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P A R T 1 1
Drugs acting on the gastrointestinal system
Contraindications and cautions
Antidiarrhoeal drugs should not be given to anyone
with known allergy to the drug or any of its compo-
nents
to prevent hypersensitivity reactions
. Caution
should be used in pregnancy and breastfeeding
because
of the potential adverse effects to the fetus or baby
. Care
should also be taken in individuals with any history
of GI obstruction, acute abdominal conditions,
which
could be exacerbated by the effects of the drugs,
or
diarrhoea due to poisonings,
which could be worsened
by slowing of the GI tract, allowing increased time for
absorption of the poison;
or with hepatic impairment,
which could alter the metabolism of the drugs.
Renal function should be monitored before starting
treatment and during treatment (more frequent mon-
itoring may be required in renal impairment); cardiac
hypersensitivity reactions have been reported; caution is
required if there is a previous history of myocarditis or
pericarditis, irrespective of cause.
Adverse effects
The adverse effects associated with antidiarrhoeal drugs,
such as constipation, distension, abdominal discomfort,
nausea, vomiting, dry mouth and even toxic mega
colon, are related to their effects on the GI tract. Other
adverse effects that have been reported include fatigue,
weakness, dizziness and skin rash. Opium derivatives
are also associated with lightheadedness, sedation,
euphoria, hallucinations and respiratory depression
related to effect on the opioid receptors.
Drug–drug interactions
Drug interactions vary depending on the antidiarrhoeal
agent. Consult the drug package insert for specific
interactions.
Prototype summary: Loperamide
Indications:
Control and symptomatic relief of acute,
non-specific diarrhoea and chronic diarrhoea
associated with irritable bowel syndrome; reduction
of volume of discharge from ileostomies.
Actions:
Inhibits intestinal peristalsis through direct
effects on the longitudinal and circular muscles of
the intestinal wall, slowing motility and movement
of water and electrolytes.
Pharmacokinetics:
Route
Onset
Peak
Oral (capsule)
Varies
5 hours
T
1/2
:
10.8 hours; metabolised in the liver and
excreted in urine and faeces.
Adverse effects:
Abdominal pain, distension or
discomfort; dry mouth; nausea; constipation;
dizziness; tiredness; drowsiness.
CRITICAL THINKING SCENARIO
Traveller’s diarrhoea
THE SITUATION
P.F. received an all-expenses-paid trip to Thailand to
celebrate his graduation from university. He was very
excited about getting away for a week of sun and fun, and
arranged to stay in the same hotel as two college friends
who were also celebrating. The three men had a wonderful
time visiting the beaches, bars and nightclubs in the area.
On the third day of the trip, P.F. began experiencing nausea,
some vomiting and a low-grade fever. Several hours later
he began experiencing intense cramping and diarrhoea.
For the next 2 days, P.F. felt so ill he was unable to leave
his hotel room. The next morning, he arranged for an
emergency trip home.
CRITICAL THINKING
What is probably happening to P.F.?
Think about the
gastrointestinal reflexes and explain the underlying cause
for his signs and symptoms.
What treatment should be started now?
What could have been done to prevent this problem from
occurring?
What possible drug therapy might have been helpful
for P.F.?
DISCUSSION
P.F. is probably experiencing the common disorder
called traveller’s diarrhoea. This disorder occurs when
pathogens found in the food and water of a foreign
environment are ingested. (Because these pathogens
are commonly found in the environment, they do not
normally cause problems for the people who live in the
area.) When the pathogen, usually a strain of
Escherichia
coli
, enters a host that is not accustomed to the bacteria,
it releases enterotoxins and sets off an intestinal–intestinal
reaction in the host.
The intestinal–intestinal reaction results in a reduction
of activity above the point of irritation (which causes
