Urinary antiseptics · Anti-infectives
Nitrofurantoin
Macrobid · Macrodantin · Macrobid® · MacroBID®(brand names vary by market)
Treats the urine and nothing else, and needs working kidneys to get there.
Also: oral liquid
Mechanism
Reduced by bacterial flavoproteins into reactive intermediates that damage ribosomes, DNA and metabolic enzymes at once. Multiple simultaneous targets are why resistance has stayed remarkably low for decades.
Indications
- Uncomplicated cystitis
- Recurrent urinary tract infection prophylaxis
Formulations
Dosing concepts
The renal paradox
It works by being concentrated in urine. As clearance falls, urinary concentrations drop below therapeutic while systemic exposure rises, so it becomes less effective and more toxic at the same time.
Not for the kidney
It achieves no useful tissue concentration, so pyelonephritis and any suspicion of bacteraemia rule it out entirely.
Concepts, not prescribing instructions. Practice numbers live in current references and local protocols.
Pharmacokinetics
- Bioavailability
- Well absorbed, especially with food
- Half-life
- ~20 min in plasma; the relevant concentration is urinary
- Elimination
- renal: Rapidly concentrated into urine, which is both the mechanism and the limitation
Adverse effects
- NauseacommonReduced by taking it with food.
- Brown or dark urinecommonHarmless and worth mentioning so it does not alarm.
- Acute pulmonary reactionseriousFever, cough and breathlessness within days; a hypersensitivity phenomenon that resolves on stopping.
- Chronic pulmonary fibrosisseriousWith long-term prophylaxis, insidious and sometimes irreversible.
- Peripheral neuropathyseriousEspecially in renal impairment and with prolonged use.
- Hemolysis in G6PD deficiencyserious
Contraindications & precautions
Contraindications
- Creatinine clearance below roughly 30 mL/min
- Pyelonephritis or suspected bacteraemia
- Term pregnancy and neonates
- G6PD deficiency
Precautions
- Long-term prophylaxis
- Older adults
- Existing lung disease
Interactions
No curated interaction entries yet for this agent.
Monitoring
Renal function before prescribing
It determines both efficacy and safety, in the same direction
Respiratory symptoms on long-term use
Pulmonary toxicity, which is easy to attribute to age or to the chest itself
Whether prophylaxis is still needed
Long courses accumulate the serious toxicities
Counselling points
- Take it with food to reduce nausea and improve absorption.
- Your urine may turn dark brown; this is expected.
- Report any new cough or breathlessness.
- Finish the course.
Clinical pearls
- Prescribing it for pyelonephritis treats the urine while the kidney infection continues. It is a bladder drug in the most literal sense.
- The most common inappropriate use is in an older patient whose eGFR has quietly fallen below the threshold, where it is simultaneously ineffective and more likely to cause neuropathy.
References & review
- Nitrofurantoin product monograph (consult the current version for your market) (monograph)
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. (reference-work)
Last reviewed 2026-08-16 · jurisdiction: global · drug information changes; verify against current references before practice use.