Nitrofurantoin Dosing for Postcoital UTI Prophylaxis
For premenopausal women with recurrent UTIs temporally related to sexual activity, nitrofurantoin 50 mg or 100 mg should be taken as a single dose within 2 hours after intercourse for 6-12 months. 1
Specific Dosing Regimen
Nitrofurantoin 50 mg as a single postcoital dose is the established prophylactic regimen, taken within 2 hours of sexual intercourse 1, 2
Nitrofurantoin 100 mg as a single postcoital dose is an alternative option that has also demonstrated efficacy in clinical trials 2
The duration of prophylaxis should be 6-12 months, with periodic assessment and monitoring 1
Evidence Supporting Postcoital Prophylaxis
The Journal of Urology guidelines specifically recommend low-dose postcoital antibiotics for premenopausal women whose recurrent UTIs are associated with sexual activity, as this strategy significantly reduces recurrence rates while minimizing adverse events compared to daily prophylaxis 1.
A landmark study of 25 premenopausal women demonstrated that postcoital prophylaxis reduced UTI episodes from 70 infections during 8 months of observation to only 4 infections during 12.5 months of treatment—with only 1 breakthrough infection occurring on nitrofurantoin therapy 2. This represents a dramatic reduction in infection burden.
Antibiotic Selection Considerations
Nitrofurantoin is preferred over fluoroquinolones and cephalosporins for prophylaxis due to antimicrobial stewardship principles 1
Alternative postcoital prophylactic agents include trimethoprim-sulfamethoxazole 40/200 mg or trimethoprim 100 mg, though antibiotic choice should account for the patient's prior organism susceptibility profile and drug allergies 1
Rotating antibiotics at 3-month intervals can be considered to avoid selection of antimicrobial resistance 1
Mechanism and Timing
Sexual intercourse is a major factor inducing recurrent UTIs in premenopausal women, typically within 24 hours, by transferring pre-existing introital bacteria into the bladder 2. The postcoital prophylactic dose prevents bacterial establishment before infection can develop.
Non-Antibiotic Alternatives
Methenamine hippurate and/or lactobacillus-containing probiotics may be offered to patients who desire non-antibiotic alternatives 1
Cranberry prophylaxis is a conditional recommendation for patients preferring non-antimicrobial options 1
Common Pitfalls to Avoid
Do not use daily prophylaxis when infections are clearly postcoital—intermittent postcoital dosing is equally effective with fewer adverse events and less antibiotic exposure 1, 2
Do not treat asymptomatic bacteriuria in women with recurrent UTIs, as this fosters antimicrobial resistance and paradoxically increases recurrence rates 1
Do not classify these patients as having "complicated UTI" unless true complicating factors exist (structural abnormalities, immunosuppression, pregnancy), as misclassification leads to unnecessary broad-spectrum antibiotics 1
Monitoring and Duration
Some women remain on postcoital prophylaxis for years to maintain benefit without adverse events, though continuing prophylaxis beyond 12 months is not evidence-based 1. Periodic reassessment should occur at 6-12 month intervals to determine if prophylaxis remains necessary.