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Bladder discomfort related to an indwelling catheter can exacerbate postoperative pain. It mimics overactive bladder syndrome that is resistant to conventional opioid therapy. Muscarinic receptor antagonists are effective for treatment of the overactive bladder. The aim of this study was to assess the efficacy of oxybutynin in the management of postoperative pain after radical prostatectomy.Forty-six ASA I or II men undergoing radical retropubic prostatectomy under general anaesthesia were randomly assigned to two groups, in a double-blind fashion: Group O and Group P (n = 23 each). Group O and Group P received, respectively, sublingual oxybutynin 5 mg or placebo every 8 h during the 24 h after surgery. A 16F Foley catheter was placed during the vesico-urethral anastomosis and the balloon inflated with 10 ml of saline. Pain was assessed in the recovery room starting 10 min after extubation using a 100-point visual analogue scale (VAS). The patients were asked to specify whether pain was related to the surgical incision or bladder pain. Standardized postoperative analgesia included acetaminophen and tramadol administered via a patient-controlled analgesia system.The incidence of bladder catheter pain was 65% (15 of 23 patients) in Group P compared with 17% (4 of 23 patients) in Group O (P0.01). Overall VAS scores at rest were significantly lower in Group O. Cumulative tramadol consumption was 322.9(124.3) mg [mean(sd)] in Group P and 146(48) mg in Group O (P0.01). No oxybutynin-related side-effects were reported.Sublingual oxybutynin is an effective treatment for postoperative pain after radical retropubic prostatectomy and produces a significant reduction in tramadol requirements.