against your local formulary and the latest BNF before prescribing.
Related
Curated clinical cross-links plus same-class fallbacks.
Calculators