Antimuscarinic agents such as hyoscyamine and dicyclomine, a nonspecific smooth muscle relaxant with antimuscarinic activity, have been used to treat symptoms associated with irritable bowel syndrome (IBS). Controlled studies assessing the efficacy of these medications are lacking. One study using hyoscyamine alone and in combination with ispaghula husk, lorazepam, or both showed nonsignificant improvement in symptoms alone or in some of the combinations, but the symptoms were not described in detail.
In another study, patients given dicyclomine 40 mg four times daily showed significant improvement in all components of the physicians’ global evaluation (pain, tenderness, bowel habits, overall condition) as well as in patients’ self-assessment from baseline when compared to placebo.
The usual adult dose of hyoscyamine is 0.15 to 0.3 mg four times per day or 0.125 mg every four hours as needed. Both regimens may be titrated to a maximum of 1.5 mg per day. Anticholinergic side effects such as dry mouth, mydriasis, urinary retention, and constipation are common. Increased ocular tension, dizziness, insomnia, and other central nervous system (CNS) effects have also been reported.
The incidence and severity of adverse effects may increase with increased doses. Adverse effects due to dicyclomine would be expected to be similar. Sixty-nine percent of patients taking dicyclomine in the aforementioned study reported adverse effects, including dry mouth, dizziness, and blurred vision; 12 patients required dose reduction and seven withdrew. Initiating treatment of 20 mg four times daily may produce the desired therapeutic effect while abating the adverse effects. The dose can be titrated to the maximum recommended of 160 mg per day.