In recent years, botulinum-A neurotoxin has increasingly been used to manage lower urinary tract symptoms, including overactive bladder and detrusor overactivity (DO), either due to neurogenic or idiopathic etiology. Techniques, doses, and preparation vary. Although many studies have reported promising results regarding efficacy, potential adverse effects, particularly urinary retention, have been less comprehensively reported. We performed a critical review of published studies evaluating botulinum treatment for overactive bladder and its reported effects on voiding function. Acute urinary retention is recognized, though rare. Chronic urinary retention is inconsistently reported; it appears to be more common in neurogenic DO, but it can occur in idiopathic DO. Increased postvoid residuals have been reported by several studies, but the clinical significance that investigators attach to the observation varies. The detrimental effect of retention on quality of life can be considerable. Accordingly, patients should be fully counseled on the risks of urinary retention and trained in intermittent catheterization before the procedure. Postvoid residual assessment should be part of follow-up and patients should be warned of possible presentations.