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.)