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 sedative 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 dopamine2 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.

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