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P A R T 1 1
Drugs acting on the gastrointestinal system
O
ne of the most common and most uncomfortable com-
plaints encountered in clinical practice is that of nausea
and vomiting. Vomiting is a complex reflex reaction
to various stimuli (see Chapter 56). In some cases of
overdose or poisoning, it may be desirable to induce
vomiting to rapidly rid the body of a toxin. This can be
accomplished by physical stimuli, often to the back of
the throat. In some cases, gastric lavage is used to clear
the contents of the stomach.
In many clinical conditions, the reflex reaction of
vomiting is not beneficial in ridding the body of any
toxins but is uncomfortable and even clinically haz-
ardous to the person’s condition. In such cases, an
antiemetic
is used to decrease or prevent nausea and
vomiting. Antiemetic agents can be centrally acting or
locally acting, and they have varying degrees of effec-
tiveness. See Figure 59.1 for sites of action of antiemetics.
Box 59.1 highlights important considerations related to
use of antiemetics across the lifespan.
ANTIEMETIC AGENTS
Drugs used in managing nausea and vomiting are called
antiemetics (Table 59.1). All of them work by reducing
the hyperactivity of the vomiting reflex in one of two
ways: locally, to decrease the local response to stimuli
that are being sent to the medulla to induce vomiting, or
centrally, to block the chemoreceptor trigger zone (CTZ)
or suppress the vomiting centre directly. The locally
acting antiemetics may be antacids, local anaesthetics,
adsorbents, protective drugs that coat the GI mucosa,
or drugs that prevent distension and stretch stimulation
of the GI tract. These agents are often reserved for use
in mild nausea. Many of these drugs are discussed in
Chapter 57.
Centrally acting antiemetics can be classified into
several groups: phenothiazines, non-phenothiazines,
anticholinergics/antihistamines, serotonin (5-HT
3
)
receptor blockers and substance P/neurokinin 1 receptor
antagonists.
5-HT
3
receptor blockers
work here:
dolasetron,
granisetron, ondansetron,
palonosetron and
tropisetron
Phenothiazine
antiemetics work here:
chlorpromazine,
prochlorperazine
Non-phenothiazines
also work here:
metoclopramide,
domperidone
Dronabinol, nabilone
work here
Substance P/neurokinin
1 receptor antagonists
work here:
aprepitant,
fosaprepitant
CTZ
FIGURE 59.1
Sites of action of emetics/
antiemetics, CTZ, chemoreceptor
trigger zone.
