Urinary antibacterial (nitrofuran) Pregnancy: Contraindicated in pregnant women at term (38 to 42 weeks gestation), during labour and delivery, or when the onset of labour is imminent, because of the possibility of haemolytic anaemia in the infant. Published epidemiological studies of first-trimester use have reported inconsistent findings for major birth defects and cannot definitively establish the presence or absence of risk; no fetotoxicity was observed in animal reproduction studies at up to 6 times the recommended human dose.

Nitrofurantoin

Brand names: Macrobid, Macrodantin, Nitrofurantoin MR

Nitrofurantoin is an antibiotic used for lower urinary tract infection and, at low dose, for UTI prophylaxis.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 50 mg to 100 mg

Route: Oral

Frequency: Four times a day

US labelling for nitrofurantoin oral suspension (25 mg/5 mL) — no UK SPC (eMC) was available in the source bundle, so the regimen must be verified against the UK SPC. The lower dosage level is recommended for uncomplicated urinary tract infections. For long-term suppressive therapy in adults, a reduction of dosage to 50 mg to 100 mg at bedtime may be adequate; the benefits of long-term suppressive therapy should be balanced against the increased potential for systemic toxicity and for the development of antibacterial resistance. Administer with food to improve drug absorption and, in some patients, tolerance. Continue therapy for one week or for at least 3 days after sterility of the urine is obtained; continued infection indicates the need for reevaluation.

Paediatric dose

Route: Oral

Frequency: Four divided doses per 24 hours

Max: 40 kg or greater — see adult dose

US labelling: 5 mg/kg to 7 mg/kg of body weight per 24 hours, given in four divided doses, in paediatric patients aged 1 month and older (a range is stated, so no single per-kg figure is given). Contraindicated in paediatric patients younger than 1 month of age because of the possibility of haemolytic anaemia due to immature erythrocyte enzyme systems (glutathione instability). Administer with food. For long-term suppressive therapy in paediatric patients, doses as low as 1 mg/kg per 24 hours, given as a single dose or in two divided doses, may be adequate. Verify against a children's formulary.

Dose adjustments

Renal

Contraindicated in patients who have anuria, oliguria, or significant impairment of renal function (creatinine clearance under 60 mL per minute or clinically significant elevated serum creatinine), due to an increased risk of toxicity resulting from impaired excretion of the drug.

Dose auto-extracted from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

US labelling (FDA)

Reference — US labelling, may differ from UK

2 DOSAGE & ADMINISTRATION Adult Patients: 50 mg to 100 mg four times a day -the lower dosage level is recommended for uncomplicated urinary tract infections. (2.2) Pediatric Patients: 5 mg/kg to 7 mg/kg of body weight per 24 hours, given in four divided doses (contraindicated under one month of age). (2.3) 2.1 Recommended Dosage and Administration in Adult Patients The recommended dosage is 50 mg to 100 mg of nitrofurantoin four times a day. For long-term suppressive therapy in adults, a reduction of dosage to 50 mg to 100 mg at bedtime may be adequate . The benefits of long-term suppressive therapy should be balanced against the increased potential for systemic toxicity and for the …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2024-03-29. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Contraindications

Known hypersensitivity to nitrofurantoin
Previous history of cholestatic jaundice/hepatic dysfunction associated with nitrofurantoin
Anuria, oliguria, or significant impairment of renal function (creatinine clearance under 60 mL per minute or clinically significant elevated serum creatinine)
Pregnant patients at term (38 to 42 weeks gestation), during labour and delivery, or when the onset of labour is imminent
Paediatric patients younger than 1 month of age (risk of haemolytic anaemia)

Side effects

Pulmonary reactions — acute, subacute or chronic pulmonary hypersensitivity reactions (malaise, dyspnoea on exertion, cough, altered pulmonary function; chronic reactions generally after six months or more of continuous treatment)
Hypersensitivity reactions, including serious and occasionally fatal anaphylactic reactions
Hepatotoxicity (including hepatitis)
Peripheral neuropathy
Haemolytic anaemia
Clostridioides difficile-associated diarrhoea

Interactions

Antacids containing magnesium trisilicate — reduce both the rate and extent of nitrofurantoin absorption; if coadministration cannot be avoided, monitor for lack of efficacy
Uricosuric drugs such as probenecid and sulfinpyrazone — inhibit renal tubular secretion of nitrofurantoin, increasing serum levels (toxicity) and decreasing urinary levels (reduced efficacy); monitor for adverse reactions
Laboratory interference — may cause a false-positive urinary glucose result with Benedict's or Fehling's copper reduction methods; use enzymatic glucose oxidase based tests

Clinical monograph

How it works

It is concentrated in the urine, where its reactive metabolites damage bacterial DNA, ribosomal proteins and other macromolecules; this urinary concentration is why it treats lower UTI but not systemic or upper-tract infection.

Prescribing in practice

Effectiveness depends on adequate renal function and urinary concentration — avoid when eGFR is low, where it is both less effective and more toxic.
Avoid at term in pregnancy and in babies under about 3 months because of the risk of neonatal haemolysis.
Rare but serious pulmonary and hepatic reactions can occur, particularly with long-term prophylactic use.

Monitoring

Short courses need no routine monitoring; with long-term prophylaxis review renal, pulmonary and hepatic status periodically.

Counselling the patient

Take it with food to improve absorption and reduce nausea.
It can turn the urine dark yellow or brown, which is harmless.
Report new breathlessness, persistent cough, or yellowing of the skin or eyes.

Evidence & guidelines

Nitrofurantoin is a first-line option for uncomplicated lower UTI where eGFR is adequate, per NICE/PHE antimicrobial guidance.

Reference: NICE NG112 UTIs; PHE Antimicrobial Guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways