Overview of Antiemetics

Study Notes

Overview of Antiemetics

Vomiting (emesis) is the expulsion of gastric contents through the mouth, triggered by various factors, including infections, food intolerances, motion sickness, and treatments like chemotherapy.
Nausea precedes vomiting and is characterized by a queasy sensation.
Prescription antiemetics should be used only after identifying the cause of vomiting, as they may obscure underlying issues.
Severe vomiting leading to dehydration or electrolyte imbalances prompts the use of antiemetics.
The brain's vomiting mechanism involves two centers: the chemoreceptor trigger zone (CTZ) and the vomiting center in the medulla.
Nonprescription antiemetics include antihistamines, bismuth subsalicylate, and phosphorated carbohydrate solutions.
Prescription antiemetics comprise multiple classes, including antihistamines, anticholinergics, dopamine antagonists, serotonin antagonists, and glucocorticoids.
Nonprescription drugs like bismuth subsalicylate directly suppress vomiting by acting on the gastric mucosa.

Pharmacodynamics of Antihistamines

Antihistamines function by blocking H1 receptors, similar to anticholinergics, thereby inhibiting vestibular stimulation.
Nausea and vomiting result from stimulation of the vestibular and reticular systems, which antihistamines counteract by blocking cholinergic stimulation.
These medications differ from H2 receptor blockers, which are used for gastric acid control.
Meclizine is an antihistamine employed specifically for motion sickness, dizziness, and vertigo.

Pharmacodynamics of Anticholinergics

Anticholinergics act by binding to acetylcholine receptors in the vestibular nuclei of the brain, blocking nausea signals to the CTZ.
By obstructing reticular formation receptors, they prevent nausea signals from reaching the vomiting center.
Anticholinergics help reduce acute GI symptoms by causing dehydration of GI secretions and decreasing intestinal muscle spasms.
Scopolamine is the primary anticholinergic antiemetic, effective for motion sickness and postoperative nausea, by correcting the imbalance between acetylcholine and norepinephrine in balance control areas of the brain.

Pharmacodynamics of Dopamine Antagonists

Dopamine antagonists, including phenothiazines, butyrophenones, and benzodiazepines, alleviate nausea by blocking dopamine D2 receptors in the CTZ.
Phenothiazines, like chlorpromazine and prochlorperazine, are effective for nausea linked to surgery, anesthesia, and cancer treatments.
Promethazine, the most commonly prescribed antiemetic, was initially used as an antihistamine and also alleviates motion sickness.
Select benzodiazepines help manage nausea and vomiting related to cancer chemotherapy, with diazepam being one of the preferred options.

Description

This quiz covers the mechanisms and classifications of antiemetics used to treat vomiting and nausea. It discusses the importance of identifying the underlying causes of these symptoms and highlights both prescription and nonprescription options. Gain insight into how different antiemetics work and their appropriate use.