This open-label, parallel-group randomized controlled trial compares two prophylaxis strategies for recurrent urinary tract infection (rUTI) in postmenopausal women: (1) nightly oral nitrofurantoin 100 mg for 6 months versus (2) a vaginal tablet containing Lactobacillus with ultra-low-dose estriol (Gynoflor: nightly for 14 days, then twice weekly to month 6). Participants (≥40 years) are randomized 1:1 in computer-generated blocks and followed for 6 months. The primary endpoint is the proportion with rUTI recurrence within 6 months. Secondary endpoints include time to first recurrence, antibiotic resistance in breakthrough infections, change in lower urinary tract symptoms (Thai RUTISS), and urinary microbiome measures (Lactobacillus dominance and community diversity by qPCR and 16S rRNA). Key assessments occur at baseline and month 6 (urinalysis, culture, urinary microbiome sampling, and kidney function). Adherence and adverse events are captured via twice-weekly phone/Line contacts. Approximately 100 participants (50 per arm) will be enrolled as a feasibility-sized pilot.