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Chapter 33: Drugs Used for the Treatment of Bowel Disorders

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A 23-year-old woman with a history of type 1 diabetes since age 6 presents with nausea and vomiting associated with gastroparesis and delayed gastric emptying. Which of the following medications would be the most suitable treatment of the nausea and vomiting in this patient?

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a. Dephenoxylate

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b. Metoclopramide

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c. Ondansetron

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d. Dronabinol

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e. Loperamide

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Answer is b. Metoclopramide is a dopamine (DA) receptor antagonist that is commonly used to treat the symptoms of delayed gastric emptying. Another drug that might be used is cisapride although it is available only through a limited use program because of its incidence of serious cardiovascular toxicities. Erythromycin is also a possible alternative.

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A 64-year-old woman is receiving ondansetron for the prophylactic treatment of emesis secondary to chemotherapy. Ondansetron’s main mechanism of action is antagonism of

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a. central μ opioid receptors.

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b. peripheral H2 receptors.

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c. central dopamine receptors.

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d. central 5-HT3 receptors.

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e. peripheral muscarinic receptors.

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Answer is d. Ondansetron is a central 5-HT3 receptor antagonist. Table 33-3 shows a general classification of anti-emetic drugs.

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Table 33-3. General Classification of Anti-emetic Agents
ANTI-EMETIC CLASS EXAMPLES TYPE OF VOMITING MOST EFFECTIVE AGAINST
5-HT3 receptor antagonistsa Ondansetron Cytotoxic drug induced emesis
Centrally acting dopamine receptor antagonists

Metoclopramideb

Promethazinec

Cytotoxic drug induced emesis
Histamine H1 receptor antagonists Cyclizine Vestibular (motion sickness)
Muscarinic receptor antagonists Hyoscine (scopolamine) Motion sickness
Neurokinin receptor antagonists Aprepitant Cytotoxic drug induced emesis (delayed vomiting)
Cannabinoid receptor agonists Dronabinol Cytotoxic drug induced emesis

aThe most effective agents for chemotherapy-induced nausea and vomiting are the 5-HT3 antagonists and metoclopramide. In addition to their use as single agents, they are often combined with other drugs to improve efficacy as well as reduce the incidence of side effects.

bAlso has some peripheral activity at 5-HT3 receptors.

cAlso has some antihistaminic and anticholinergic activity.

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A 35-year-old woman has severe irritable bowel syndrome characterized by frequent and profuse diarrhea. She is being treated with alosetron because she has not responded to other forms of antidiarrheal therapy. The physician treating this patient must diligently monitor for

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a. ischemic colitis.

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b. congestive ...

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