Antiemetics

• Are drugs used to prevent or treat nausea and vomiting.

Nausea is an unpleasant sensation of abdominal discomfort accompanied by a desire to vomit.

Vomiting is the expulsion of stomach contents through the mouth Nausea may occur without vomiting and vomiting may occur without prior nausea, but the two symptoms most often occur together.

• Vomiting occurs when the vomiting center in the medulla oblongata is stimulated. Dopamine and acetylcholine play a major role in stimulating the vomiting center. To a certain extent, vomiting is a protective mechanism which can result from various noxious stimuli.

Drugs used in nausea and vomiting belong to several different therapeutic classifications.

• Most antiemetic agents relieve nausea and vomiting by acting on the vomiting center, CTZ, cerebral cortex, vestibular apparatus, or a combination of these.
• Antiemetic drugs are generally more effective in prophylaxis than treatment. Antiemetic drugs include:

Phenothiazines (neuroleptics) such as chlorpromazine

• Acts on CTZ and vomiting center
• Block dopamine receptors
• Are effective in prevention or treating nausea and vomiting induced by drugs, radiation therapy, surgery and most other stimuli

(e.g. pregnancy).

• Are generally ineffective in motion sickness.

Antihistamines – such as promothazine, dimehydrinate etc.

–     Are especially effective in prevention and treatment of motion sickness (but they may cause concurrent drowsiness, that may be troublesome for travellers)

Miscellaneous antiemetics

Metoclopramide has both central and peripheral antiemetic effects. Centrally, metoclopoamide antagonizes the action of dopamine.

Peripherally metoclopoamide stimulates the release of acetylcholine, which in turn, increases the rate of gastric emptying (used in esophapeal reflux)

Indication as chlorpromazine

• Scopolamine, an anticholinergic drug is very effective in reliving nausea & vomiting associated with motion sickness.
• Ondansetron- is serotonin antagonist (5-HT₃ receptors) found on the afferent fibers of the vagus nerve and in parts of the brain associated with CTZ.
• Controls chemical induced vomiting and nausea)