This patient should begin low-dose antimicrobial prophylaxis. She has recurrent urinary tract infection (UTI), which is defined as three UTIs in the previous 12 months or two UTIs in the previous 6 months. Recurrent UTI affects 20% to 30% of women. She has already tried a modifiable behavioral practice by voiding after sexual intercourse. Scientific evidence is lacking for most behavioral strategies, but they are low risk and worth attempting. Low-dose antimicrobial prophylaxis is an effective intervention to manage frequent, recurrent, acute, uncomplicated UTI. It may be administered daily or every other day, generally at bedtime, or as postcoital prophylaxis. The initial duration of prophylaxis is generally 6 months; however, 50% of women experience recurrence by 3 months after discontinuation of prophylaxis. If this occurs, prophylaxis should be reinstated for 1 or 2 years with reassessment after that time. Because continuous prophylaxis may result in unnecessary antimicrobial use, an alternative strategy is patient self-diagnosis and self-treatment at the start of symptoms.

Methenamine salts produce formaldehyde, which acts as a bacteriostatic agent without affecting microbial susceptibility to antibiotic agents. They are well tolerated and effective. However, carcinogenic potential is a concern if they are used at high doses for a prolonged duration. They can be used for up to 1 week to prevent UTI in patients without urinary tract abnormalities, but not long term.

Vitamin C (ascorbic acid) has not been shown to be effective and is therefore not recommended for the prevention of UTI based on current evidence.

Anatomic or functional abnormalities of the urinary tract should be excluded as a cause of recurrent UTI in men and postmenopausal women. In premenopausal women, however, the yield for this indication is low. Therefore, urethroscopic assessment of this patient would not be indicated.