Antiemetic and emetic drugs
Antiemetics and
emetics are two groups of drugs with opposingactions. Antiemetic drugs
decrease nausea, reducing the urge to vomit. Emetic drugs, which are derived
from plants, produce vom-iting.
The major antiemetics are:
§ antihistamines, including buclizine,
cyclizine, dimenhydrinate, diphenhydramine, hydroxyzine hydrochloride,
hydroxyzine pamoate, meclizine, and trimethobenzamide
§ phenothiazines, including chlorpromazine,
perphenazine, prochlorperazine maleate, promethazine, and thiethylperazine
maleate
§ serotonin 5-HT3 receptor antagonists, including dolasetron, granisetron, and
ondansetron.
Ondansetron is currently the antiemetic of choice
in the United States.
The pharmacokinetic properties of antiemetics may
vary slightly.
Oral antihistamine antiemetics are absorbed well
from the GI tract and are metabolized primarily by the liver. Their inactive
metabo-lites are excreted in urine.
Phenothiazine antiemetics and serotonin 5-HT3 receptor antag-onists are absorbed well,
extensively metabolized by the liver, and excreted in urine and stool.
The action of antiemetics may vary.
The mechanism of action that produces the
antiemetic effect of antihistamines is unclear.
Phenothiazines produce their antiemetic effect by
blocking the dopaminergic receptors in the chemoreceptor trigger zone in the
brain. (This area of the brain, near the medulla, stimulates the vomiting
center in the medulla, causing vomiting.) These drugs may also directly depress
the vomiting center.
The serotonin 5-HT3 receptor antagonists block serotonin stimula-tion centrally in the chemoreceptor
trigger zone and peripherally in the vagal nerve terminals, both of which
stimulate vomiting.
The uses of antiemetics may vary.
Antihistamines are specifically used for nausea and vomiting caused by
inner ear stimulation. As a consequence, these drugs prevent or treat motion
sickness. They usually prove most effecive when given before activities that
produce motion sickness and are much less effective when nausea or vomiting has
already begun.
Phenothiazine antiemetics and serotonin 5-HT3 receptor antago-nists control severe nausea and
vomiting from various causes. They’re used when vomiting becomes severe and
potentially haz-ardous, such as postsurgical or viral nausea and vomiting. Both
types of drugs are also prescribed to control the nausea and vom-iting
resulting from chemotherapy and radiotherapy. (See Otherantiemetics.)
Antiemetics may have many significant interactions.
§ Antihistamines and phenothiazines can produce
additive CNS depression and sedation when taken with CNS depressants, such as
barbiturates, tranquilizers, antidepressants, alcohol, and opi-oids.
§ Antihistamines can cause additive
anticholinergic effects, such as constipation, dry mouth, vision problems, and
urine retention, when taken with anticholinergic drugs, including tricyclic
antide-pressants, phenothiazines, and antiparkinsonian drugs.
§ Phenothiazine antiemetics taken with
anticholinergic drugs in-crease the anticholinergic effect and decrease the antiemetic
ef-fects.
§ Droperidol used with phenothiazine
antiemetics increases the
§ risk of extrapyramidal (abnormal involuntary
movements) effects. (See Adverse
reactions to antiemetics.)
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