How the dose is calculated
Nitrofurantoin is dosed in mg/kg/day. Treatment of an active uncomplicated UTI uses a higher dose split four times a day; long-term prophylaxis against recurrent UTI uses a much lower once-nightly dose.
Typical dosing by indication
| Indication | Dose | Frequency | Max |
|---|---|---|---|
| UTI treatment | 6 mg/kg/day | every 6h (4×/day) | 400 mg/day |
| UTI prophylaxis | 1.5 mg/kg once nightly | once nightly | 100 mg/day |
Nitrofurantoin only achieves effective concentrations in the urine, not in blood or tissue — it treats uncomplicated lower urinary tract infection only, and is not appropriate for pyelonephritis or suspected systemic infection. It is also contraindicated in infants under 1 month and avoided near term in pregnancy, and requires caution in significant renal impairment (reduced efficacy and increased toxicity risk).
Clinical use
- Nitrofurantoin is a first-line option for uncomplicated cystitis because of low resistance rates and minimal impact on gut flora compared to broader-spectrum agents.
- Not suitable for suspected pyelonephritis or any infection beyond the bladder — it doesn't reach adequate tissue/blood concentrations outside the urinary tract. In febrile infants and young children, where renal involvement is more likely than isolated cystitis, an agent with reliable tissue penetration should be used instead.
- Low-dose once-nightly prophylaxis is used for recurrent UTI in selected patients, typically as a time-limited course reviewed periodically.
- Reduced efficacy and increased toxicity risk occur with significant renal impairment (reduced urinary concentration) — avoid or use with specialist input in this setting.
- Use with caution in anaemia, G6PD deficiency (haemolysis risk), vitamin B deficiency (peripheral neuropathy risk), and diabetes mellitus (higher baseline neuropathy and renal-impairment risk).
- Common brand/formulation names include Nitrofurantoin Mono, Macrobid (Mono/Mac, dual-release), and Mono-MCR — all deliver the same active drug; check the specific product's dosing interval, since dual-release formulations are often dosed twice daily rather than four times daily.
Frequently asked questions
What is nitrofurantoin used for?
Nitrofurantoin treats uncomplicated urinary tract infections (cystitis) and is also used at a lower dose for long-term prevention of recurrent UTIs — it is not used for kidney infections or infections outside the urinary tract.
Why can't nitrofurantoin be used for a kidney infection?
Nitrofurantoin only reaches effective concentrations in the urine, not in blood or kidney tissue, so it can't adequately treat pyelonephritis or any infection beyond the bladder.
Is nitrofurantoin safe for newborns?
No — it's contraindicated in infants under 1 month of age because of a risk of haemolytic anaemia related to immature enzyme systems.
Can nitrofurantoin be used in kidney disease?
It's generally avoided or used with caution in significant renal impairment, since reduced kidney function lowers the urinary drug concentration (reducing effectiveness) while increasing systemic toxicity risk.
Is nitrofurantoin safe in pregnancy?
It's commonly used for UTI in pregnancy, but is generally avoided near term (38-42 weeks gestation, and during labour/delivery) because of a theoretical risk of neonatal haemolytic anaemia if given shortly before birth, and some guidance also advises caution in the first trimester. Confirm current local/national guidance and the specific gestational age before prescribing.
What are the side effects of nitrofurantoin?
Common side effects include nausea, headache, and urine discolouration (harmless brown/dark yellow tint). Less common but important effects include pulmonary reactions (acute or chronic, especially with prolonged use), hepatotoxicity, and peripheral neuropathy (more likely with renal impairment, prolonged use, diabetes, or vitamin B deficiency) — report new breathlessness, cough, jaundice, or numbness/tingling promptly.
How many days should nitrofurantoin be taken for a UTI?
Typically 5-7 days for uncomplicated UTI treatment (shorter courses of 3 days are sometimes used in adult women per some guidelines), while prophylaxis against recurrent UTI is a much lower once-nightly dose continued for a longer, time-limited period (often 3-6 months) reviewed periodically — confirm the exact duration for the specific indication and patient.
References
- Gupta K, et al. International Clinical Practice Guidelines for Uncomplicated Cystitis (IDSA/ESCMID). Clin Infect Dis. 2011.
- British National Formulary for Children (BNFc) — Nitrofurantoin.
- American Academy of Pediatrics Red Book — Urinary Tract Infection.