Constipation

Laxatives promoting defecation include:

bulking agents (e.g. bran, methylcellulose): increase bulk matter within the lumen, thereby stimulating stretch receptors in the intestinal wall and indirectly stimulating muscle contractility

water-retaining agents: saline (e.g. Epsom salts) and osmotic agents (lactulose) are poorly absorbed from the gastrointestinal tract and lead to retention of water within the lumen by osmosis, thereby facilitating bulk mass and transit

stimulant (irritant) laxatives (e.g. Senna, bisacodyl, phenolphthalein): promote water and electrolyte accumulation in the colon and stimulate muscle motility by stimulating the enteric nervous system

faecal softeners (e.g. docusate) or lubricants(e.g. liquid paraffin): improve transit of contents within the colon (Fig. 3.19.1).

Increasing water and fibre content in the diet will also improve transit time within the gastrointestinal tract.

Prokinetic (motility-promoting) drugs

Agents that increase muscle motility of the gastrointestinal tract (prokinetic) but without laxative actions are used in a number of motility disorders, including gastrointestinal reflux and gastric stasis, where increased gastric emptying is desirable. Domperidone and metoclopromide are dopamine D₂ antagonists used in the treatment of nausea and emesis (Ch. 20). Domperidone also increases gastric motility by a poorly defined mechanism and metoclopromide possibly acts on serotonin 5HT₄ receptors on gastrointestinal intrinsic neurons to increase acetylcholine release, thereby increasing motility and gastric emptying. Cisapride also stimulates acetylcholine release from intrinsic neurons in the myenteric plexus via a 5HT₄ receptor-dependent mechanism, thereby promoting gastric motility. Domperidone does not cross the blood–brain barrier and so is preferable to metoclopromide.