McKenna's Pharmacology for Nursing, 2e

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P A R T 1 1  Drugs acting on the gastrointestinal system

Lumen of the GI tract

Bulk laxatives work here: lactulose, magnesium sulfate

Nerve plexus

Antidiarrhoeals work here: loperamide, opium

Lumen

Mucosal layer

Lubricant laxatives work here: docusate, glycerin, mineral oil

Circular muscularis mucosa

Longitudinal muscularis mucosa

Chemical stimulant laxatives work here: bisacodyl, castor oil, senna

FIGURE 58.1  Sites of action of drugs affecting gastrointestinal motility.

Kinds of laxatives include chemical stimulants (which chemically irritate the lining of the GI tract), bulk stimulants (which cause faecal matter to increase in bulk) and lubricants (which help the intestinal contents move more slowly). Newer laxatives are available for very specific needs and alter sodium absorption or affect opioid receptors in the GI tract. C hemical stimulants Chemical stimulants directly stimulate the nerve plexus in the intestinal wall, causing increased movement and the stimulation of local reflexes. Laxatives classified as chemical stimulants include bisacodyl ( Dulcolax ), castor oil (generic) and senna ( Senokot ). Therapeutic actions and indications Castor oil is rarely used nowadays; however, it is still used in some countries. All of these agents begin working at the beginning of the small intestine and increase motility throughout the rest of the GI tract by irritating the nerve plexus. Bisacodyl can be given in rectal sup- pository and bowel preparation to stimulate the activity in the lower GI tract. Senna is available orally in tablet and powder forms. Pharmacokinetics Most of these agents are only minimally absorbed and exert their therapeutic effect directly in the GI tract. Changes in absorption, water balance and electrolytes resulting from GI changes can have adverse effects on people with underlying medical conditions that are

affected by volume and electrolyte changes (see Adverse effects). They have an onset of action of 6 to 8 hours, making them preferable if one wants the drug to work overnight and see effects in the morning. Contraindications and cautions Laxatives are contraindicated with allergy to any com- ponent of the drug to prevent hypersensitivity reactions ; in acute abdominal disorders, including appendici- tis, diverticulitis and ulcerative colitis, when increased motility could lead to rupture or further exacerbation of the inflammation. Laxatives should be used with caution in heart block, coronary artery disease (CAD) or debilitation, which could be affected by the decrease in absorption and changes in electrolyte levels that can occur ; with great caution during pregnancy and breast- feeding because in some cases, stimulation of the GI tract can precipitate labour, and many of these agents cross the placenta and are excreted in breast milk. Castor oil should not be used during pregnancy because its irritant effect has been associated with induction of premature labour. Magnesium laxa- tives can cause diarrhoea in the neonate if used during breastfeeding. Adverse effects The adverse effects most commonly associated with laxatives are GI effects such as diarrhoea, abdominal cramping and nausea. Central nervous system (CNS) effects, including dizziness, headache and weakness, are not uncommon and may relate to loss of fluid and electrolyte imbalances that may accompany laxative