The investigators propose conducting a randomized controlled trial at the Men's Health Clinic to evaluate whether patients who start and continue mirabegron postoperatively after undergoing Rezum therapy experience enhanced patient-reported outcomes, improved quality of life (QoL), and comparable rates of adverse events. There are no predicted adverse events of this study. There are no identified potential harms of this study.

Benign prostatic hyperplasia (BPH) is a highly prevalent condition in men that can affect up to 80% of men by the age 80 1 . Minimally invasive surgical therapies (MISTs) are a growing area of research to provide adequate symptoms relief while avoiding potential adverse side effects. The Rezum 2 study was able to demonstrate significant improvement in symptom scores as well as uroflow parameters with preserved sexual function, and has become a valuable tool in a urologist's armamentarium for the management of LUTS/BPH. Despite the ultimate symptomatic relief from surgical management of BPH, the most common unwanted adverse event post treatment is early storage voiding symptoms (ie urgency, frequency, nocturia, urge incontinence) in 30-40% of patients 3 . Beta-3 agonists (e.g., Mirabegron) are commonly used to treat overactive bladder (OAB) symptoms, offering a more favorable side-effect profile compared to anticholinergic medications. One notable advantage of Beta-3 agonists, particularly in patients with benign prostatic hyperplasia (BPH) and lower urinary tract symptoms (LUTS), is the low risk of urinary retention 4,5 . In fact, urinary retention was not reported in initial phase III trials. The concurrent management of OAB symptoms during BPH surgery has been explored, including pharmacologic and intravesical options 3,6 . However, evidence supporting their use in patients who have undergone Rezum therapy remains limited. This single-blinded randomized controlled study is to be conducted at the Men's Health Clinic in Winnipeg, Manitoba. All eligible patients who are to undergo Rezum BPH/LUTS are to be included in the study. Patient meeting inclusion criteria and exclusion criteria (see below) will be randomized to either 1 month of B3-agonist or placebo pill. Inclusion criteria: age > 50, symptomatic LUTS with IPSS ≥ 10, prostate volume 30-80cc on any imaging modality, Qmax between 5 and 15ml/s with minimum voided volume of 125cc, frequency ≥ 8 voids/day. Exclusion criteria: PVR greater than 300ml, active UTI within past week, previous surgical intervention for BPH, documented urethral stricture disease, bladder stones, active malignancy, any confirmed or suspected neurologic disease, hypertension, known insensitivity to B3 agonist, current catheter dependence, currently taking anticholinergic, B3 agonist. Follow-up/data measurements will occur pre-procedure, immediately post procedure, 1 week, 2 weeks, and 4 weeks following procedure. At time of consultation pre-procedure, baseline symptom scores, uroflow parameters will be obtained. Symptom scores and any possible adverse events will again be measured at 1, 2 and 4 weeks. Additionally, at 4 weeks uroflow parameters are to be repeated. For this randomized controlled trial, a sample size of 44 was calculated to detect a mean difference of 3 in the OABSS score, which has been identified in prior literature as the minimal clinically significant difference. This calculation assumes a significance level of 5% and a power of 90%. Accounting for a projected crossover rate of 0% in the control arm and 12.5% in the experimental arm, the adjusted sample size increases to 58 participants.

Inclusion Criteria:

• age >50, symptomatic LUTS with IPSS ≥ 10
• prostate volume 30-80cc on any imaging modality
• Qmax between 5 and 15ml/s with minimum voided volume of 125cc
• frequency ≥ 8 voids/day.

Exclusion Criteria:

• PVR greater than 300ml
• active UTI within past week
• previous surgical intervention for BPH
• documented urethral stricture disease
• bladder stones
• active malignancy
• any confirmed or suspected neurologic disease
• hypertension
• known insensitivity to B3 agonist
• current catheter dependence
• currently taking anticholinergic or B3 agonist.