C H A P T E R 5 8
Drugs affecting gastrointestinal motility
921
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.
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.
■■
BOX 58.2
Other laxatives
Care considerations for
people receiving laxatives
Assessment: History and examination
■
■
Assess for
possible contraindications or
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.
■
■
Perform a physical examination
to establish
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.
■
■
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.
