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P A R T 1 1
Drugs acting on the gastrointestinal system
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
Antidiarrhoeals work
here:
loperamide, opium
Lubricant laxatives
work here:
docusate, glycerin,
mineral oil
Chemical stimulant
laxatives work here:
bisacodyl,
castor oil, senna
Lumen of the
GI tract
Mucosal layer
Circular
muscularis mucosa
Longitudinal
muscularis
mucosa
Nerve plexus
Lumen
Bulk laxatives work
here:
lactulose,
magnesium sulfate
FIGURE 58.1
Sites of action of drugs affecting gastrointestinal
motility.
