Classification of Antiemetics Drugs

Antiemetic Drugs: Classification, Mechanism, Uses, and Key Guidelines

Nausea and vomiting are common symptoms triggered by conditions such as motion sickness, infections, pregnancy, drug toxicity, or chemotherapy. These symptoms occur when signals from the gastrointestinal tract, vestibular system, and chemoreceptor trigger zone (CTZ) stimulate the vomiting center in the brain.

Antiemetics are medicines used to prevent or control nausea and vomiting by blocking these signals at different receptor sites. In this post from Pharma Info Nepal, we’ll explore the classification of antiemetic drugs, their mechanism of action, common uses, drugs of choice for specific conditions, and their side effects.

Classification of Antiemetics Drugs

What Are Antiemetics?

Antiemetics are a class of drugs that help prevent or relieve nausea and vomiting. They work by acting on specific receptors such as dopamine (D2), serotonin (5-HT3), histamine (H1), muscarinic (M1), and neurokinin-1 (NK1) receptors in the brain and gastrointestinal tract.

Classification of Antiemetic Drugs

1. 5-HT3 Receptor Antagonists

These medicines block serotonin receptors located in the CTZ and gastrointestinal lining.
Examples: Ondansetron, Granisetron, Palonosetron, Tropisetron
Use: Highly effective for chemotherapy- and radiation-induced nausea, as well as postoperative vomiting.

2. Dopamine (D2) Receptor Antagonists

By blocking dopamine receptors in the CTZ, these drugs reduce vomiting triggered by toxins, medications, and GI disturbances.
Examples: Metoclopramide, Domperidone, Prochlorperazine, Promethazine
Use: Drug-induced nausea, postoperative vomiting, and gastric motility disorders. Metoclopramide also has prokinetic effects.

3. H1 Receptor Antagonists (Antihistamines)

These drugs reduce vestibular input to the vomiting center and are especially helpful in motion-related sickness.
Examples: Cyclizine, Meclizine, Dimenhydrinate, Promethazine
Use: Motion sickness, vertigo, and nausea in pregnancy.

4. Muscarinic (M1) Receptor Blockers

These drugs act on muscarinic receptors in the vestibular system.
Example: Hyoscine (Scopolamine)
Use: Prevention of motion sickness.

5. NK1 Receptor Antagonists

They block neurokinin-1 receptors in the central nervous system to prevent severe vomiting.
Examples: Aprepitant, Fosaprepitant, Netupitant
Use: Delayed vomiting associated with chemotherapy, typically combined with corticosteroids and 5-HT3 blockers.

6. Corticosteroids

These are commonly used as supportive drugs to enhance the effectiveness of other antiemetics.
Examples: Dexamethasone, Methylprednisolone
Use: Chemotherapy-related and postoperative nausea (especially delayed phase).

7. Cannabinoids

Cannabinoids activate CB1 receptors in the brain to reduce vomiting and improve appetite.
Examples: Dronabinol, Nabilone
Use: Refractory nausea in chemotherapy and appetite stimulation in cancer or AIDS patients.

8. Benzodiazepines

These drugs work on GABA-A receptors to relieve anxiety and are used to control anticipatory vomiting.
Examples: Lorazepam, Diazepam
Use: Anxiety-induced nausea and anticipatory vomiting before chemotherapy.

Main Uses of Antiemetics

Antiemetic medications are essential in the management of:

• Chemotherapy- and radiation-induced nausea and vomiting
• Post-surgical vomiting
• Motion sickness and vertigo
• Gastroenteritis and other GI disorders
• Pregnancy-related nausea (morning sickness)
• Drug-induced nausea from opioids, digoxin, and other medications

Drug of Choice Highlights

• Motion sickness: Hyoscine or Promethazine
• Acute chemotherapy vomiting: Ondansetron
• Delayed chemotherapy vomiting: Aprepitant with Dexamethasone
• Postoperative vomiting: Ondansetron or Metoclopramide
• Pregnancy-related nausea: Doxylamine with Pyridoxine
• Drug-induced nausea: Metoclopramide
• Vertigo: Meclizine or Promethazine
• Anticipatory nausea: Lorazepam

Common Side Effects of Antiemetics

• 5-HT3 antagonists: Constipation, headache, QT prolongation
• D2 blockers: Sedation, extrapyramidal symptoms, hyperprolactinemia
• H1 blockers: Drowsiness, dry mouth, blurred vision
• Muscarinic blockers: Dizziness, urinary retention, dry mouth
• NK1 blockers: Fatigue, hiccups, drug interactions via CYP3A4
• Corticosteroids: Insomnia, hyperglycemia, mood changes
• Cannabinoids: Dizziness, euphoria, abuse risk
• Benzodiazepines: Sedation, dependence

Clinical Pearls

• Ondansetron is the most widely used antiemetic for both hospital and outpatient care.
• Metoclopramide should be used with caution in children and elderly patients due to EPS risk.
• Aprepitant is reserved for highly emetogenic chemotherapy and works best in combination therapy.
• Doxylamine with Pyridoxine is the safest and most effective choice for pregnancy nausea.
• Cannabinoids are not first-line agents and should only be used for resistant cases.

Final Thoughts

Antiemetic drugs are a crucial part of medical care, helping patients manage nausea and vomiting from various causes. A good understanding of their classification, mechanisms, and proper selection ensures safe and effective treatment.

For more detailed pharmacology notes and Nepal Pharmacy Council License Exam preparation, follow Pharma Info Nepal.

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