Participants randomized to this arm receive solifenacin succinate 5 mg, an oral selective M3 antimuscarinic, taken once daily for 2 months. Solifenacin reduces overactive bladder symptoms by blocking muscarinic receptor-mediated detrusor contractions.

Participants randomized to this arm receive mirabegron 50 mg, an oral β3-adrenergic agonist, taken once daily for 2 months. Mirabegron relieves overactive bladder symptoms through a non-anticholinergic mechanism, promoting detrusor relaxation during the storage phase.

Participants randomized to this arm receive mirabegron 50 mg, an oral β3-adrenergic agonist, taken once daily for 2 months. Mirabegron relieves overactive bladder symptoms through a non-anticholinergic mechanism, promoting detrusor relaxation during the storage phase.

Participants randomized to this arm receive solifenacin succinate 5 mg, an oral selective M3 antimuscarinic, taken once daily for 2 months. Solifenacin reduces overactive bladder symptoms by blocking muscarinic receptor-mediated detrusor contractions.

Patients will receive solifenacin succinate 5 mg, an oral selective M3 antimuscarinic, taken once daily for 2 months. Solifenacin reduces overactive bladder symptoms by blocking muscarinic receptor-mediated detrusor contractions.

This measure captures how much overactive bladder symptoms change after two months of treatment. The ICIQ-OAB (International Consultation on Incontinence Questionnaire - Overactive Bladder) is a validated, patient-completed questionnaire covering the core symptoms of the condition: daytime urinary frequency, night-time urination (nocturia), urgency, and urgency-related leakage. The four symptom items are summed into a total score ranging from 0 to 16, where a higher score means more severe symptoms. Participants complete the questionnaire at the start of treatment (baseline) and again at 2 months; the outcome is the change between these two time points, with a larger reduction indicating greater symptom improvement. The two treatment arms are compared using an analysis of covariance (ANCOVA) adjusted for each participant's baseline score, performed separately within the high anticholinergic burden group (Drug Burden Index ≥ 1) and the low burden group (DBI < 1).

This measure reports the proportion of participants who achieve a clinically meaningful improvement in symptoms. A participant is classified as a "responder" if their ICIQ-OAB total score falls by at least 3 points from baseline to 2 months - a reduction generally regarded as noticeable and worthwhile to patients. The outcome is the percentage of responders in each treatment arm.

This exploratory analysis examines whether the difference in effectiveness between the two treatments depends on a participant's baseline anticholinergic burden - that is, whether one treatment works comparatively better in the high-burden group (DBI ≥ 1) and the other in the low-burden group (DBI < 1). It is assessed by testing the statistical interaction between treatment arm and DBI stratum for the change in ICIQ-OAB total score at 2 months. This analysis is hypothesis-generating only; the study is not designed with sufficient statistical power to confirm such an interaction.