medicnotes.org.uk :: Anti-emetics

Anti-emetic drugs

The reflex mechanism of vomiting:

Emetic stimuli:

Chemicals in blood

Neuronal input from GI tract, labyrinth and CNS

Impulses from chemoreceptor trigger zone (CTZ), and various CNS centres, relay to the vomiting centre
Numerous chemical transmitters involved:

Histamine (H₁-receptors)
Acetylcholine (muscarinic receptors)
Dopamine (D₂ receptors)
5-Hydroxytryptamine (5-HT₃ receptors)

Antiemetic drugs:

H₁-receptor antagonists (e.g. cyclizine)
Muscarinic antagonists (e.g. hyoscine)
5-HT₃ antagonists (e.g. ondansetron)
D₂-receptor antagonists (e.g. metoclopramide)
Cannabinoids (e.g. nabilone)

H₁-receptor antagonists:

E.g. cyclizine, cinnarizine, promethazine
Little or no activity against vomiting produced by substances acting directly on the CTZ
Effective in motion sickness and against vomiting caused by substances which act locally in the stomach
Most effective if given before the onset of nausea and vomiting
May have some action in controlling it when established

Muscarinic-receptor antagonists:

E.g. hyoscine
Active against nausea and vomiting of labyrinthine origin and against vomiting due to local stimuli in the stomach
Ineffective against substances which act directly on the CTZ
Not very useful once sickness has occurred

Main side-effects:

Drowsiness
Dry mouth

5-HT receptor antagonists:

E.g. ondansetron
5-HT is released in the CNS and gut and is an important transmitter in emesis
Very effective at treating vomiting caused by cytotoxic drugs

Main side-effects:

Headache
GI disturbances

D_D-receptor antagonists:

E.g. metoclopramide
Acts in the CTZ
Is a pro-kinetic (increases gut motility)

Main side-effects:

Extrapyramidal effects
Dizziness
Hyperprolactinaemia
Amenorrhoea
Diarrhoea

Cannabinoids:

E.g. nabilone
Decreases vomiting due to agents which stimulate the CTZ
Its antiemetic effect is antagonised by naloxone (which implies that opioid receptors may be important in the action of this drug)

Main side-effects:

Drowsiness
Dizziness
Dry mouth
Mood changes


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	Anti-emetic drugs The reflex mechanism of vomiting: Emetic stimuli: Chemicals in blood Neuronal input from GI tract, labyrinth and CNS Impulses from chemoreceptor trigger zone (CTZ), and various CNS centres, relay to the vomiting centre Numerous chemical transmitters involved: Histamine (H₁-receptors) Acetylcholine (muscarinic receptors) Dopamine (D₂ receptors) 5-Hydroxytryptamine (5-HT₃ receptors) Antiemetic drugs: H₁-receptor antagonists (e.g. cyclizine) Muscarinic antagonists (e.g. hyoscine) 5-HT₃ antagonists (e.g. ondansetron) D₂-receptor antagonists (e.g. metoclopramide) Cannabinoids (e.g. nabilone) H₁-receptor antagonists: E.g. cyclizine, cinnarizine, promethazine Little or no activity against vomiting produced by substances acting directly on the CTZ Effective in motion sickness and against vomiting caused by substances which act locally in the stomach Most effective if given before the onset of nausea and vomiting May have some action in controlling it when established Muscarinic-receptor antagonists: E.g. hyoscine Active against nausea and vomiting of labyrinthine origin and against vomiting due to local stimuli in the stomach Ineffective against substances which act directly on the CTZ Not very useful once sickness has occurred Main side-effects: Drowsiness Dry mouth 5-HT receptor antagonists: E.g. ondansetron 5-HT is released in the CNS and gut and is an important transmitter in emesis Very effective at treating vomiting caused by cytotoxic drugs Main side-effects: Headache GI disturbances D_D-receptor antagonists: E.g. metoclopramide Acts in the CTZ Is a pro-kinetic (increases gut motility) Main side-effects: Extrapyramidal effects Dizziness Hyperprolactinaemia Amenorrhoea Diarrhoea Cannabinoids: E.g. nabilone Decreases vomiting due to agents which stimulate the CTZ Its antiemetic effect is antagonised by naloxone (which implies that opioid receptors may be important in the action of this drug) Main side-effects: Drowsiness Dizziness Dry mouth Mood changes
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