Bladder pain syndrome/interstitial cystitis (BPS/IC) is a difficult disease to both diagnose and treat. It is defined as an unpleasant sensation (pain, pressure, or discomfort) perceived to be related to the urinary bladder and associated with lower urinary tract symptoms for at least 6 weeks duration, in the absence of infection or other identifiable causes. Pain is the universal symptom, but many also experience symptoms of overactive bladder, possibly directly related to the mechanism of pain. Treating pain may influence the symptom of urgency, if the urge arises from a need to alleviate pain. In some patients whose pain improves with treatment, troubling overactive bladder symptoms still remain. Beta-3 adrenergic agonists have been found to decrease signaling of C-fibers in animal models. So, the investigators hypothesize that mirabegron, which is FDA-approved for treatment of overactive bladder, would also improve symptoms in patients with BPS/IC. As a selective beta-3 agonist, mirabegron acts on the beta-3 receptors found in the bladder which mediate relaxation of the detrusor muscle. It has been shown to significantly decrease the number or micturition episodes, urgency episodes, and increased mean volume of urine voided per micturition. It also has a favorable tolerability profile.