Emetics and Anti-Emetic Drugs
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Emetics
These drugs can be used to remove non-corrosive poisons from the stomach before any is absorbed. Charcoal can be given to adsorb as much poison as possible and decrease their absorption.
They act by:
Irritating the gastric mucosa, the so called "Reflux Emetics". Examples are; magnesium sulphate (epsom salts), ipecacuanha, sodium chloride, sodium carbonate (washing soda), mustard.
Stimulating the Vomiting Centre in the Medulla These drugs act by agonising neurotransmitter receptors involved in emetic pathways; they are centrally acting emetics. Apomorphine is such a drug.
Anti-Emetics
Persistent vomiting is a potentially dangerous situation for an animal to be in. It can lead to dehydration, metabolic alkalosis and electrolyte imbalances; at worst it can lead to shock. If vomiting is occuring for a reason other than ingestion of a foreign material or a gastric obstruction, such as a drug side-effect, it may be worth considering using an anti-emetic. It must be remebered that vomiting often can be controlled by withholding food, a change of diet and anthelmintic therapy and so they are only really needed if there is no improvement and the animal is becoming more ill.
Note that when vomiting is caused by excess levels of acid or gastritis, Gastroprotective Drugs can be used to reduce the vomiting.
Dopamine 2 (D2) Receptor Antagonists
One class of drugs the phenothiazines inhibit unselectively M, H1, and alpha receptors as well as D2 receptors. This means that they have additional effects, such as sedation and hypotension. These drugs are further discussed on the anaesthesia pages, but examples are acepromazine, chlorpromazine and proclorperazide.
Metoclopramide isn't a phenothiazine but also acts on the D2 receptors, it also though works on serotonin receptor. It inhibits some serotonin receptors (5-HT3) and activates others (5-HT4). This results in reducing inputs into the vomiting center and also increases gastric emptying and gastrointestinal tract motility.
5-HT3 Receptor Antagonists
An example is ondansetron, which works by directly inhibiting the chemoreceptor trigger zone and thus inhibiting the vomiting centre. Ondansetron is a serotonin 5-HT3 receptor antagonist used mainly as an antiemetic (to treat nausea and vomiting), often following chemotherapy. It affects both peripheral and central nerves. Ondansetron reduces the activity of the vagus nerve, which deactivates the vomiting center in the medulla oblongata, and also blocks serotonin receptors in the chemoreceptor trigger zone. It has little effect on vomiting caused by motion sickness, and does not have any effect on dopamine receptors or muscarinic receptors.
Muscarinic Receptor Antagonists
Examples are hyoscine and pirenzepine.
Histamine Receptor Antagonists
Examples are diphenhydramine and cyclizine.
Neurokinin (NK1) Receptor Antagonists
The new drug maropitant has become liscensed in the dog in the UK for use in treating vomiting and motion sickness.