Antiemetic Drugs – Working, Classification

Emesis

Vomiting occurs due to stimulation of the emetic (vomiting) centre situated in the medulla oblongata.

Antiemetic

Antiemetic drugs are types of chemicals that help to cure symptoms of nausea or vomiting.

This may also be used to treat nausea and vomiting caused by other medications, frequent motion sickness, infection, or stomach flu.

Work

These drugs help to block specific neurotransmitters in the body. These neurotransmitters trigger impulses such as nausea and vomiting, so blocking the impulses will help shut them down.

Emetic response

Nausea is accompanied by reduced gastric tone and peristalsis.

In the emetic response fundus and body of stomach, esophageal sphincter and esophagus relax, glottis closes, while duodenum and pyloric stomach contract in a retrograde manner. Rhythmic contractions of diaphragm and abdominal muscles then compress the stomach and evacuate its contents via the mouth. Conditions that inhibit gastric emptying predispose to vomiting.

Emetics

NOTE– The chemoreceptor trigger zone (CTZ) is an area of the medulla oblongata that receives inputs from blood-borne drugs or hormones, and communicates with other structures in the vomiting center to initiate vomiting. emetic are drugs used to evoke (produce) vomiting.

1. Act on CTZ- Apomorphine
2. Act reflexly and on CTZ : Ipecacuanha

Vomiting need to be induced only when an undesirable substance (poison) has been ingested. Powdered mustard suspension or strong salt solution may be used in emergency. They act reflexly by irritating the stomach.

1. Apomorphine :-

• It is a semisynthetic derivative of morphine; acts as a dopaminergic agonist on the CTZ.
• It should not be used if respiration is depressed, because it has inherent respiratory and CNS depressant actions.
• Oral use of apomorphine is not recommended because the emetic dose is larger, slow to act and rather inconsistent in action.
• It has a therapeutic effect in parkinsonism, but is not used due to side effects.

2. Ipecacuanha :-

• The dried root of Cephaelis ipecacuanha contains emetine and is used as syrup for inducing vomiting.
• It acts by irritating gastric mucosa as well as through CTZ

All emetics are contraindicated in :-

(a) Corrosive (acid, alkali) poisoning: risk of perforation and further injury to esophageal mucosa.

(b) CNS stimulant drug poisoning: convulsions may be precipitated.

(c) Kerosine (petroleum) poisoning: chances of aspiration of the liquid (due to low viscosity) and chemical pneumonia are high.

(d) Unconscious patient: may aspirate the vomitus, because laryngeal reflex is likely to be impaired.

(e) Morphine or phenothiazine poisoning: emetics may fail to act.

Antiemetic Drugs

Antiemetic drugs are used to suppress vomitting.

CLASSIFICATION :-

1. Anticholinergics= Hyoscine, Dicyclomine
2. H1 Antihistaminics= Promethazine, Diphenhydramine, Dimenhydrinate, Doxylamine, Meclozine (Meclizine), Cinnarizine.
3. Neuroleptics= (Antipsychotics, also known as neuroleptics, are a class of medication primarily used to manage psychosis including hallucinations or disordered thought etc) Chlorpromazine, Trifluoperazine, Prochlorperazine, Haloperidol etc.
4. Prokinetic drugs= Metoclopramide, Domperidone, Cisapride, Mosapride, Itopride
5. 5-HT, antagonists – Ondansetron, Granisetron, Palonosetron, Ramosetron

• Other drugs as; Aprepitant, Fosaprepitant Dexamethasone, Benzodiazepines, Dronabinol, Nabilone etc also Antiemetic drugs.

ANTICHOLINERGICS

1. Hyoscine –

• It is the most effective drug for motion sickness; produces sedation, dry mouth and other anticholinergic side effects.
• A transdermal patch containing 1.5 mg of hyoscine applied behind the pinna, it suppresses motion sickness while producing only mild side effects.

2. Dicyclomine –

• It has been used for prophylaxis of motion sickness and for morning sickness.

ANTIHISTAMINICS

• Some Antihistamines are Antiemetic.
• They are useful mainly in motion sickness and to a lesser extent in morning sickness, postoperative and some other forms of vomiting.

1. Promethazine, diphenhydramine, dimenhydrinate = These drugs afford protection of motion sickness for 4-6 hours, but produce sedation and dryness of mouth.
2. Promethazine theoclate
3. Doxylamine= It is a sedative H1 Antihistaminics with prominent Anticholinergic activity. Marketed in combination with pyridoxine, it is specifically promoted for ‘morning sickness‘ (vomiting of early pregnancy).
4. Meclozine (meclizine)= It is less sedative and longer-acting; protects against sea sickness for nearly 24 hours.
5. Cinnarizine= It is an antivertigo drug having antimotion sickness property.

PROKINETIC DRUGS

Drugs which enhances gastrointestinal motility by increasing the frequency or strength of contractions but without disrupting their rhythm.

TO KNOW ABOUT MORE DRUGS, READ DRUGS IN MODERN PHARMACOLOGY.