Antispasmodics are the mainstay of OTC treatment of IBS and research trials show some improvement in abdominal pain and cramp. The smooth muscle relaxants alverine, peppermint oil and mebeverine and the antimuscarinic hyoscine are used and available OTC. They work by a direct effect on the smooth muscle of the gut, causing relaxation and thus reducing abdominal pain.

If these are to be tried and have not been previously used, the patient should see an improvement within a few days of starting treatment and should be asked to return in a week or so to monitor progress. It may be worth trying a different antispasmodic if the first has not worked.

• Alverine citrate is given in a dose of 60-120mg (one or two capsules) up to three times a day. It is taken with water and should not be chewed. Side-effects are rare, but nausea, dizziness, pruritus, rash and headache have occasionally been reported. The drug should not be recommended for pregnant or breastfeeding women or for children under 12 years. Alverine citrate is also available in a combination product with sterculia.
• Peppermint oil has been used for many years as an aid to digestion and has an antispasmodic effect. Capsules containing 0.2ml of the oil are taken in a dose of one or two capsules three times a day, 15-30 minutes before meals. They are enteric coated, with the intention that the peppermint oil is delivered beyond the stomach and upper small bowel. The capsules should not be chewed as not only will this render the treatment ineffective, but it will also cause irritation of the mouth and oesophagus. This treatment should not be taken by children. Occasionally, peppermint oil causes heartburn and so is best avoided in patients who already suffer from this problem. Perianal irritation may occasionally occur with peppermint oil. Allergic reactions can also occur but are rare. Rash, headache and muscle tremor have been reported.
• Mebeverine hydrochloride is used at a dose of 135mg three times a day. The dose should be taken 20 minutes before meals. The drug is not to be recommended for pregnant or breastfeeding women, for people under 18 years of age or patients with porphyria (which is very rare). Adverse effects are rare and include allergic reactions, angio-oedema, rash and urticaria.
• Hyoscine butylbromide 10mg tablets can be used in adults and children aged over six years. On starting treatment, adults should take one tablet three times a day, increasing if necessary
  to two tablets four times a day. The main adverse effects are antimuscarinic/anticholinergic and common, which may limit its usefulness. These include dry mouth, urinary symptoms, blurred vision and constipation. Beware of combining with prescribed drugs with anticholinergic effects (such as antidepressants and oxybutynin) as hyoscine may intensify these.