Nitrofurantoin acts in the urine, so it depends on renal excretion. In reduced kidney function less reaches the bladder and more accumulates systemically. UK prescribing guidance advises avoiding it below an eGFR of 45 mL/min/1.73 m², allowing only short treatment courses with caution between 30 and 44 — so long-term prophylaxis needs an eGFR of 45 or more.
With long-term use, rare but serious lung and liver reactions can occur, which is another reason to review prophylaxis regularly.
- Check eGFR before starting nitrofurantoin and periodically during long-term use.
- Avoid long-term nitrofurantoin prophylaxis when eGFR is below 45.
- Ask about new breathlessness or cough in patients on long-term nitrofurantoin.
- Consider liver tests if a patient on long-term nitrofurantoin becomes unwell.
Why it matters
Older patients on long-term prophylaxis often lose kidney function without the prescription being revisited.
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