Laxative drugs

Laxative drugs

Laxatives are used to treat constipation (defined as passing hard stools less frequently than normal for the patient). The aetiology of constipation is varied and can be a symptom of organic disease, or can be a side effect of certain drug treatments (e.g. opioid-induced constipation, OIC). Laxatives are of clinical value in the treatment of irritable bowel syndrome (IBS), OIC, as part of anthelmintic treatment or to clear the alimentary tract before surgery and radiological procedures. Abuse of laxatives may lead to hypokalaemia.

The mechanism of action of laxatives is varied, being physical or biochemical in nature:

Bulk-forming laxatives improve stool formation by adding bulk to the diet. This group includes wheat bran-based dietary supplements, methylcellulose, ispaghula husk and stercullia. Bulk-forming laxatives are useful in the management of IBS, chronic diarrhoea associated with diverticular disease, and in patients with colostomy, ileostomy, haemorrhoids and anal fissure. Can also be used as an adjunct in the treatment of ulcerative colitis.

Stimulant laxatives for example, bisacodyl, sodium picosulphate, anthroquinines such as senna, and parasympathomimetics such as bethanechol chloride (a muscarinic cholinergic receptor agonist), neostigmine and pyridostigmine bromide (both acetylcholinesterase inhibitors) increase intestinal motility. Over use can cause diarrohea and hypokalaemia.

Faecal softeners ease the passage of stool in the gut. Bulk-forming laxatives, non-ionic surfactant agents, glycerol and arachis oil all have softening properties. Non-ionic stool surfactants (e.g., docusate) lower the surface tension of the stool and so allow water to enter the stool more easily. Glycerol and arachis oil act as both stool softeners and stool lubricants.

Osmotic laxatives commonly contain polyethylene glycol (PEG), nonabsorbable synthetic disaccharides (e.g., lactulose and sorbitol), or salts (e.g., sodium sulphate, potassium sulphate, and magnesium sulphate or magnesium citrate) as the active ingredient. These agents increase water in the large intestine to soften the stool and promote bowel movement. Balanced electrolyte solutions containing PEG or high-dose saline preparations are often used as bowel clearing agents prior to colonoscopy, colonic surgery or radiological examination. Hyperosmotic salt preparations should be avoided or used with caution in patients with renal insufficiency, heart failure, end-stage liver disease, electrolyte imbalance, or taking drugs that alter renal blood flow or electrolyte excretion.    

Other laxative drugs include linactolide (an oral guanylate cyclase C receptor agonist, see Busby et al. (2010)), lubiprostone (a chloride channel activator) and prucalopride (a serotonin 5HT4 receptor agonist).