• Follow local guidelines when available. Otherwise:
• For empirical treatment, prescribe either:
• Trimethoprim 200 mg twice daily, for 3 days, or
• Nitrofurantoin 50 mg four times daily, or 100 mg (modified-release) twice daily, for 3 days.
• If the woman has been treated with trimethoprim recently (up to a year previously), consider prescribing nitrofurantoin instead of trimethoprim.

Choice of antibiotic

• The reasons for preferring trimethoprim and nitrofurantoin as first-line options for treating cystitis are discussed in Choice of antibiotic.

Considering nitrofurantoin when trimethoprim has been used recently

• Nitrofurantoin may be preferable for empirical prescribing when the woman has recently used trimethoprim because there is evidence that uropathogens are more likely to be resistant to trimethoprim if it has been used recently (up to the past year). The evidence is not clear enough to recommend precise thresholds of exposure.
• Trimethoprim is not preferred when nitrofurantoin has previously been used because there is no evidence that previous treatment with nitrofurantoin increases the chance that future infections will be resistant organisms. Furthermore, laboratory studies find that nitrofurantoin-resistant Escherichia coli reproduce substantially less effectively than nitrofurantoin-sensitive E. coli (in other words, nitrofurantoin resistance imposes a high fitness cost on the organism) [Sandegren et al, 2008].