Anti-Emetic Drugs

Antiemetic Drugs are used to prevent and treat nausea and vomiting. The most common uses of antiemetic drugs include

• Gastroenteritis.
• Motion sickness.
• Morning sickness (first trimester).
• Chemotherapy-induced nausea and vomiting (CINV).
• Postoperative nausea and vomiting. 

Pathophysiology Of Vomiting

• The higher center stimulates the vomiting center in the brain through bad smell, sight, and fear. 
• The second stimulus comes from the vestibular apparatus (balance) connected to the cerebellum, which activates the vomiting Center. 
• The toxins or some drugs in the blood vessels also trigger the stimulation of the vomiting center. 
• Outside the blood-brain barrier, there is a zone called the chemoreceptor trigger zone (CTZ). Toxins and drugs can also activate this zone, which activates the vomiting Center. Receptors in this zone, including D2, Muscularinic, 5HT3, and Neurokinin1 (NK1), activate the vomiting center. 
• The nucleus tractus-solitarius (NTS) is also connected to the vomiting Center. 
• NTS can be activated by neurotransmitters released from the nerve terminal, which can cause nausea and vomiting. This is controlled by the cannabinoid1 (CB1) receptor, which decreases the release of neurotransmitters. Hence, it is suggested to stimulate CB1 to control nausea and vomiting. 
• Pain in any condition causes the release of “substance P,” which activates NK1. 

Targets, uses, and effects of Antiemetic Drugs

Mechanism Of Action

Metoclopramide acts on the peripheral and central regions of the brain.  

Action of Metclopromide on the Peripheral regions of the brain:

• It enters the brain by crossing the blood-brain barrier. In the periphery, it blocks D2, which is responsible for the antiemetic effect. 
• It also has 5HT4 agonistic property that helps in prokinetic properties used to manage GERD (Gastroesophageal reflux disease). 
• The drug also produces 5HT3 antagonistic properties at high doses.  
• Prokinetic property:
  • In GERD patients, the esophageal sphincter does not work properly, and acid reflux back into the esophagus. Metoclopramide increases esophageal sphincter tone, gastric emptying, and small intestinal motility, collectively known as prokinetic properties.

Action of Metclopromide on the central region of the brain

• When metoclopramide enters the brain, it blocks the D2 receptor, where dopamine and acetylcholine are in balance. However, D2 Blockers stop the dopamine action, and acetylcholine becomes higher in concentration. The patient may develop extrapyramidal symptoms, leading to acute dystonia and oculogyric crisis. 
• These conditions are usually seen in younger populations, but blocking D2 in the elderly can lead to Parkinson’s symptoms. 
• In this condition, central anticholinergics such as benzhexol or promethazine are given to manage the levels of acetylcholine. 
• Due to the central action of metoclopramide, it is not commonly used nowadays. 
• Domperidone does not cross BBB, so no risk of extrapyramidal symptoms. 
• Hyoscine can be given in two ways; oral tablet or transdermal patch.

1. The oral tablet is taken 1-2 hours before the journey and works for a shorter journey (4-6 hours). 
2. The patch is applied 6-8 hours before the journey and works for 3 days. 

• Recently, the combination of Netupitant and Palonosetron has been approved for early and late CINV.

Frequently Asked Questions

Q: What used to prevent motion sickness?

Answer: Promethazine 

Q: What activates the vomiting center?

Answer: Neurokinin1

Q: Which Cannabinoid 1 receptor Agonist is used as an antiemetic drug?

Answer: Nabilone  

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