McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 5 8 Drugs affecting gastrointestinal motility

and changes in electrolyte levels that can occur ; caution should be used during pregnancy and breastfeeding 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. Adverse effects The adverse effects most commonly associated with lubricant laxatives are GI effects such as diarrhoea, abdominal cramping and nausea. In addition, leakage and staining may be a problem when liquid paraffin is used and the stool cannot be retained by the external sphincter. CNS effects, including dizziness, headache and weakness, are not uncommon and may relate to loss of fluid and electrolyte imbalances that may accom- pany laxative use. Sweating, palpitations, flushing and even fainting have been reported after laxative use. These effects are less likely to happen with the lubri- cant laxatives than with the chemical or mechanical stimulants. Clinically important drug–drug interactions Frequent use of liquid paraffin can interfere with absorp- tion of the fat-soluble vitamins A, D, E and K. Prototype summary: Liquid paraffin Indications: Short-term relief of constipation; to prevent straining when it is clinically undesirable; to remove ingested poisons from the lower GI tract; an adjunct in anthelmintic therapy when it is desirable to flush helminths from the GI tract. Actions: Forms a slippery coat on the contents of the intestinal tract; less water is absorbed out of the bolus, and the bolus is less likely to become hard or impacted. Pharmacokinetics: Not absorbed systemically. T 1/2 : Not absorbed systemically. Adverse effects: Diarrhoea; abdominal cramps; bloating; perianal irritation; dizziness; interference with absorption of the fat-soluble vitamins A, D, E and K; leakage of stool and staining. O ther laxatives Another drug that does not fit into the categories usually used for laxatives has been approved recently for the treatment of opioid-induced constipation. This drug is discussed in Box 58.2.

■■ BOX 58.2  Other laxatives

A drug that does not fit into the categories usually used for laxatives has been approved for the treatment of specific forms of constipation. • Methylnaltrexone ( Relistor ) was approved in 2008 for the treatment of opioid-induced constipation in people with advanced disease who are receiving palliative care and are no longer responsive to traditional laxatives. Opioids bind to various receptors in the body, including the μ-receptors, which leads to decreased GI motility and constipation. People on long-term opioid treatment frequently have a very difficult time with constipation. Methylnaltrexone is a selective antagonist to opioid binding at the μ-receptor. It does not cross the blood–brain barrier and therefore acts specifically at peripheral opioid receptor sites, like the GI tract, but does not affect the analgesic effects of opioids in the CNS. This drug is given by daily subcutaneous injections. It reaches peak levels in ½ hour and is eliminated primarily unchanged in the urine. The half-life of the drug is about 8 hours. People may experience abdominal pain, flatulence, nausea, dizziness and diarrhoea. Severe or continued diarrhoea should be reported. Use of this drug beyond 4 months has not been studied. cautions : history of allergy to laxative to prevent hypersensitivity reaction ; faecal impaction or intestinal obstruction, which could be exacerbated by increased GI activity ; acute abdominal pain, nausea or vomiting, which could represent an underlying medical condition ; and current status of pregnancy or breastfeeding, which could be contraindications or require cautious use. baseline data before beginning therapy and during therapy to determine the effectiveness of the drug and to evaluate for any adverse effects associated with drug therapy. ■ ■ Perform a physical examination to establish ■ ■ Inspect the skin for rash to monitor for adverse reactions. ■ ■ Assess the person’s neurological status, including level of orientation and affect, to evaluate any CNS effects of the drug. ■ ■ Obtain a baseline pulse rate to assess for any cardiovascular effects of the drug. ■ ■ Assess bowel elimination patterns, including the person’s perception of normal frequency, actual frequency and stool characteristics, to determine the need for therapy. Care considerations for people receiving laxatives Assessment: History and examination ■ ■ Assess for possible contraindications or