These drugs block cholinergic transmission at parasympathetic postganglionic nerve endings and cause smooth muscle to relax. The synthetic anti-muscarinics dicyclomine and propantheline are probably the most useful in IBS, but therapeutic efficacy is often limited by other anticholinergic effects. The drugs are contraindicated in patients with glaucoma and prostatism, and should be avoided in patients with gastro-oesophageal reflux.
Mebeverine is a reserpine derivative which has a direct effect on colonic muscle activity, especially, it appears, on colonic hypermotility. As it does not possess antimuscarinic activity, it does not exhibit the troublesome unwanted effects of that group of drugs.
Alverine and peppermint oil also have direct smooth muscle relaxing activity.
A trial of low dose amitryptiline (10-25 mg at night) is worthwhile in patients who do not respond to antispasmodics, and associated depression will be helped by conventional doses of this or other antidepressants. Relaxation therapy, hypnotherapy and cognitive behaviour therapy have a place in selected cases.
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