1. BULK-FORMING 2. OSMOTIC3. LUBRICANT AND FAECAL SOFTENERS4. STIMULANT (CATHARTICS)Indications for use
Bulk-formingBulk-forming laxatives such as methylcellulose act by drawing water into the colon. This increases the moisture content of the stool and causes the faeces to expand. The increased bulk stimulates receptors in the colonic mucosa, promoting peristalsis.These laxatives are useful for treating constipation, where the faeces are dry and hard (Hopkins, 1999). Increased fluid intake is essential to prevent intestinal obstruction. Bulk-forming laxatives are useful postpartum, in the elderly and in debilitated patients, as well as in treating diverticulosis and irritable bowel syndrome. They are useful for managing patients with a colostomy, ileostomy, haemorrhoids and fissure. Bulk laxatives usually take a few days to work and should not be taken immediately before bedtime.OsmoticOsmotic laxatives such as lactulose, macrogols and magnesium salts act by reducing the absorption of fluid from the small and large intestines. The increased fluid distends the colon and promotes peristalsis and softening of the faeces. Osmotic laxatives are used to treat simple constipation and for bowel cleansing/preparation before surgery or investigative procedures.LubricantLubricant laxatives such as liquid paraffin and arachis oil create a barrier between the faeces and the colon wall. This prevents fluid loss from faeces, keeping them soft and easing passage through the intestine. Lubricants are useful in conditions where straining on defaecation should be avoided, such as angina, aneurysm, stroke, hernia and post-abdominal surgery. The faecal-softening effect is also useful in preventing discomfort or tearing in the case of haemorrhoids (Edmunds, 2003).StimulantLaxatives such as senna, dantron (danthron) and bisacodyl increase gut motility by nerve stimulation. Senna preparations cause primary stimulation of the colon nerves, while bisacodyl stimulates sensory nerves in intestinal mucosa. Stimulant laxatives are used to treat constipation arising from prolonged bedrest or poor dietary habits. They are also used for bowel preparation. Dantron is limited to terminally ill patients, owing to possible carcinogenic effects (Medicines Control Agency, 2000).Pharmacokinetics
Laxatives are administered orally or rectally (in suppositories or enemas). When given orally, most reach the intestine unchanged. In general, laxatives are minimally absorbed and are excreted predominantly in the faeces.Bulk laxatives are mainly hydrophilic vegetable substances of a mucilaginous nature (Hopkins, 1999). Polysaccharides in these drugs are converted into osmotically active metabolites that draw water into the colon. This helps form a bulky emollient gel that distends the colon and promotes peristalsis (Katzung, 2004).Lubricant laxatives are minimally absorbed and distributed to the mesenteric lymph nodes, intestinal mucosa, liver and spleen. They are metabolised by the liver and excreted in faeces.Stimulant laxatives are minimally absorbed, metabolised in the liver and excreted in urine and faeces. The osmotic laxative, lactulose, is metabolised by bacteria in the colon to form lactic and acetic acids and promotes formation of softer faeces of low pH. Saline compounds (Epsom salts, milk of magnesia) result in the excretion of sodium and magnesium ions in urine.Cautions and contraindications
Laxatives should be seen as a short-term solution to constipation. They are available over the counter and subject to overuse, which can disrupt the body's natural emptying rhythm (Edmunds, 2003). Abuse may lead to hypokalaemia and an atonic non-functioning colon.Patients with chronic renal insufficiency should avoid osmotic laxatives containing magnesium or potassium. Laxatives containing sodium are contraindicated in pregnancy and in patients with cardiac conditions (Hopkins, 1999). Bulk-forming laxatives with a high sugar content are contraindicated in patients with diabetes mellitus.Adverse reactions
All classifications of laxative can cause abdominal cramps, which may be accompanied by nausea and vomiting. Allergic reactions, including pruritis, urticaria and rhinitis, can also occur.Fluid and electrolyte disturbances may occur with hyperosmolar laxatives.Bulk-forming laxatives can cause abdominal obstruction or impaction if they are taken without adequate fluids. Long-term use of lubricant laxatives may impair absorption of nutrients and fat-soluble vitamins (A, D, E, K) (Katzung, 2004).Stimulant laxatives may produce muscle weakness and, if used excessively, could cause irritable bowel syndrome. Prolonged diarrhoea may lead to hyponatraemia (low concentrations of sodium in the blood), hypokalaemia and dehydration.INDICATIONS FOR USE
Laxatives are indicated for the treatment of constipation in:- Illness and pregnancy- Elderly patients- Decreased peristalsis due to medicationAnd for:- Bowel cleansing in preparation for surgery or diagnostic procedures such as X-ray or endoscopy or colonoscopy- Expulsion of parasites (such as worms) after anthelmintic treatment- Elimination of toxic material following ingestion of poison
|Downie, G., Mackenzie, J., Williams, A. (2003)Pharmacology and Medicines Management for Nurses (3rd edn). London: Churchill Livingstone.Edmunds, W.E. (2002)Introduction to Clinical Pharmacology (4th edn). St Louis, Mo: Mosby.Hopkins, S.J., Kelly, J.C. (1999)Drugs and Pharmacology for Nurses (13th edn). London: Churchill Livingstone.Katzung, B.G. (2004)Basic and Clinical Pharmacology (9th edn). Maidenhead: Appleton and Lange.Medicines Control Agency. (2000)Danthron restricted to constipation in the terminally ill. Current Problems in Pharmacovigilance. 26: 4.|
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