Intravesical bacillus Calmette-Guérin is used to decrease recurrence rates of nonmuscle invasive urothelial carcinoma. Irritative urinary symptoms are a common side effect of treatment and frequently limit treatment tolerance. While anticholinergic medications may be used for symptom prophylaxis, to our knowledge they have not been evaluated in a randomized controlled trial. A total of 50 bacillus Calmette-Guérin naïve patients were randomized to 10 mg extended release oxybutynin daily or placebo starting the day before 6 weekly bacillus Calmette-Guérin treatments. A questionnaire assessing urinary symptoms (frequency, burning on urination, urgency, bladder pain, hematuria), systemic symptoms (flu-like symptoms, fever, arthralgia) and medication side effects (constipation, blurred vision, dry mouth) was recorded daily throughout the therapeutic course. A linear mixed repeated measures model tested the differences between each point and baseline score. The treatment group had a greater increase in urinary frequency and burning on urination compared to placebo (p = 0.004 and p = 0.04, respectively). There were no significant differences between groups for other urinary symptoms, which increased in severity after bacillus Calmette-Guérin but concomitantly returned to baseline in both groups. The treatment group experienced increases in fever, flu-like symptoms, dry mouth and constipation compared to placebo (p <0.0001, p = 0.0008, p = 0.045 and p = 0.001, respectively). There were otherwise no significant differences in nonurinary symptoms or medication adverse reactions. Oxybutynin increased urinary frequency and burning on urination compared to placebo in patients receiving intravesical bacillus Calmette-Guérin treatment. Our results do not support the routine use of oxybutynin as prophylaxis against urinary symptoms during bacillus Calmette-Guérin therapy.