antispasmodics
Summary
Antispasmodics are drugs that relieve smooth muscle spasm of the gut, chiefly antimuscarinics such as dicyclomine and hyoscyamine and direct smooth muscle relaxants such as peppermint oil. They are used for the cramping abdominal pain of irritable bowel syndrome.
Detail
Two mechanisms are represented. Antimuscarinic agents block M3 receptors, reducing acetylcholine-driven contraction, and carry systemic anticholinergic effects that limit their use, particularly in older adults. Direct relaxants act on smooth muscle calcium handling; peppermint oil blocks calcium channels in intestinal smooth muscle and has reasonable evidence in irritable bowel syndrome, with heartburn as the main adverse effect, mitigated by enteric-coated preparations. Antispasmodics are symptomatic only and belong within a broader IBS strategy: soluble fibre and dietary modification including a low FODMAP trial, loperamide or a bile acid sequestrant for diarrhoea-predominant disease, polyethylene glycol or linaclotide and lubiprostone for constipation-predominant disease, and low-dose tricyclic antidepressants or SSRIs for pain, which act on visceral hypersensitivity rather than mood. Alosetron, a 5-HT3 antagonist, is restricted to severe refractory diarrhoea-predominant IBS in women because of ischaemic colitis risk. Antispasmodics should be avoided in suspected bowel obstruction and in inflammatory bowel disease, where they can precipitate toxic megacolon.
Sources
- Katzung Basic and Clinical Pharmacology
- Harrison's Principles of Internal Medicine
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