**1. Introduction**

Since the early 19th century, patients who exhibited chronic bladder pain, urinary frequency, and urgency without evidence of urinary tract infection or bladder stones have been diagnosed with interstitial cystitis (IC) [1]. As our understanding of IC has progressed over the past 200 years, its name and definition have changed many times.[1] Because patients with IC may not experience bladder inflammation, the term "interstitial cystitis/bladder pain syndrome" (IC/BPS) is considered to be more suitable and is widely used in current guidelines [2,3]. IC/BPS has attracted considerable attention among urologists. A recent epidemiology study revealed that the prevalence of IC/BPS is 4.2% for the high-sensitivity definition, whereas it is 1.9% for the high-specificity definition [4]. Although the prevalence has shown an increasing trend in recent years [4], the treatment of patients with IC/BPS remains challenging for urologists. Current guidelines recommend the use of oral medications, such as amitriptyline and cimetidine, as well as intravesical instillation therapy (i.e., heparin and hyaluronic acid) to treat patients with IC/BPS [2]. However, no single treatment is effective for all patients with IC/BPS, and symptomatic relapse after the abovementioned treatments is common among patients with IC/BPS. Thus, patients with IC/BPS need a more effective treatment with greater durability.

For complicated lower urinary-tract diseases (LUTDs), treatment options are sometimes limited. Over the past two decades, biotoxins (e.g., resiniferatoxin and capsaicin) have gradually been included as possible treatment modalities for complicated LUTDs [5,6]. Botulinum toxin (BoNT), which is produced by the bacterium *Clostridium botulinum* and related species, is regarded as the most potent poisonous neurotoxin worldwide [7]. In 1988, Dykstra et al. first used a BoNT type A (BoNT-A) injection to treat patients with spinal-cord injury and detrusor–sphincter dyssynergia [8]. Today, BoNT-A is widely used to treat various types of complicated LUTDs, and many clinical trials have proven its efficacy [9]. For patients with IC/BPS, the efficacy of traditional conservative treatments is typically insufficient [2]. In recent years, the treatment of IC/BPS has shown great progress with the use of the intravesical BoNT-A injection, and laboratory studies have demonstrated bladder improvement after BoNT-A injection [10]. This review summarizes the possible pathomechanisms of using BoNT-A for treatment of IC/BPS in the bladder and examines the practical issues and long-term efficacy of such treatment.
