Urinary incontinence is a common problem that is often treated with oxybutynin chloride, an anticholinergic drug. Potential adverse effects of this medication include constipation and drowsiness, but dry mouth is reported to be its most common and troubling side effect. Versi and colleagues studied the occurrence of dry mouth in patients taking oxybutynin for the treatment of urinary incontinence, with special interest given to the differences between long-acting and immediate-release formulations of the drug.

The study subjects were noninstitutionalized men and women who reported having seven to 45 episodes of urge incontinence per week and who were known to have responded well to anticholinergic medications previously. Patients with serious medical conditions, contraindications to the use of oxybutynin and postvoid residual urine volumes of more than 100 mL were excluded from the study.

After initial assessment, the patients were randomly assigned to treatment with immediate-release or controlled-release oxybutynin, beginning with a dosage of 5 mg per day. The dosage was increased by 5 mg per day every seven days until the optimal balance was obtained between control of incontinence and tolerability of side effects. The maximal dosage was 20 mg per day. Once the optimal dose was established, it was maintained for at least one week. Patients kept symptom diaries and records of urinary incontinence episodes.

The 111 patients randomized to treatment with controlled-release oxybutynin were comparable in all important variables to the 115 patients randomized to receive immediate-release oxybutynin. Overall, the average age of patients was 59 years; most were white and female. Seven patients in each group discontinued medication, but only one patient cited dry mouth as contributing to this decision.

Both treatments were highly effective. The number of urge incontinence episodes decreased by 83 percent in the patients treated with controlled-release oxybutynin and by 76 percent in the patients who received immediate-release oxybutynin. Two other patients left the study (lack of efficacy and personal reasons).

The overall rates of dry mouth for the two groups were not significantly different: 47.7 percent for the patients given the controlled-release formulation and 59.1 percent for those given the immediate-release preparation. The cumulative proportion of patients reporting moderate or severe dry mouth was dose-dependent and was higher at each dosage for the immediate-release preparation: 5 mg per day, 3.6 percent for the controlled-release formulation and 7.0 percent for the immediate-release formulation; 10 mg per day, 8.5 percent for the controlled-release formulation and 25.6 for the immediate-release formulation; 15 mg per day, 19.4 percent for the controlled-release formulation and 38.9 percent for the immediate-release formulation; and 20 mg per day, 39.5 for the controlled-release formulation and 45 percent for the immediate-release formulation.

The authors concluded that moderate to severe dry mouth is a common, dose-related side effect of oxybutynin therapy. The precise mechanism by which controlled-release oxybutynin preparations may reduce dry mouth symptoms is unknown. The lower rates associated with these preparations may be related to metabolism in the colon rather than the small intestine.

editor's note: Thanks to aggressive advertising on television and in selected magazines, many patients are now requesting the “overactive bladder medication.” Taking a detailed history to clarify the urinary symptoms and the patient's expectations can frequently be revealing, and many patients are surprised that an examination or additional tests may be necessary to make an accurate assessment. Anticholinergic medications are a common first-choice therapy in patients with genuine urge incontinence. This study demonstrates that moderate to severe dry mouth is common in patients treated with anticholinergic drugs. Dry mouth can be distressing and can impair nutrition or even communication. Patients should be cautioned about this side effect and instructed about its management. In addition, dry mouth is only one marker of anticholinergic activity. Patients at risk for other adverse effects, especially blurring of vision, should be carefully monitored.—a.d.w.