(Mode of Action of Drugs)

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Chapter

1.4

Chapter

7.2 Emetics & Antiemetic Agents

(Drugs used in A(Drugs used in A(Drugs used in A(Drugs used in A(Drugs used in A

EMETICS

Vomiting or emesis occurs due to stimulation
of vomiting/emetic centre in medulla oblon-
gata. The chemoreceptor trigger zone (CTZ)
and nucleus tractus solitarius (NTS) are relays
for the afferent impulses arising from GIT,
throat and other viscera. There are different
drugs e.g. morphine, digitalis glycosides, apo-
morphine etc. which stimulate the CTZ. While
certain agents e.g. chlorpromazine and certain
antihistaminics depress it. Vomiting due to ir-
ritants in the GIT especially upper region does
not involve CTZ and is mediated directly by
the vomiting centre. Histamine (H 1 ), seroto-
nin (5-HT 3 ), dopamine (D 2 ) cholinergic (M)
and opioid (μ) receptors on CTZ and NTS are
involved in inducing vomiting. Impulses from
vestibular centre also lead to stimulation of
vomiting centre via H 1 and M receptors.

The drugs that produce or evoke
vomiting are known as emetics. The most
common compounds used are apomorphine
and ipecacuanha.

APOMORPHINE

It is a semisynthetic opioid and act as
dopami-nergic agonist on the CTZ. It is

given by subcuta-neous/IM route in the dose

of 6 mg and it produces vomiting within 15

minutes. Apomorphine induced vomiting

can be antagonized by chlorpromazine.

Side effects include tremors, restless-

ness and in toxic doses it may cause convul-

sions and respiratory depression.

IPECACUANHA

It is used as tincture and syrup containing

alkaloid emetine obtained from plant

Cephaelis ipeca-cuanha. It act on both gastric

mucosa and CTZ.

The emetics are used mainly in poisoning

when gastric lavage facilities are not

available. But in certain poisoning e.g.

kerosene poisoning, corrosive acid or alkali

poisoning, emetics are contraindicated. They

are also not advisable in unconscious patients

as they may aspirate vomitus.

ANTIEMETICS

These are the drugs which are used to prevent

vomiting. They are classified in table 7.2.1.

The detailed pharmacology of phe-

nothiazines e.g. chlorpromazine, triflupro-

Emetics & Antiemetic

Agents