See anticholinergics in the Appendix. Detailed tables show doses, recom-
mendations, expectations, side effects, contraindications, and more; available on
the book’s Web site (see URL in Appendix).
Dopamine antagonists
Dopamine antagonists suppress emesis by blocking dopamine receptors in the
CTZ. These include phenothiazines, butyrophenones, and metoclopramide.
Common side effects are extrapyramidal symptoms (EPS)—caused by blocking
the dopamine receptors—and hypotention. Phenothiazines are the largest group
of antiemetics. They are also used for anxiety (see Chapter 15). Dopamine
antagonists suppress emesis by blocking dopamine receptors. They act by
inhibiting the CTZ. Not all phenothiazines are effective antiemetics. The dose is
generally smaller when phenothiainerase is used as an antiemetic. Promethazine
(Phenergan) is a phenothiazine. It was introduced as an antihistamine with seda-
tive side effects and can be used for motion sickness.
Phenothiazines and the miscellaneous antiemetics such as benzquinamide,
diphenidol, metclopramide, and trimethobenzamide act on the CTZ center.
Chlorpromazine (Thorazine) and prochlorperazine edisylate (Compazine) are
tranquilizers used for both psychosis and vomiting.
Butyrophenones include haloperidol (Haldol) and droperidol (Inapsine).
They block dopamine 2 receptors in the CTZ. Extrapyramidal side effects (EPS)
are likely to occur if these drugs are used over an extended period of time.
Hypotension can also occur.
Metoclopramide (Reglan) suppresses emesis by blocking the dopamine and
serotonin receptors in the CTZ. High doses can cause sedation and diarrhea. The
occurrence of EPS is more prevalent in children than adults.
Benzodiazepines
Benzodiazepines indirectly control nausea and vomiting. Lorazepam (Ativan) is
the choice drug in this category and may be given with metoclopramide.
Serotonin antagonists
Serotonin antagonists suppress nausea and vomiting by blocking the serotonin
receptors in the CTZ and the afferent vagal nerve terminals in the upper GI tract.
Two serotonin antagonists—ondanestron (Zofran) and granisetron (Kytril)—are
effective in suppressing chemotherapy-induced emesis. They do not block the
dopamine receptors. Therefore, they do not cause EPS. Zofran is commonly
used to prevent and treat post-operative nausea and vomiting.
CHAPTER 18 Gastrointestinal System^333