Modes of action

• Phenothiazines increase the threshold for vomiting at the chemoreceptor trigger zone via central DA2-dopaminergic blockade; at higher doses there may also be some effect on the vomiting centre.

• Metoclopramide acts centrally and by increasing gastric motility.

• The exact mechanism of cyclizine action is unknown. It increases lower oesophageal sphincter tone and may inhibit the midbrain emetic centre.

• Ondansetron is a highly selective 5HT3 receptor antagonist; its precise mode of action is unknown but may act both centrally and peripherally.

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