General concepts of the use of emetics and antiemetics

Apomorphine - dopamine agonist

Effective in dogs, NOT in cats

PO, IV, SC, conjunctival and gingival membranes - not IM

Vomiting in 5-10 minutes

Does not work well in sedated and motionless animals

Ropinirole - dopamine agonist

Effective in dogs only

Administered as eye drops

Results in emesis within 30 minutes of administration in 95% of the animals

Adverse effects of Ropinirole

Transient hyperemia of the eye, ocular discharge, protrusion of the 3rd eyelid, and

blepharospasm.

Transient mild lethargy and increased heart rate

α2-adrenergic agonists - xylazine and dexmedetomidine

Effective in cats - stimulates the CRTZ

Profound sedation and hypotension: monitor the patient

Hydrogen peroxide(3% max): stimulates vomiting via the 9th cranial nerve

administer cautiously, especially in cats, because aspiration of hydrogen peroxide foam causes severe aspiration pneumonia

5-HT3 Receptor Antagonists: Ondansetron

blocks serotonin receptors and is used for vomiting caused by chemotherapy, radiation, and postoperative recovery

Dopamine Antagonists: Metoclopramide

blocks dopamine receptors in the brain's trigger zone and treats gastrointestinal and drug-induced emesis

Antihistamines and Anticholinergics: Meclizine or scopolamine

reduce inner ear and vestibular excitability → ideal for motion sickness and vertigo

Antiemetic drug targets

Neuronal pathways involved in nausea and vomiting: Motion sickness

vestibular system

ACh-M1-receptor antagonists

H2-receptor antagonists

Neuronal pathway involved in nausea and vomiting: CRTZ and/or vomiting

center

D2-receptor antagonists

H2-receptor antagonists

5-HT3-receptor antagonists

NK1-receptor antagonists

α2-receptor antagonists (cats)

Acepromazine Targets:

Dopamine antagonists

α-adrenergic antagonist in the vomiting center

Acepromazine uses

Broad-spectrum antiemetic effects

Acepromazine adverse effects

Sedation, altered involuntary movements

Vasodilation

Atropine targets

Non-selective muscarinic antagonists

Atropine uses

Effective in cases of vestibular stimulation and CRTZ stimulation

Atropine adverse effects

Xerostomia (dry mouth), decreased stomach emptying, ileus, urinary retention, and constipation

Diphenhydramine Target

Histamine receptors(antihistaminergic)

Also has antimuscarinic effects(atropine-like)

Diphenhydramine uses

Broad-spectrum antiemetic

Diphenhydramine adverse effects

Safe, but can lead to sedation

metoclopramide Target

Inhibition of D2 dopamine transmission

Inhibits serotonin (5-HT3) receptors

Metoclopramide uses

Vomiting (gastrointestinal disease)

Peripheral effects on the GI tract - increased emptying of the stomach and upper duodenum

Metoclopramide adverse effects

Excitement and other behavioral changes are observed

Ondansetron and dolasetron target

5-HT3 antagonists

short half-life - administer SC

Ondansetron and dolasetron uses

Cancer chemotherapy, gastroenteritis, pancreatitis, and inflammatory bowel disease

Ondansetron and dolasetron adverse effects

Uncommon and include constipation, sleepiness, or head shaking

Neurokinin-1 antagonists: Maropitant targets

Interacts with the vomiting center - very effective

First-pass effects in dogs, not cats - SC administration

Maropitant uses

Broad-spectrum antiemetic - good safety profile

maropitant adverse effects

Analgesia - decreases coughing